61.

Quantum transport in random alloys with intermetallic needle-shape precipitates 

Elham Sharafedini, Hossein Hamzehpour, and Mohammad Alidoust

Phys. Rev. E 112, 034128 (2025). [PDF]

Employing Monte Carlo sampling and a self-consistent multiscale numerical approach, we obtain a parametric simple formula for the effective electrical charge conductivity of composite systems containing randomly distributed intermetallic needle-shape precipitates. This formula, derived from extensive fits to computational results, accounts for various experimentally relevant parameters, including the externally applied voltage difference 𝑉 across the sample, the band gap 𝜇 and density 𝜙 of the precipitates, the density of intermetallic materials, and the orientation of the needle-shape precipitates/order parameter 𝑠. Employing this formula, we determine the charge conductance threshold activation functionalities for each parameter versus the other parameters. This parametric formula can serve as a tool for analyzing and designing composite systems.

60.

 Signatures of enhanced spin-triplet superconductivity induced by interfacial properties 

Chenghao Shen, Jong E. Han, Thomas Vezin, Mohammad Alidoust, and Igor Žutić

Phys. Rev. B 110, 104514 (2024). [PDF]

While spin-triplet pairing remains elusive in nature, there is a growing effort to realize proximity-induced equal-spin triplet superconductivity in junctions with magnetic regions or an applied magnetic field and common s-wave superconductors. To enhance such spin-triplet contribution, it is expected that junctions with a weak interfacial barrier and strong spin-orbit coupling are desirable. Intuitively, a weak interfacial barrier enables a robust proximity-induced superconductivity and strong spin-orbit coupling promotes spin mixing, converting spin-singlet into spin-triplet superconductivity. In contrast, we reveal a nonmonotonic spin-triplet contribution with the strength of the interfacial barrier and spin-orbit coupling. This picture is established by considering different signatures in conductance and superconducting correlations, as well as by performing self-consistent calculations. As a result, we identify a strongly enhanced spin-triplet superconductivity, realized for an intermediate strength of interfacial barrier and spin-orbit coupling. In junctions with magnetic regions, an enhanced spin-triplet superconductivity leads to a large magnetoanisotropy of conductance and superconducting correlations. This picture of an enhanced spin-triplet superconductivity is consistent with experiments demonstrating a huge increase in the conductance magnetoanisotropy, which we predict can be further enhanced at a finite bias.

59.

Phase jumps in Josephson junctions with time-dependent spin–orbit coupling 

David Monroe, Chenghao Shen, Dario Tringali, Mohammad Alidoust, Tong Zhou, and Igor Žutić

Appl. Phys. Lett. 125, 012601 (2024). [PDF]

Planar Josephson junctions (JJs), based on common superconductors and III–V semiconductors, are sought for Majorana states and fault-tolerant quantum computing. However, with gate-tunable spin–orbit coupling (SOC), we show that the range of potential applications of such JJs becomes much broader. The time-dependent SOC offers unexplored mechanisms for switching JJs, accompanied by the 2pi-phase jumps and the voltage pulses corresponding to the single-flux-quantum transitions, key to high-speed and low-power superconducting electronics. In a constant applied magnetic field, with Rashba and Dresselhaus SOC, anharmonic current-phase relations, calculated microscopically in these JJs, yield a nonreciprocal transport and superconducting diode effect. Together with the time-dependent SOC, this allows us to identify a switching mechanism at no applied current bias, which supports fractional-flux-quantum superconducting circuits and neuromorphic computing.

58.

 Statistical quantum conductance of porous and random alloys 

Elham Sharafedini, Hossein Hamzehpour, and Mohammad Alidoust

Appl. Phys. Lett. 123, 172104 (2023). [PDF]

Performing statistical evaluations, coupling Schrödinger's equation and Poisson's equation self-consistently, and employing an iterative fitting process, we have obtained a simple parametric formula for the charge conductance of nonmagnetic two-phase porous and random alloys. The formula exhibits remarkable agreement in describing the response of a system to an applied voltage difference, system size, bandgap, and density of conductive grains as parameters. Exploiting the obtained formula, we parametrically determine the activation threshold functionality of each parameter to other parameters where the charge conductance switches “on” and “off.” The results of our study can be directly utilized to guide experiments.

57.

Multiscale statistical quantum transport in porous media and random alloys with vacancies 

Elham Sharafedini, Hossein Hamzehpour, and Mohammad Alidoust

J. Appl. Phys. 133, 035102 (2023). [PDF]

We have developed a multi-scale self-consistent method to study the charge conductivity of a porous system or a metallic matrix alloyed by randomly distributed nonmetallic grains and vacancies by incorporating Schrödinger's equation and Poisson's equation. To account for the random distribution of the nonmetallic grains and clusters within the alloy system, we have used an uncorrelated white-noise Monte-Carlo sampling to generate numerous random alloys and statistically evaluate the charge conductance. We have performed a parametric study and investigated various electrical aspects of random porous and alloy systems as a function of the inherent parameters and density of the random grains. Our results find that the charge conductance within the low-voltage regime shows a highly nonlinear behavior against voltage variations in stark contrast to the high-voltage regime where the charge conductance is constant. The former finding is a direct consequence of the quantum scattering processes. The results reveal the threshold to the experimentally observable quantities, e.g., voltage difference, so that the charge current is activated for values larger than the threshold. The numerical study determines the threshold of one quantity as a function of the remaining quantities. Our method and results can serve to guide future experiments in designing circuital elements, involving this type of random alloy system.

56.

Tunable planar Josephson junctions driven by time-dependent spin-orbit coupling (Diode effect, φ0 state, and gate-induced GHz SOC changes in circuits with anharmonic CPR) 

David Monroe, Mohammad Alidoust, and Igor Zutic

Phys. Rev. Applied (Letter) 18, L031001 (2022). [PDF]

Integrating conventional superconductors with common III-V semiconductors provides a versatile platform to implement tunable Josephson junctions (JJs) and their applications. We propose that with gate-controlled time-dependent spin-orbit coupling, it is possible to strongly modify the current- phase relations and Josephson energy and provide a mechanism to drive the JJ dynamics, even in the absence of any bias current. We show that the transition between stable phases is realized with a simple linear change in the strength of the spin-orbit coupling, while the transition rate can exceed the gate-induced electric field GHz changes by an order of magnitude. The resulting interplay between the constant effective magnetic field and changing spin-orbit coupling has direct implications for superconducting spintronics, controlling Majorana bound states, and emerging qubits. We argue that topological superconductivity, sought for fault-tolerant quantum computing, offers simpler applications in superconducting electronics and spintronics.

55.

Machine-learned model Hamiltonian and strength of spin-orbit interaction in strained Mg2X (X = Si, Ge, Sn, Pb) 

Mohammad Alidoust, Erling Rothmund, and Jaakko Akola
J. Phys.: Condens. Matter 34, 365701 (2022). [PDF]

Machine-learned multi-orbital tight-binding (MMTB) Hamiltonian models have been developed to describe the electronic characteristics of intermetallic compounds Mg2Si,Mg2Ge,Mg2Sn, and Mg2Pb subject to strain. The MMTB models incorporate spin-orbital mediated interactions and they are calibrated to the electronic band structures calculated via density functional theory (DFT) by a massively parallelized multi-dimensional Monte-Carlo search algorithm. The results show that a machine-learned five-band tight-binding model reproduces the key aspects of the band structures in the entire Brillouin zone. The five-band model reveals that compressive strain localizes the contribution of the 3s orbital of Mg to the conduction bands and the outer shell p orbitals of X (X = Si,Ge,Sn,Pb) to the valence bands. In contrast, tensile strain has a reversed effect as it weakens the contribution of the 3s orbital of Mg and the outer shell p orbitals of X to the conduction bands and valence bands, respectively. The π bonding in the Mg2X compounds is negligible compared to the σ bonding components, which follow the hierarchy |σsp| > |σpp| > |σss|, and the largest variation against strain belongs to σpp. The five-band model allows for estimating the strength of spin-orbit coupling (SOC) in Mg2X and obtaining its dependence on the atomic number of X and strain. Further, the band structure calculations demonstrate a significant band gap tuning and band splitting due to strain. A compressive strain of −10% can open a band gap at the Γ point in metallic Mg2Pb, whereas a tensile strain of +10% closes the semiconducting band gap of Mg2Si. A tensile strain of +5% removes the three-fold degeneracy of valence bands at the Γ point in semiconducting Mg2Ge. The presented MMTB models can be extended for various materials and simulations (band structure, transport, classical molecular dynamics), and the obtained results can help in designing devices made of Mg2X.

54.

Controllable nonreciprocal optical response and handedness-switching in magnetized spin orbit coupled graphene 

Mohammad Alidoust and Klaus Halterman

Phys. Rev. B 105, 045409 (2022). [PDF]

Starting from a low-energy effective Hamiltonian model, we theoretically calculate the dynamical optical conductivity and permittivity tensor of a magnetized graphene layer with Rashba spin-orbit coupling (SOC). Our results reveal a transverse Hall conductivity correlated with nonreciprocal longitudinal conductivity. Further analysis illustrates that for intermediate magnetization strengths, the relative magnitudes of the magnetization and SOC can be identified experimentally by two well-separated peaks in the dynamical optical response (both the longitudinal and transverse components) as a function of photon frequency. Moreover, the frequency-dependent permittivity tensor is obtained for a wide range of chemical potentials and magnetization strengths. Employing experimentally realistic parameter values, we calculate the circular dichroism of a representative device consisting of magnetized spin-orbit coupled graphene and a dielectric insulator layer, backed by a metallic plate. The results reveal that this device has different relative absorptivities for right-handed and left-handed circularly polarized electromagnetic waves. It is found that the magnetized spin-orbit coupled graphene supports strong handedness-switchings, effectively controlled by varying the chemical potential and magnetization strength with respect to the SOC strength.

53.

Comparison of Optical Response from DFT Random Phase Approximation and Low-Energy Effective Model: Strained Phosphorene (DFT-RPA needs to be revisted) 

Mohammad Alidoust, Erlend E. Isachsen, Klaus Halterman, and Jaakko Akola

Phys. Rev. B 104, 115144 (2021). [PDF]

The engineering of the optical response of materials is a paradigm that demands microscopic-level accuracy and reliable predictive theoretical tools. Here we compare and contrast the dispersive permittivity tensor, using both a low-energy effective model and density functional theory (DFT). As a representative material, phosphorene subject to strain is considered. Employing a low-energy model Hamiltonian with a Green's function current-current correlation function, we compute the dynamical optical conductivity and its associated permittivity tensor. For the DFT approach, first-principles calculations make use of the first-order random phase approximation. Our results reveal that although the two models are generally in agreement within the low-strain and low-frequency regime, the intricate features associated with the fundamental physical properties of the system and optoelectronics devices implementation such as band gap, Drude absorption response, vanishing real part, absorptivity, and sign of permittivity over the frequency range show significant discrepancies. Our results suggest that the random phase approximation employed in widely used DFT packages should be revisited and improved to be able to predict these fundamental electronic characteristics of a given material with confidence. Furthermore, employing the permittivity results from both models, we uncover the pivotal role that phosphorene can play in optoelectronics devices to facilitate highly programable perfect absorption of electromagnetic waves by manipulating the chemical potential and exerting strain and illustrate how reliable predictions for the dielectric response of a given material are crucial to precise device design.

52.

Supercurrent diode effect, spin torques, and robust zero-energy peak in planar half-metallic trilayers 

Klaus Halterman, Mohammad Alidoust, Ross Smith, Spencer Starr

Phys. Rev. B 105, 104508 (2022). [PDF]

We consider a Josephson junction with F1/F2/F3 ferromagnetic trilayers in the ballistic regime, where the magnetization in each ferromagnet Fi(i=1,2,3), can have arbitrary orientations and magnetization strengths. The trilayers are sandwiched between two s-wave superconductors with a macroscopic phase difference Δφ. A broad range of magnetization strengths of the central F2 layer are considered, from an unpolarized normal metal (N) to a half-metallic phase, supporting only one spin species. Our results reveal that when the magnetization configuration in F1/F2/F3 has three orthogonal components, a supercurrent can flow at Δφ=0, and a strong second harmonic in the current-phase relation appears. Upon increasing the magnetization strength in the central ferromagnet layer up to the half-metallic limit, the self-biased current and second harmonic component become dramatically enhanced, and the critical supercurrent reaches its maximum value. The higher harmonics in the current-phase relations can be controlled by the relative magnetization orientations, with negligible current damping compared to the corresponding F1/N/F3 counterparts. For a broad range of exchange field strengths in the central ferromagnet F2, the ground state of the system can be tuned to an arbitrary phase difference φ0 by rotating the magnetization in the outer ferromagnet F3. For intermediate exchange field strengths in F2, a φ0 state can arise that creates a superconducting diode effect, whereby Δφ can be tuned to create a one-way dissipationless current flow. The density of states demonstrates the emergence of zero energy peaks for the mutually orthogonal magnetization configurations, which is strongest in the half-metallic phase.

51.

Spin transfer torque and anisotropic conductance in spin orbit coupled graphene 

Morteza Salehi, Razieh Beiranvand, and Mohammad Alidoust

arXiv:2104.09039. [PDF]

We theoretically study spin-transfer torque (STT) in a graphene system with spin-orbit coupling (SOC). We consider a graphene-based junction where the spin-orbit coupled region is sandwiched between two ferromagnetic (F) segments. The magnetization in each ferromagnetic segment can possess arbitrary orientations. Our results show that the presence of SOC results in anisotropically modified STT, magnetoresistance, and charge conductance as a function of relative magnetization misalignment in the F regions. We have found that within the Klein regime, where particles hit the interfaces perpendicularly, the spin-polarized Dirac fermions transmit perfectly through the boundaries of an F-F junction (i.e., with zero reflection), regardless of the relative magnetization misalignment and exert zero STT. In the presence of SOC, however, due to band structure modification, a nonzero STT reappears. Our findings can be exploited for experimentally examining proximity-induced SOC into a graphene system.

50.

Cubic spin-orbit coupling and anomalous Josephson effect in planar junctions (Diode effect, φ0 state, f-wave Majorana zero mode, anharmonic current phase relation, cubic Rashba-Dresselhaus SOC) 

Mohammad Alidoust, Chenghao Shen, and Igor Zutic

Phys. Rev. B (Letter) 103, 060503 (2021). [PDF]

Spin-orbit coupling in two-dimensional systems is usually characterized by Rashba and Dresselhaus spin-orbit coupling (SOC) linear in the wave vector. However, there is a growing class of materials which instead support dominant SOC cubic in the wave vector (cSOC), while their superconducting properties remain unexplored. By focusing on Josephson junctions in Zeeman field with superconductors separated by a normal cSOC region, we reveal a strongly anharmonic current-phase relation and complex spin structure. An experimental cSOC tunability enables both tunable anomalous phase shift and supercurrent, which flows even at the zero-phase difference in the junction. A fingerprint of cSOC in Josephson junctions is the f -wave spin-triplet superconducting correlations, important for superconducting spintronics and supporting Majorana bound states.

49.

Supergap and subgap enhanced currents in asymmetric S1/F/S2 Josephson junctions 

Mohammad Alidoust and Klaus Halterman

Phys. Rev. B 102, 224504 (2020). [PDF]

We have theoretically studied the supercurrent profiles in three-dimensional normal metal and ferromagnetic Josephson configurations, where the magnitude of the superconducting gaps in the superconducting leads are unequal, i.e., Δ1 ≠ Δ2 , creating asymmetric S1/N/S2 and S1/F/S2 systems. Our results reveal that by increasing the ratio of the superconducting gaps Δ2/Δ1, the critical supercurrent in a ballistic S1/N/S2 system can be enhanced by more than 100 % and reaches a saturation point, or decays away, depending on the junction thickness, magnetization strength, and chemical potential. The total critical current in a diffusive S1/N/S/2 system was found to be enhanced by more than 50 % parabolically and reaches saturation by increasing one of the superconducting gaps. In a uniform ferromagnetic junction, the supercurrent undergoes reversal by increasing Δ2/Δ1>1. Through decomposing the total supercurrent into its supergap and subgap components, our results illustrate their crucial relative contributions to the Josephson current flow. It was found that the competition of subgap and supergap currents in a S1/F/S2 junction results in the emergence of second harmonics in the current-phase relation. In contrast to a diffusive asymmetric Josephson configuration, the behavior of the supercurrent in a ballistic system with Δ2/Δ1=1 can be properly described by the subgap current component only, in a wide range of parameter sets, including Fermi level mismatch, magnetization strength, and junction thickness. Interestingly, when Δ2/Δ1>1, our results have found multiple parameter sets where the total supercurrent is driven by the supergap component. Therefore, our comprehensive study highlights the importance of subgap and supergap supercurrent components in both the ballistic and diffusive regimes. We focus on experimentally accessible material and geometric parameters that can lead to advancements in cryogenic devices based on Josephson junction architectures that utilize supergap currents, which are less sensitive to temperature compared to the subgap current.

48.

 Josephson effect in graphene bilayers with adjustable relative displacement 

Mohammad Alidoust, Antti-Pekka Jauho, and Jaakko Akola

Phys. Rev. Res. (Rapid Comm.) 2, 032074 (2020). [PDF]

The Josephson current is investigated in a superconducting graphene bilayer where the pristine graphene sheets can make in-plane or out-of-plane displacements with respect to each other. The superconductivity can be of intrinsic nature, or due to a proximity effect. The results demonstrate that the supercurrent responds qualitatively differently to relative displacement if the superconductivity is due to either intralayer or interlayer spin-singlet electron-electron pairing, thus providing a tool to distinguish between the two mechanisms. Specifically, both the AA and AB stacking orders are studied with antiferromagnetic spin alignment. For the AA stacking order with intralayer and on-site pairing no current reversal is found. In contrast, the supercurrent may switch its direction as a function of the in-plane displacement and out-of-plane interlayer coupling for the cases of AA ordering with interlayer pairing and AB ordering with either intralayer or interlayer pairing. In addition to sign reversal, the Josephson signal displays many characteristic fingerprints which derive directly from the pairing mechanism. Thus, measurements of the Josephson current as a function of the graphene bilayer displacement open up means for achieving a deeper insight of the superconducting pairing mechanism.

47.

Strain-engineered widely-tunable perfect absorption angle in black phosphorus from first-principles: A multi-scale approach  

Mohammad Alidoust, Klaus Halterman, Douxing Pan, Morten Willatzen, and Jaakko Akola
Phys. Rev. B 102, 115307 (2020). [PDF]

Using the density functional theory of electronic structure, we compute the anisotropic dielectric response of black phosphorus, solve Maxwell’s equations, and study the electromagnetic response of a layered structure comprising a film of black phosphorus stacked on a metallic substrate. Our results reveal that a small compressive or tensile strain, ~ 4%, exerted either perpendicular or in the plane to the black phosphorus growth direction, efficiently controls the epsilon-near-zero response, and allows a perfect absorption tuning from low-angle of the incident beam θ = 0◦ to high values θ ≈ 90◦ while switching the energy flow direction. Incorporating spatially inhomogeneous strain model, we also find that for certain thicknesses of the black phosphorus, near-perfect absorption can be achieved through controlled variations of the in-plane strain. These findings can serve as guidelines for designing largely tunable perfect electromagnetic wave absorber devices.

46.

Density functional simulations of pressurized Mg-Zn and Al-Zn alloys 

Mohammad Alidoust, David Kleiven and Jaakko Akola

Phys. Rev. Materials 4, 045002 (2020). [PDF]

Binary Mg-Zn and Al-Zn alloys have been investigated theoretically under static isotropic pressure. The stable phases of these binaries on both initially hexagonal-close-packed (hcp) and face-centered-cubic (fcc) lattices have been determined by utilizing an iterative approach that uses a configurational cluster expansion method, Monte Carlo search algorithm, and density functional theory (DFT) calculations. Based on 64-atom models, it is shown that the most stable phases of the Mg-Zn binary alloy under ambient condition are MgZn3, Mg19Zn45, MgZn, and Mg34Zn30 for the hcp lattice, and MgZn3 and MgZn for the fcc lattice, whereas the Al-Zn binary is energetically unfavorable throughout the entire composition range for both the hcp and fcc lattice symmetries under all pressure conditions. By applying an isotropic pressure in the hcp lattice, Mg19 Zn45 turns into an unstable phase at P ≈ 10 GPa, a new stable phase Mg3Zn appears at P > 20 GPa, and Mg34Zn30 becomes unstable for P > 30 GPa. For the fcc lattice, the Mg3Zn phase weakly touches the convex hull at P > 20 GPa while the other stable phases remain intact up to ≈120 GPa. Furthermore, making use of the obtained DFT results, the bulk modulus has been computed for several compositions up to pressure values on the order of ≈120 GPa. The findings suggest that one can switch between Mg-rich and Zn-rich early-stage clusters simply by applying external pressure. Zn-rich alloys and precipitates are more favorable in terms of stiffness and stability against external deformation.

45.

Critical supercurrent and φ0 state for probing a persistent spin helix 

Mohammad Alidoust

Phys. Rev. B 101, 155123 (2020). [PDF]

We theoretically study the profile of a supercurrent in two-dimensional Josephson junctions with Rashba-Dresselhaus spin-orbit interaction (RDSOI) in the presence of a Zeeman field. Two types of RDSOIs are considered that might be accessible in GaAs quantum wells and zinc-blende materials. Through investigating self-biased supercurrent (so called φ0-Josephson state), we obtain explicit expressions for the functionality of the φ0 state with respect to RDSOI parameters (α,β) and in- plane Zeeman field components (hx,hy). Our findings reveal that, when the chemical potential (μ) is high enough compared to the energy gap (∆) in superconducting electrodes, i.e., μ ≫ ∆, RSOI and DSOI with equal strengths (|α| = |β|) cause vanishing φ0 state independent of magnetization and the type of RDSOI. A Zeeman field with unequal components, i.e., |hx| ≠ |hy|, however, can counteract and nullify the destructive impact of equal-strength RDSOIs (for one type only), where μ ∼ ∆, although |hx| = |hy| can still eliminate the φ0 state. Remarkably, in the μ ∼ ∆ limit, the φ0 state is proportional to the multiplication of both components of an in-plane Zeeman field, i.e., hxhy, which is absent in the μ ≫ ∆ limit. Furthermore, our results of critical supercurrents demonstrate that the persistent spin helices can be revealed in a high enough chemical potential regime μ ≫ ∆, while an opposite regime, i.e., μ ∼ ∆, introduces an adverse effect. In the ballistic regime, the “maximum” of the critical supercurrent occurs at |α| = |β| and the Zeeman field can boost this feature. The presence of disorder and nonmagnetic impurities change this picture drastically so the “minimum” of the critical supercurrent occurs at and around the symmetry lines |α| = |β|. We show that the signature of persistent spin helices explored in disordered systems originate from the competition of short-range spin-singlet and long-range spin-triplet supercurrent components. Our study uncovers delicate details of how the interplay of RDSOI and a Zeeman field manifests in the φ0 state and critical supercurrent. Relying on the fact that the φ0 state is accessible regardless of the amount of nonmagnetic impurities and disorder, our results can provide guidelines for future experiments to confirm the presence of persistent spin helices, determine the type of SOI, and reliably extract SOI parameters in a system, which might be helpful in devising spin-orbit-coupled spintronics devices and ultra sensitive spin-transistor technologies.

44.

Evolution of Pair Correlation Symmetries and Supercurrent Reversal in Tilted Weyl Semimetals 

Mohammad Alidoust and Klaus Halterman

Phys. Rev. B 101, 035120 (2020). [PDF]

We study the effective symmetry profiles of superconducting pair correlations and the flow of charge supercurrent in ballistic Weyl semimetal systems with a tilted dispersion relation. Utilizing a microscopic method in the ballistic regime and starting from both opposite-pseudospin and equal-pseudospin phonon-mediated opposite-spin electron-electron couplings, we calculate the anomalous Green's function to study various superconducting pair correlations that Weyl semimetal systems may develop. The momentum-space profile reveals that by properly manipulating the parameters of Weyl semimetal systems, including the tilting parameter, the effective symmetry class of even-parity s-wave (odd-parity p-wave) superconducting correlations can be converted into a d-wave (f-wave) symmetry class that consists of equal-pseudospin and opposite-pseudospin channels. We also find that the supercurrent in a ballistic Weyl Josephson junction can be made to vanish or switch directions, depending on the tilt of the Weyl cones, in addition to the relevant parameters characterizing the Weyl semimetal and junction. We show that inversion symmetry breaking terms introduce transitions that result in the appearance of self-biased current at zero difference between the macroscopic phases of the superconducting segments, creating a phi0 Josephson state. Weyl semimetal systems are shown to offer several experimentally tunable parameters to control the induction of higher harmonics into the current phase relations.

43.

Waveguide modes in Weyl semimetals with tilted Dirac cones 

Klaus Halterman and Mohammad Alidoust

Opt. Express 27, 36164 (2019). [PDF]

Wetheoreticallystudyunattenuatedelectromagneticguidedwavemodesincentrosym- metric Weyl semimetal layered systems. By solving Maxwell’s equations for the electromagnetic fields and using the appropriate boundary conditions, we derive dispersion relations for propagat- ing modes in a finite-sized Weyl semimetal. Our findings reveal that for ultrathin structures and proper Weyl cones tilts, extremely localized guided waves can propagate along the semimetal interface over a certain range of frequencies. This follows from the anisotropic nature of the semimetal where the diagonal components of the permittivity can exhibit a tunable epsilon-near- zero response. From the dispersion diagrams, we determine experimentally accessible regimes that lead to high energy-density confinement in the Weyl semimetal layer. Furthermore, we show that the net system power can vanish all together, depending on the Weyl cone tilt and frequency of the electromagnetic wave. These effects are seen in the energy transport velocity, which demonstrates a substantial slowdown in the propagation of electromagnetic energy near critical points of the dispersion diagrams. Our results can provide guidelines in designing Weyl semimetal waveguides that can offer efficient control in the velocity and direction of energy flow.

42.

Symmetry of superconducting correlations in displaced bilayers of graphene 

Mohammad Alidoust, Morten Willatzen and Antti-Pekka Jauho

Phys. Rev. B 99, 155413 (2019). [PDF]

Using a Green's function approach, we study phonon-mediated superconducting pairing symmetries that may arise in bilayer graphene where the monolayers are displaced in-plane with respect to each other. We consider a generic coupling potential between the displaced graphene monolayers, which is applicable to both shifted and commensurate twisted graphene layers; study intralayer and interlayer phonon-mediated BCS pairings; and investigate AA and AB(AC) stacking orders. Our findings demonstrate that at the charge neutrality point, the dominant pairings in both AA and AB stackings with intralayer and interlayer electron-electron couplings can have even-parity s-wave class and odd-parity p-wave class of symmetries with the possibility of invoking equal-pseudospin and odd-frequency pair correlations. At a finite doping, however, the AB (and equivalently AC) stacking can develop pseudospin-singlet and pseudospin-triplet d-wave symmetry, in addition to s-wave, p-wave, f-wave, and their combinations, while the AA stacking order, similar to the undoped case, is unable to host the d-wave symmetry. When we introduce a generic coupling potential, applicable to commensurate twisted and shifted bilayers of graphene, d-wave symmetry can also appear at the charge neutrality point. Inspired by a recent experiment where two phonon modes were observed in a twisted bilayer graphene, we also discuss the possibility of the existence of two-gap superconductivity, where the intralayer and interlayer phonon-mediated BCS picture is responsible for superconductivity. These analyses may provide a useful tool in determining the superconducting pairing symmetries and mechanism in bilayer graphene systems.

41.

Control of superconducting pairing symmetries in monolayer black phosphorus 

Mohammad Alidoust, Morten Willatzen and Antti-Pekka Jauho

Phys. Rev. B 99, 125417 (2019) - Kaleidoscope. [PDF]

Motivated by recent experimental progress, we study the effect of mechanical deformations on the superconducting pairing symmetries in monolayer black phosphorus (MBP). Starting with phonon-mediated intervalley spin-singlet electron-electron pairing and making use of realistic band parameters obtained through first-principles calculations, we show that the application of weak mechanical strain in the plane of MBP can change the effective s-wave and p-wave symmetry of the superconducting correlations into effective d-wave and f-wave symmetries, respectively. This prediction of a change in the pairing symmetries might be experimentally confirmed through angular dependence high-resolution tunneling spectroscopy, the Meissner effect, and critical temperature experiments. The idea of manipulating the superconducting symmetry class by applying planar mechanical strain can be extended to other anisotropic materials as well, and may help in providing important information of the symmetries of the order parameter, perhaps even in some high-Tc superconductors.

40.

Self-biased current, magnetic interference response, and superconducting vortices in tilted Weyl semimetals with disorder 

Mohammad Alidoust

Phys. Rev. B 98, 245418 (2018). [PDF]

We have generalized a quasiclassical model for Weyl semimetals with a tilted band in the presence of an externally applied magnetic field. This model is applicable to ballistic, moderately disordered, and samples containing a high density of nonmagnetic impurities. We employ this formalism and show that a self-biased supercurrent, creating a {\phi}0-junction, can flow through a triplet channel in Weyl semimetals. Furthermore, our results demonstrate that multiple supercurrent reversals are accessible through varying junction thickness and parameters characterizing Weyl semimetals. We discuss the influence of different parameters on the Fraunhofer response of charge supercurrent, and how these parameters are capable of shifting the locations of proximity-induced vortices in the triplet channel.

39.

Fraunhofer response and supercurrent spin switching in black phosphorus with strain and disorder 

Mohammad Alidoust, Morten Willatzen and Antti-Pekka Jauho

Phys. Rev. B 98, 184505 (2018). [PDF]

We develop theory models for both ballistic and disordered superconducting monolayer black phosphorus devices in the presence of magnetic exchange field and strain. The ballistic case is studied through a microscopic Bogoliubov-de Gennes formalism while for the disordered case we formulate a quasiclassical model. Utilizing the two models, we theoretically study the response of supercurrent to an externally applied magnetic field in two-dimensional black phosphorus Josephson junctions. Our results demonstrate that the response of the supercurrent to a perpendicular magnetic field in ballistic samples can deviate from the standard Fraunhofer interference pattern when the Fermi level and mechanical strain are varied. This finding suggests the combination of chemical potential and strain is an efficient external knob to control the current response in highly sensitive strain-effect transistors and superconducting quantum interference devices. We also study the supercurrent in a superconductor-ferromagnet-ferromagnet-superconductor junction where the magnetizations of the two adjacent magnetized regions are uniform with misaligned orientations. We show that the magnetization misalignment can control the excitation of harmonics higher than the first harmonic sin(phi) (in which phi is the phase difference between the superconductors) in supercurrent and constitutes a full spin switching current element. Finally, we discuss possible experimental implementations of our findings. We foresee our models and discussions could provide guidelines to experimentalists in designing devices and future investigations.

38.

Induced Energy Gap in Finite-Sized Superconductor/Ferromagnet Hybrids 

Klaus Halterman and Mohammad Alidoust

Phys. Rev. B 98, 134510 (2018). [PDF]

We theoretically study self-consistent proximity effects in finite-sized systems consisting of ferromagnet (F) layers coupled to an s-wave superconductor (S). We consider both SF1F2 and SH nanostructures, where the F1F2 bilayers are uniformly magnetized, and the ferromagnetic H layer possesses a helical magnetization profile. We find that when the F1F2 layers are weakly ferromagnetic, a hard gap can emerge when the relative magnetization directions are rotated from parallel to antiparallel. Moreover, the gap is most prominent when the thicknesses of F1 and F2 satisfy dF1≤dF2, respectively. For the SH configuration, increasing the spatial rotation period of the exchange field can enhance the induced hard gap. Our investigations reveal that the origin of these findings can be correlated with the propagation of quasiparticles with wavevectors directed along the interface. To further clarify the source of the induced energy gap, we also examine the spatial and energy resolved density of states, as well as the spin-singlet, and spin-triplet superconducting correlations, using experimentally accessible parameter values. Our findings can be beneficial for designing magnetic hybrid structures where a tunable superconducting hard gap is needed.

37.

Strain-engineered Majorana Zero Energy Modes and φ0 Josephson State in Black Phosphorus (Phosphorene) 

Mohammad Alidoust, Morten Willatzen and Antti-Pekka Jauho

Phys. Rev. B 98, 085414 (2018). [PDF]

We develop a theory for strain control of Majorana zero energy modes and Josephson effect in black phosphorus (BP) devices proximity coupled to a superconductor. Employing realistic values for the band parameters subject to strain, we show that the strain closes the intrinsic band gap of BP, however the proximity effect from the superconductor reopens it and creates Dirac and Weyl nodes. Our results illustrate that Majorana zero energy flat bands connect the nodes within the band-inverted regime in which their associated density of states is localized at the edges of the device. In a ferromagnetically mediated Josephson configuration, the exchange field induces super-harmonics into the supercurrent phase relation in addition to a φ0 phase shift, corresponding to a spontaneous supercurrent, and strain offers an efficient tool to control these phenomena. We analyze the experimental implications of our findings, and show that they can pave the way for creating a rich platform for studying two-dimensional Dirac and Weyl superconductivity.

36.

Epsilon-Near-Zero Response and Tunable Perfect Absorption in Weyl Semimetals 

Klaus Halterman, Mohammad Alidoust, and Alexander Zyuzin 

Phys. Rev. B 98, 085109 (2018). [PDF]

We theoretically study the electromagnetic response of type-I and type-II centrosymmetric Weyl metals. We derive an anisotropic permittivity tensor with off-diagonal elements to describe such gyrotropic media. Our findings reveal that for appropriate Weyl cones tilts, the real part of the transverse component of the permittivity can exhibit an epsilon-near-zero response. The tilt parameter can also control the amount of loss in the medium, ranging from lossless to dissipative when transitioning from type-I to type-II. Similarly, by tuning either the frequency of the electromagnetic field or the chemical potential in the system, an epsilon-near-zero response can appear as the permittivity of the Weyl semimetal transitions between positive and negative values. Employing the obtained permittivity tensor, we consider a setup where the Weyl semimetal is deposited on a perfect conductive substrate and study the refection and absorption characteristics of this layered configuration. We show that by choosing the proper geometrical and material parameters, devices can be created that perfectly absorb electromagnetic energy over a wide angular range of incident electromagnetic waves.

35.

Half-metallic superconducting triplet spin multivalves 

Mohammad Alidoust and Klaus Halterman 

Phys. Rev. B 97, 064517 (2018). [PDF]

We study spin switching effects in finite-size superconducting multivalve structures. We examine F1-F2-S-F3 and F1-F2-S-F3-F4 hybrids where a singlet superconductor (S) layer is sandwiched among ferromagnet (F) layers with differing thicknesses and magnetization orientations. Our results reveal a considerable number of experimentally viable spin-valve configurations that lead to on-off switching of the superconducting state. For S widths on the order of the superconducting coherence length ξ0, noncollinear magnetization orientations in adjacent F layers with multiple spin axes leads to a rich variety of triplet spin-valve effects. Motivated by recent experiments, we focus on samples where the magnetizations in the F1 and F4 layers exist in a fully spin-polarized half-metallic phase, and calculate the superconducting transition temperature, spatially and energy resolved density of states, and the spin-singlet and spin-triplet superconducting correlations. Our findings demonstrate that superconductivity in these devices can be completely switched on or off over a wide range of magnetization misalignment angles due to the generation of equal-spin and opposite-spin triplet pairings.

34.

Spontaneous supercurrent and φ0 phase shift parallel to magnetized topological insulator interfaces 

Mohammad Alidoust and Hossein Hamzehpour 

Phys. Rev. B 96, 165422 (2017). [PDF]

Employing a Keldysh-Eilenberger technique, we theoretically study the generation of a spontaneous supercurrent and the appearance of the φ0 phase shift parallel to uniformly in-plane magnetized superconducting interfaces made of the surface states of a three-dimensional topological insulator. We consider two weakly coupled uniformly magnetized superconducting surfaces where a macroscopic phase difference between the s-wave superconductors can be controlled externally. We find that, depending on the magnetization strength and orientation on each side, a spontaneous supercurrent due to the φ0 states flows parallel to the interface at the nanojunction location. Our calculations demonstrate that nonsinusoidal phase relations of current components with opposite directions result in maximal spontaneous supercurrent at phase differences close to π. We also study the Andreev subgap channels at the interface and show that the spin-momentum locking phenomenon in the surface states can be uncovered through density of states studies. We finally discuss realistic experimental implications of our findings.

33.

Nonlocal Andreev entanglements and triplet correlations in graphene with spin-orbit coupling 

Razieh Beiranvand, Hossein Hamzehpour, and Mohammad Alidoust 

Phys. Rev. B 96, 161403(Rapid Comm.) (2017). [PDF]

Using a wavefunction Dirac Bogoliubov-de Gennes method, we demonstrate that the tunable Fermi level of a graphene layer in the presence of Rashba spin orbit coupling (RSOC) allows for producing an anomalous nonlocal Andreev reflection and equal spin superconducting triplet pairing. We consider a graphene junction of a ferromagnet-RSOC-superconductor-ferromagnet configuration and study scattering processes, the appearance of spin triplet correlations, and charge conductance in this structure. We show that the anomalous crossed Andreev reflection is linked to the equal spin triplet pairing. Moreover, by calculating current cross-correlations, our results reveal that this phenomenon causes negative charge conductance at weak voltages and can be revealed in a spectroscopy experiment, and may provide a tool for detecting the entanglement of the equal spin superconducting pair correlations in hybrid structures.

32.

Magnetization-control and transfer of spin-polarized Cooper pairs into a half-metal manganite 

Anand Srivastava, Linde A. B. Olde Olthof, Angelo Di Bernardo, Sachio Komori, Mario Amado, Carla Palomares-Garcia, Mohammad Alidoust, Klaus Halterman, Mark G. Blamire, Jason W. A. Robinson 

Phys. Rev. Applied (2017). [PDF]

The pairing state and critical temperature (Tc) of a thin s-wave superconductor (S) on two or more ferromagnets (F) are controllable through the magnetization-alignment of the F layers. Magnetization misalignment can lead to spin-polarized triplet pair creation, and since such triplets are compatible with spin-polarized materials they are able to pass deeply into the F layers and so, cause a decrease in Tc. Various experiments on S/F1/F2 "triplet spin-valves" have been performed with the most pronounced suppression of Tc reported in devices containing the half-metal ferromagnet (HMF) CrO2 (F2) albeit using out-of-plane magnetic fields to tune magnetic non-collinearity [Singh et al., Phys. Rev. X 5, 021019 (2015)]. Routine transfer of spin-polarized triplets to HMFs is a major goal for superconducting spintronics so as to maximize triplet-state spin-polarization. However, CrO2 is chemically unstable and out-of-plane fields are undesirable for superconductivity. Here, we demonstrate magnetization-tuneable pair conversion and transfer of spin-polarized triplet pairs to the chemically stable mixed valence manganite La2/3Ca1/3MnO3 in a pseudo spin-valve device using in-plane magnetic fields. The results match microscopic theory and offer full control over the pairing state.