arXiv subjects

Maxim Breitkreiz

Publications and source records attributed to Maxim Breitkreiz.

18 recordsLinked to original sources

arXiv2026

Spin preservation in screw-symmetric molecules

In electronic transport through long molecules, spin is expected to be preserved only when the dwell time is much shorter than the characteristic spin-mixing timescale $\hbar/Δ$, where $Δ$ is the magnitude of a spin-dependent potential, such as spin-orbit coupling. We show that, in molecules featuring discrete screw symmetry, spin preservation can be enhanced far beyond this timescale owing to strong spin separation in quasi-momentum. This spin fidelity in long molecules is consistent with chirality-induced spin selectivity (CISS), suggesting spin-dependent transport in long, chiral molecules with amplified spin-splitting mechanisms. We provide analytical derivation of the enhanced spin preservation and test it on tight-binding models, which confirm that the effect gradually weakens when the screw symmetry is broken or changes from discrete to continuous. Furthermore, we perform transport simulations to show that a strong magnetoresistance trace of symmetry-protected spin fidelity emerges in a spin-valve setup with two magnetic leads, which we propose as an experimentally accessible signature.

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arXiv2026

Tunneling amplifies chirality-induced spin selectivity and explains its current-direction invariance

We propose a minimal model for chirality-induced spin selectivity (CISS) in dc transport through insulating chiral molecules, based on quantum tunneling and interaction-induced spin splitting. As a concrete realization of the latter, we consider a weak Zeeman interaction of the particle spin with the current-induced magnetic field, recently shown to occur in helical molecules. We show that quantum tunneling, combined with dissipation, amplifies the effect, so that even such a small spin-dependent perturbation can yield spin polarizations on the order of 100\% across a wide range of applied bias voltages. Furthermore, our tunneling scenario naturally reproduces the characteristic CISS symmetry of the current-voltage dependence -- namely, the invariance of the spin-polarization sign under reversal of the current direction -- while fully respecting Onsager's reciprocity relations.

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arXiv2026

Step-Edge Anomaly in Topological Metals

Bulk-boundary correspondence guarantees the presence of robust, anomalous states on the boundary of topological matter. The edges of a two-dimensional Chern insulator harbor one-dimensional chiral states, which have a conductance $n\, e^2/h$, where $n$ is an integer that is solely determined by the bulk. In this work we show that step edges on the surface of three-dimensional topological metals have a robust conductance $K\, e^2/h$, where $K$ is also fixed by the bulk and assumes non-integer values. We explain this prediction on the basis of the topology of gapless systems, exemplify it on a lattice model, and connect to recent experimental observations of enhanced density of states at step-edges in topological metals.

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arXiv2025

Magnetotransport across Weyl semimetal grain boundaries

A clean interface between two Weyl semimetals features a universal, field-linear tunnel magnetoconductance of $(e^2/h)N_\mathrm{ho}$ per magnetic flux quantum, where $N_\mathrm{ho}$ is the number of chirality-preserving topological interface Fermi arcs. In this work we show that the linearity of the magnetoconductance is robust with to interface disorder. The slope of the magnetoconductance changes at a characteristic field strength $B_\mathrm{arc}$ -- the field strength for which the time taken to traverse the Fermi arc due to the Lorentz force is equal to the mean inter-arc scattering time. For fields much larger than $B_\mathrm{arc}$, the magnetoconductance is unaffected by disorder. For fields much smaller than $B_\mathrm{arc}$, the slope is no longer determined by $N_\mathrm{ho}$ but by the simple fraction $N_\mathrm{L} N_\mathrm{R}/(N_\mathrm{L}+N_\mathrm{R})$, where $N_\mathrm{L}$ and $N_\mathrm{R}$ are the numbers of Weyl-node pairs in the left and right Weyl semimetal, respectively. We also consider the effect of spatially correlated disorder potentials, where we find that $B_\mathrm{arc}$ decreases exponentially with increasing correlation length. Our results provide a possible explanation for the recently observed robustness of the negative linear magnetoresistance in grained Weyl semimetals.

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arXiv2024

Axionic Instability of Periodic Weyl-Semimetal Superstructures

Weyl-semimetal superstructures with a spiraling position of a pair of Weyl nodes of opposite chirality can host a chiral-symmetry preserving Fermi-arc metal state, where the chirality is carried by cylindrical Fermi surfaces, electron- and hole-like depending on the chirality. The Fermi surfaces nest at vanishing momentum separation (zero nesting vector) at the electron-hole-compensation energy because the nesting is topologically protected by vanishing spatial overlap of any pair of equal-momentum opposite-chirality states. In this work we show that the nesting and Coulomb interaction drive a spontaneous chiral symmetry breaking in such a Fermi arc metal, which leads to a dynamical axion insulator state but without breaking translational symmetry (no charge-density-wave order) as in a conventional Weyl semimetal. As for material realization, we discuss magnetically doped Bi$_2$Se$_3$, for which the Weyl-node positions depend on the order of the magnetic dopands. In this case, the axionic condensation can itself stabilize a spiral order of the magnetization, and hence the spiraling node positions, even if the magnetic interaction is intrinsically ferromagnetic.

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arXiv2023

Quantum Oscillation Signatures of Fermi Arcs in Tunnel Magnetoconductance

Fermi-arc surface states of Weyl semimetals exhibit a unique combination of localization to a surface and connectivity to the bulk Weyl fermions that can move along the localization direction. We predict anomalous quantum-oscillation signatures of Fermi arcs in the tunnel mangetoconductance across an interface between two Weyl semimetals. These oscillations stem from a momentum-space analog of Aharonov-Bohm interference of electrons moving along the interface Fermi arcs, driven by an external magnetic field normal to the interface. The Fermi arcs' connectivity to the bulk enables their characterization via transport normal to the interface, while their localization manifests in a strong field-angle anisotropy of the oscillations. This combination distinguishes these anomalous oscillations from conventional Shubnikov-de Haas oscillations and makes them identifiable even in complex oscillation spectra of real materials.

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arXiv2023

Fermi-arc metals

We predict a novel metallic state of matter that emerges in a Weyl-semimetal superstructure with spatially varying Weyl-node positions. In the new state, the Weyl nodes are stretched into extended, anisotropic Fermi surfaces, which can be understood as being built from Fermi arc-like states. This "Fermi-arc metal" exhibits the chiral anomaly of the parental Weyl semimetal. However, unlike in the parental Weyl semimetal, in the Fermi-arc metal the "ultra-quantum state", in which the anomalous chiral Landau level is the only state at the Fermi energy, is already reached for a finite energy window at zero magnetic field. The dominance of the ultra-quantum state implies a universal low-field ballistic magnetoconductance and the absence of quantum oscillations, making the Fermi surface "invisible" to de Haas-van Alphen and Shubnikov-de Haas effects, although it signifies its presence in other response properties.

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arXiv2023

Magnetic Breakdown and Chiral Magnetic Effect at Weyl-Semimetal Tunnel Junctions

We investigate magnetotransport across an interface between two Weyl semimetals whose Weyl nodes project onto different interface momenta. Such an interface generically hosts Fermi arcs that connect Weyl nodes of identical chirality in different Weyl semimetals (homochiral connectivity) -- in contrast to surface Fermi arcs that connect opposite-chirality Weyl nodes within the same Weyl semimetal (heterochiral connectivity). We show that electron transport along the arcs with homochiral connectivity, in the presence of a longitudinal magnetic field, leads to a universal longitudinal magnetoconductance of $e^2/h$ per magnetic flux quantum. Furthermore, a weak tunnel coupling can result in a close encounter of two homochiral-connectivity Fermi arcs, enabling magnetic breakdown. Above the breakdown field the interface Fermi arc connectivity is effectively heterochiral, leading to a saturation of the conductance.

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arXiv2021

Chiral Anomaly Trapped in Weyl Metals: Nonequilibrium Valley Polarization at Zero Magnetic Field

In Weyl semimetals the application of parallel electric and magnetic fields leads to valley polarization -- an occupation disbalance of valleys of opposite chirality -- a direct consequence of the chiral anomaly. In this work, we present numerical tools to explore such nonequilibrium effects in spatially confined three-dimensional systems with a variable disorder potential, giving exact solutions to leading order in the disorder potential and the applied electric field. Application to a Weyl-metal slab shows that valley polarization also occurs without an external magnetic field as an effect of chiral anomaly "trapping": Spatial confinement produces chiral bulk states, which enable the valley polarization in a similar way as the chiral states induced by a magnetic field. Despite its finite-size origin, the valley polarization can persist up to macroscopic length scales if the disorder potential is sufficiently long ranged, so that direct inter-valley scattering is suppressed and the relaxation then goes via the Fermi-arc surface states.

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arXiv2021

Equilibrium current in a Weyl-semimetal - superconductor heterostructure

A heterostructure consisting of a magnetic Weyl semimetal and a conventional superconductor exhibits an equilibrium current parallel to the superconductor interface and perpendicular to the magnetization. Analyzing a minimal model, which as a function of parameters may be in a trivial magnetic insulator phase, a Weyl semimetal phase, or a three-dimensional weak Chern insulator phase, we find that the equilibrium current is sensitive to the presence of surface states, such as the topological Fermi-arc states of the Weyl semimetal or the chiral surface states of the weak Chern insulator. While there is a nonzero equilibrium current in all three phases, the appearance of the surface states in the topological regime leads to a reversal of the direction of the current, compared to the current direction for the trivial magnetic insulator phase. We discuss the interpretation of the surface-state contribution to the equilibrium current as a real-space realization of the superconductivity-enabled equilibrium chiral magnetic effect of a single chirality, predicted to occur in bulk Weyl superconductors.

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arXiv2020

Parabolic Hall Effect due to Co-Propagating Surface Modes

Real-space separations of counter-moving states to opposite surfaces or edges are associated with different types of Hall effects, such as the quantum-, spin-, or the anomalous Hall effect. Some systems provide the possibility to separate a fraction of countermovers in a completely different fashion: Surface states propagating all in the same direction, balanced by counter-moving bulk states, realized, e.g., in Weyl metals with intrinsically or extrinsically broken inversion and time-reversal symmetries. In this work we show that these co-propagating surface modes are associated with a novel Hall effect --- a parabolic potential profile in the direction perpendicular to and in its magnitude linear in the applied field. While in 2D systems the parabolic potential profile is measurable directly, in 3D the resulting voltage between bulk and surface is measurable in the geometry of a hollow cylinder. Moreover, the parabolic Hall effect leads to characteristic signatures in the longitudinal conductivity.

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arXiv2019

Large contribution of Fermi arcs to the conductivity of topological metals

Surface-state contributions to the dc conductivity of most homogeneous metals exposed to uniform electric fields are usually as small as the system size is large compared to the lattice constant. In this work, we show that surface states of topological metals can contribute with the same order of magnitude as the bulk even in large systems. This effect is intimately related to the intrinsic anomalous Hall effect, in which an applied voltage induces chiral surface-state currents proportional to the system size. Unlike the anomalous Hall effect, the large contribution of surface states to the dc conductivity is also present in time-reversal invariant Weyl semimetals, where the surface states come in counter-propagating time-reversed pairs. While the Hall voltage vanishes in the presence of time-reversal symmetry, the twinned chiral surface currents develop similarly as in the time-reversal broken case. For this effect to occur, the relaxation length associated with scattering between time-reversed partner states needs to be larger than the separation of contributing surfaces, which results in a characteristic size dependence of the resistivity and a highly inhomogeneous current-density profile across the sample.

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arXiv2017

Linear Response of Zero-Resistance States

A two-dimensional electron system in the presence of a magnetic field and microwave irradiation can undergo a phase transition towards a zero-resistance state. A widely used model predicts the zero-resistance state to be a domain state, which responds to applied dc voltages or dc currents by slightly changing the domain structure. Here we propose an alternative response scenario, according to which the domain pattern remains unchanged. Surprisingly, a fixed domain pattern does not destroy zero resistance, provided that the resistance is direction independent. Otherwise, if the symmetry of the domain pattern allows a direction dependence of the resistance, the domain state can be dissipative. We give examples for both situations and simulate the response behavior numerically.

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arXiv2016

Interrupted orbital motion in density-wave systems

In conventional metals, electronic transport in a magnetic field is characterized by the motion of electrons along orbits on the Fermi surface, which usually causes an increase in the resistivity through averaging over velocities. Here we show that large deviations from this behavior can arise in density-wave systems close to their ordering temperature. Specifically, enhanced scattering off collective fluctuations can lead to a change of direction of the orbital motion on reconstructed pockets. In weak magnetic fields, this leads to linear magnetoconductivity, the sign of which depends on the electric-field direction. At a critical magnetic field, the conductivity crosses zero for certain directions, signifying a thermodynamic instability of the density-wave state.

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arXiv2014

Resistive anisotropy due to spin-fluctuation scattering in the nematic phase of iron pnictides

The large in-plane anisotropy of the resistivity is a hallmark of the nematic state of the iron pnictides. Solving the Boltzmann transport equation, we show that the prominent doping dependence as well as the large values of the anisotropy can be well explained by momentum-dependent spin-fluctuation scattering without assuming anisotropic impurity states. Due to the forward-scattering corrections, the hot spots contribute to the resistive anisotropy even in the case of strong spin fluctuations, which makes large values of the anisotropy possible. The ellipticity of the electron pockets plays an important role in explaining the dominance of positive values of the anisotropy, i.e., larger resistivity in the direction with weaker spin fluctuations, throughout the doping range.

cond-mat.str-el↗

arXiv2014

Transport in multiband systems with hot spots on the Fermi surface: Forward-scattering corrections

Multiband models with hot spots are of current interest partly because of their relevance for the iron-based superconductors. In these materials, the momentum-dependent scattering off spin fluctuations and the ellipticity of the electron Fermi pockets are responsible for anisotropy of the lifetimes of excitations around the Fermi surface. The deep minima of the lifetimes---the so-called hot spots---have been assumed to contribute little to the transport as is indeed predicted by a simple relaxation-time approach. Calculating forward-scattering corrections to this approximation, we find that the effective transport times are much more isotropic than the lifetimes and that, therefore, the hot spots contribute to the transport even in the case of strong spin-fluctuation scattering. We discuss this effect on the basis of an analytical solution of the Boltzmann equation and calculate numerically the temperature and doping dependence of the resistivity and the Hall, Seebeck, and Nernst coefficients.

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arXiv2013

Semiclassical theory of speckle correlations

Coherent wave propagation in random media results in a characteristic speckle pattern, with spatial intensity correlations with short-range and long-range behavior. Here, we show how the speckle correlation function can be obtained from a ray picture for two representative geometries: A chaotic cavity and a random waveguide. Our calculation allows us to study the crossover between a "ray limit" and a "wave limit", in which the Ehrenfest time $τ_E$ is larger or smaller than the typical transmission time $τ_D$, respectively. Remarkably, long-range speckle correlations persist in the ray limit $τ_E \gg τ_D$.

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arXiv2013

Transport anomalies due to anisotropic interband scattering

Unexpected transport behavior can arise due to anisotropic single-particle scattering in multiband systems. Specifically, we show within a semiclassical Boltzmann approach beyond the relaxation-time approximation that anisotropic scattering between electronlike and holelike Fermi surfaces generically leads to negative transport times, which in turn cause negative magnetoresistance, an extremum in the Hall coefficient, and a reduction of the resistivity. The anisotropy required for this to occur decreases with increasing mismatch between the Fermi-surface radii.

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