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Amplifiers based on Josephson junctions allow for a fast and noninvasive readout of superconducting qubits. Motivated by the ongoing progress toward the realization of fault-tolerant qubits based on Majorana bound states, we investigate the topological counterpart of the Josephson bifurcation amplifier. We predict that the bifurcation dynamics of a topological Josephson junction driven in the appropriate parameter regime may be used as an additional tool to detect the emergence of Majorana bound states. [ABSTRACT FROM AUTHOR]

We present evidence for the existence and control of bulk-surface coupling in Bi2Te2Se nanoribbons. Our magnetoresistance measurements reveal that the number of coherent channels contributing to quantum interference in the nanoribbons changes abruptly when the film thickness exceeds the bulk phase relaxation length. We interpret this observation as an evidence for bulk-mediated coupling between metallic states located on opposite surfaces. This hypothesis is supported by additional magnetoresistance measurements conducted under a set of gate voltages and in a parallel magnetic field, the latter of which alters the intersurface coupling in a controllable way. [ABSTRACT FROM AUTHOR]

We present a quantitative microscopic theory of the disorder- and phonon-induced coupling between surface and bulk states in doped topological insulator films. We find a simple mathematical structure for the surface-to-bulk scattering matrix elements and confirm the importance of bulk-surface coupling in transport and photoemission experiments, assessing its dependence on temperature, carrier density, film thickness, and particle-hole asymmetry. [ABSTRACT FROM AUTHOR]

We develop an approximate theory of phonon-induced topological insulation in Dirac materials. In the weak-coupling regime, long-wavelength phonons may favor topological phases in Dirac insulators with direct and narrow band gaps. This phenomenon originates from electron-phonon matrix elements, which change qualitatively under a band inversion. A similar mechanism applies to weak Coulomb interactions and spin-independent disorder; however, the influence of these on band topology is largely independent of temperature. As applications of the theory, we evaluate the temperature dependence of the critical thickness and the critical stoichiometric ratio for the topological transition in CdTe/HgTe quantum wells and in BiTl(S1-δSeδ)2, respectively. [ABSTRACT FROM AUTHOR]

Spin transfer torques (STTs) occur when electric currents travel through inhomogeneously magnetized systems and are important for the motion of magnetic textures such as domain walls. Since superconductors are easy-plane ferromagnets in particle-hole (charge) space, it is natural to ask whether any charge duals of STT phenomena exist therein. We find that the superconducting analog of the adiabatic STT vanishes in a bulk superconductor with a momentum-independent order parameter, while the superconducting counterpart of the nonadiabatic STT does not vanish. This nonvanishing superconducting torque is induced by heat (rather than charge) currents and acts on the charge (rather than spin) degree of freedom. It can become significant in the vicinity of the superconducting transition temperature, where it generates a net quasiparticle charge and alters the dispersion and linewidth of low-frequency collective modes. [ABSTRACT FROM AUTHOR]

Phase slips in a one-dimensional closed array of Josephson junctions hybridize the persistent current states and plasmon branches of excitations. The interference between phase slips passing through different junctions of the array makes the hybridization sensitive to the charges of the superconducting islands comprising the array. This in turn results in the Aharonov-Casher effect for plasmons, which in the absence of phase slips are insensitive to island charges. [ABSTRACT FROM AUTHOR]

We show that electron-phonon interactions can alter the topological properties of Dirac insulators and semimetals, at both zero and nonzero temperature. Contrary to the common belief that increasing temperature always destabilizes topological phases, our results highlight instances in which phonons may lead to the appearance of topological surface states above a crossover temperature in a material that has a topologically trivial ground state. [ABSTRACT FROM AUTHOR]

We evaluate quantum corrections to conductivity in an electrically gated thin film of a three-dimensional topological insulator. We derive approximate analytical expressions for the low-field magnetoresistance as a function of bulk doping and bulk-surface tunneling rate. Our results reveal parameter regimes for both weak localization and weak antilocalization, and include diffusive Weyl semimetals as a special case. [ABSTRACT FROM AUTHOR]

When an electron with well-defined momentum tunnelsinto a nonchiral Luttinger liquid, it breaks up into two separate wave packets that carry fractional charges and move in opposite directions. A direct observation of this phenomenon has proven elusive, mainly due to single-particle and plasmon backscattering caused by measurement probes. This paper theoretically introduces two topological insulator devices that are naturally suited for detecting fractional charges and their velocities directly and in a noninvasive fashion. [ABSTRACT FROM AUTHOR]

The magnetization dynamics of a ferromagnet subjected to an electrical current are described by an extension of the Landau-Lifshitz-Gilbert equation that contains two additional terms, the adiabatic and nonadiabatic spin-transfer torques. First-principles calculations of the nonadiabatic spin-transfer torque parameter ß for bcc iron and fcc nickel show that ß is related to and typically of the same order as α, the damping constant, but is distinct from it. Calculations as a function of the ratio of spin-dependent scattering rates show that (1) the minimum of the damping constant as a function of scattering rate does not change significantly, and (2) when the polarization of the current approaches zero, ß can become large but the implied domain-wall velocity does not. [ABSTRACT FROM AUTHOR]

We develop a theoretical proposal for the charge-Kondo effect in mesoscopic normal-superconductor-normal heterostructures, where the superconducting gap exceeds the electrostatic charging energy. Charge-Kondo correlations in these devices alter the conventional temperature dependence of Andreev reflection and electron cotunneling. We predict typical Kondo temperatures of ≳10 mK, and suggest experimental architectures that combine superconducting charge qubits with semiconducting nanowires at cryogenic temperatures. [ABSTRACT FROM AUTHOR]

We present a theoretical study of interacting electron-hole pairs located on a magnetized surface of a strong topological insulator (TI). The excitonic energy levels and the optical absorption on such a surface display unique and potentially measurable features such as (i) an enhanced binding energy for excitons whose total angular momentum is aligned with the magnetic exchange field, (ii) a stark dependence of the optical absorption on the direction of the magnetic exchange field as well as on the chirality of the incident light, and (iii) a tunable center-of-mass motion of spinful excitons induced by particle-hole asymmetry in the exchange field. Our predictions are relevant to surfaces of magnetically doped TIs or surfaces coated with magnetic films, in addition to TI nanowires placed under longitudinal magnetic fields. [ABSTRACT FROM AUTHOR]

It has been recently predicted that the interplay between Coulomb interactions and Berry curvature can produce interesting optical phenomena in topologically nontrivial two-dimensional insulators. Here, we present a theory of the interband optical absorption for three-dimensional, doped Weyl semimetals. We find that the Berry curvature, Coulomb interactions, and the nonlinearity in the single-particle energy spectrum can together enable a light-induced valley polarization. We support and supplement our numerical results with an analytical toy model calculation, which unveils topologically nontrivial Mahan excitons with nonzero vorticity. [ABSTRACT FROM AUTHOR]

The interplay between topological phases of matter and dissipative baths constitutes an emergent research topic with links to condensed matter, photonic crystals, cold atomic gases, and quantum information. While recent studies suggest that dissipative baths can induce topological phases in intrinsically trivial quantum materials, the backaction of topological invariants on dissipative baths is overlooked. By exploring this backaction for a centrosymmetric Dirac insulator coupled to phonons, we show that the linewidths of bulk optical phonons can reveal electronic band inversions. This result is the first known example where topological phases of an open quantum system may be detected by measuring the bulk properties of the surrounding environment. [ABSTRACT FROM AUTHOR]

We demonstrate that the charge-spin conversion efficiency of topological insulators (TI) can be experimentally determined by injecting spin-polarized tunneling electrons into a TI. Through a comparative study between bismuth selenide and bismuth antimony telluride, we verified the topological-surface-state origin of the observed giant spin signals. By injecting energetic electrons into bismuth selenide, we further studied the energy dependence of the effective spin polarization at the TI surface. The experimentally verified large spin polarization, as well as our calculations, provides new insights into optimizing TI materials for near room-temperature spintronic applications. [ABSTRACT FROM AUTHOR]

The phonon magnetochiral effect consists of a nonreciprocity in the velocity or attenuation of acoustic waves when they propagate parallel and antiparallel to an external magnetic field. The first experimental observation of this effect in the bulk has been reported recently in a chiral magnet and ascribed to the hybridization between acoustic phonons and chiral magnons. Here, we predict a potentially measurable phonon magnetochiral effect of electronic origin in chiral Weyl semimetals. Caused by the Berry curvature and the orbital magnetic moment, this effect is enhanced for longitudinal phonons by the chiral anomaly. [ABSTRACT FROM AUTHOR]

Recent theoretical work has established the presence of hidden spin and orbital textures in nonmagnetic materials with inversion symmetry. Here, we propose that these textures can be detected by nuclear magnetic resonance (NMR) measurements carried out in the presence of an electric field. In crystals with hidden polarizations, a uniform electric field produces a staggered magnetic field that points to opposite directions at atomic sites related by spatial inversion. As a result, the NMR resonance peak corresponding to inversion partner nuclei is split into two peaks. The magnitude of the splitting is proportional to the electric field and depends on the orientation of the electric field with respect to the crystallographic axes and the external magnetic field. As a case study, we present a theory of electric-field-induced splitting of NMR peaks for Se77,Te125, and Bi209 in Bi2Se3 and Bi2Te3. In conducting samples with current densities of ≃106A/cm2, the splitting for Bi can reach 100kHz, which is comparable to or larger than the intrinsic width of the NMR lines. In order to observe the effect experimentally, the peak splitting must also exceed the linewidth produced by the Oersted field. In Bi2Se3, this requires narrow wires of radius ≲1μm. We also discuss other potentially more promising candidate materials, such as SrRuO3 and BaIr2Ge2, whose crystal symmetry enables strategies to suppress the linewidth produced by the Oersted field. [ABSTRACT FROM AUTHOR]

Discovered in high-energy physics, the chiral anomaly has recently made way to materials science by virtue of Weyl semimetals (WSM). Thus far, the main efforts to probe the chiral anomaly in WSM have concentrated on electronic phenomena. Here, we show that the chiral anomaly can have a large impact in the A1 phonons of enantiomorphic WSM. In these materials, the chiral anomaly produces an unusual magnetic-field-induced resonance in the effective phonon charge, which in turn leads to anomalies in the phonon dispersion, optical reflectivity, and the Raman scattering. [ABSTRACT FROM AUTHOR]

Motivated by recent nuclear magnetic resonance (NMR) experiments, we present a microscopic sp³ tight-binding model calculation of the NMR shifts in bulk Bi2Se3 and Bi2Te3. We compute the contact, dipolar, orbital and core polarization contributions to the carrier-density-dependent part of the NMR shifts in 209Bi, 125Te, and 77Se. The spin-orbit coupling and the layered crystal structure result in a contact Knight shift with strong uniaxial anisotropy. Likewise, because of spin-orbit coupling, dipolar interactions make a significant contribution to the isotropic part of the NMR shift. The contact interaction dominates the isotropic Knight shift in 209Bi NMR, even though the electronic states at the Fermi level have a rather weak s-orbital character. In contrast, the contribution from the contact hyperfine interaction to the NMR shift of 77Se and 125Te is weak compared to the dipolar and orbital shifts therein. In all cases, the orbital shift is at least comparable to the contact and dipolar shifts, while the shift due to core polarization is subdominant (except for Te nuclei located at the inversion centers). By artificially varying the strength of spin-orbit coupling, we evaluate the evolution of the NMR shift across a band inversion but find no clear signature of the topological transition. [ABSTRACT FROM AUTHOR]

Intraband and interband contributions to the current-driven spin-orbit torque in magnetic materials lacking inversion symmetry are theoretically studied using the Kuboformula. In addition to the current-driven fieldlike torque TFL=τFLm×uso (uso being a unit vector determined by the symmetry of the spin-orbit coupling), we explore the intrinsic contribution arising from impurity-independent interband transitions and producing an anti-damping-like torque of the form TDL=τDLm×(uso×m). Analytical expressions are obtained in the model case of a magnetic Rashba two-dimensional electron gas, while numerical calculations have been performed on a dilute magnetic semiconductor (Ga,Mn)As modeled by the Kohn-Luttinger Hamiltonian exchange coupled to the Mn moments. Parametric dependencies of the different torque components and similarities to the analytical results of the Rashba two-dimensional electron gas in the weak disorder limit are described. [ABSTRACT FROM AUTHOR]

Topological insulators constitute a new class of materials with an energy gap in the bulk and peculiar metallic states on the surface. We report on new features resulting from the bulk electronic structure, based on a comprehensive nuclear magnetic resonance (NMR) study of 77Se on Bi2Se3 and Cu0.15Bi2Se3 single crystals. First, we find two resonance lines and show that they originate from the two inequivalent Se lattice sites. Second, we observe unusual field-independent linewidths and attribute them to an unexpectedly strong internuclear coupling mediated by bulk electrons. In order to support this interpretation, we present a model calculation of the indirect internuclear coupling and show that the Bloembergen-Rowland coupling is much stronger than the Ruderman-Kittel-Kasuya-Yosida coupling. Our results call for a revision of earlier NMR studies and add information concerning the bulk electronic properties. [ABSTRACT FROM AUTHOR]

In two-dimensional insulators with time-reversal (TR) symmetry, a nonzero local Berry curvature of low-energy massive Dirac fermions can give rise to nontrivial spin and charge responses, even though the integral of the Berry curvature over all occupied states is zero. In this Letter, we present a new effect induced by the electronic Berry curvature. By studying electron-phonon interactions in BaMnSb_{2}, a prototype two-dimensional Dirac material possessing two TR-related massive Dirac cones, we find that the nonzero local Berry curvature of electrons can induce a phonon angular momentum. The direction of this phonon angular momentum is locked to the phonon propagation direction, and thus we refer to it as "phonon helicity" in a way that is reminiscent of electron helicity in spin-orbit-coupled electronic systems. We discuss possible experimental probes of such phonon helicity.

We develop a general method to evaluate the Kondo temperature in a multilevel quantum dot that is weakly coupled to conducting leads. Our theory reveals that the Kondo temperature is strongly enhanced when the intradot energy-level spacing is comparable or smaller than the charging energy. We propose an experiment to test our result, which consists of measuring the size dependence of the Kondo temperature.

When a ferromagnet is deposited on the surface of a topological insulator the topologically protected surface state develops a gap and becomes a two-dimensional quantum Hall liquid. We demonstrate that the Hall current in such a liquid, induced by an external electric field, can have a dramatic effect on the magnetization dynamics of the ferromagnet by changing the effective anisotropy field. This change is dissipationless and may be substantial even in weakly spin-orbit coupled ferromagnets. We study the possibility of dissipationless current-induced magnetization reversal in monolayer-thin, insulating ferromagnets with a soft perpendicular anisotropy and discuss possible applications of this effect.