arXiv subjects

Reinhold Egger

Publications and source records attributed to Reinhold Egger.

At least 55 records · Page 3Linked to original sources

arXiv2022

Readout of parafermionic states by transport measurements

Recent experiments have demonstrated the possibility of inducing superconducting pairing into counterpropagating fractional quantum Hall edge modes. This paves the way for the realization of localized parafermionic modes, non-Abelian anyons that share fractional charges in a nonlocal way. We show that, for a pair of isolated parafermions, this joint degree of freedom can be read by conductance measurements across standard metallic electrodes. We propose two complementary setups. We investigate first the transport through a grounded superconductor hosting two interacting parafermions. In the low-energy limit, its conductance peaks reveal their shared fractional charge yielding a three-state telegraph noise for weak quasiparticle poisoning. We then examine the two-terminal electron conductance of a blockaded fractional topological superconductor, which displays a characteristic $e/3$ periodicity of its zero-bias peaks in the deep topological regime, thus signalling the presence of parafermionic modes.

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arXiv2022

Transport, refraction and interface arcs in junctions of Weyl semimetals

We study the low-energy single-electron transport across a junction of two magnetic Weyl semimetals, in which the anisotropy axes are tilted one respect to the other. Using a two-band model with a potential step, we compute the transmission factor for normal and Klein tunneling and the refraction properties of the interface as a function of the tilt angle. We show that the interface acts as a beam splitter, separating electrons with different chiralities. We also characterize interface states, only appearing for finite tilt angle, which connect the projection of the Fermi surfaces on the two sides of the junction, and we discuss transport effects due to their interplay with Fermi arcs.

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arXiv2022

Phonon-limited Transport and Fermi Arc Lifetime in Weyl Semimetals

Weyl semimetals harbor topological Fermi-arc surface states which determine the nontrivial charge current response to external fields. We here study the quasiparticle decay rate of Fermi arc states arising from their coupling to acoustic phonons, as well as the phonon-limited conductivity tensor for a clean Weyl semimetal slab. Using the phonon modes for an isotropic elastic continuum with a deformation potential coupling to electrons, we determine the temperature dependence of the quasiparticle decay rate, both near and far away from the arc termination points. By solving the coupled Boltzmann equations for the bulk and arc state distribution functions in the slab geometry, we show how the linear response conductivity depends on key parameters such as the temperature, the chemical potential, the geometric shape of the Fermi arcs, or the slab width. The chiral nature of Fermi arc states causes an enhancement of the longitudinal conductivity along the chiral direction at low temperatures, together with a 1/T^2 scaling regime at intermediate temperatures without counterpart for the conductivity along the perpendicular direction.

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arXiv2022

Violation of the Wiedemann-Franz law in the Topological Kondo model

We study the thermal transport through a Majorana island connected to multiple external quantum wires. In the presence of a large charging energy, we find that the Wiedemann-Franz law is nontrivially violated at low temperature, contrarily to what happens for the overscreened Kondo effect and for nontopological junctions. For three wires, we find that the Lorenz ratio is rescaled by a universal factor 2/3 and we show that this behavior is due to the presence of localized Majorana modes on the island.

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arXiv2021

Phonon-induced magnetoresistivity of Weyl semimetal nanowires

We study longitudinal magnetotransport in disorder-free cylindrical Weyl semimetal nanowires. Our theory includes a magnetic flux $Φ$ piercing the nanowire and captures the finite curvature of the Fermi arc in the surface Brillouin zone through a boundary angle $α$. Electron backscattering by acoustic phonons via the deformation potential causes a finite resistivity which we evaluate by means of the semiclassical Boltzmann approach. We find that low-energy transport is dominated by surface states, where transport observables are highly sensitive to the angle $α$ and to Aharonov-Bohm phases due to $Φ$. A generic subband dispersion relation allows for either one or two pairs of Fermi points. In the latter case, intra-node backscattering is possible and implies a parametrically larger resistivity than for a single Fermi point pair. As a consequence, large and abrupt resistivity changes take place across the transition points separating parameter regions with a different number of Fermi point pairs in a given subband.

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arXiv2020

Electrical Access to Ising Anyons in Kitaev Spin Liquids

We show that spin-spin correlations in a non-Abelian Kitaev spin liquid are associated with a characteristic inhomogeneous charge density distribution in the vicinity of $\mathbb{Z}_2$ vortices. This density profile and the corresponding local electric fields are observable, e.g., by means of surface probe techniques. Conversely, by applying bias voltages to several probe tips, one can stabilize Ising anyons ($\mathbb{Z}_2$ vortices harboring a Majorana zero mode) at designated positions, where we predict a clear Majorana signature in energy absorption spectroscopy.

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arXiv2020

Non-Abelian Berry phase for open quantum systems: Topological protection vs geometric dephasing

We provide a detailed theoretical analysis of the adiabatic evolution of degenerate open quantum systems, where the dynamics is induced by time-dependent fluctuating loop paths in control parameter space. For weak system-bath coupling, the fluctuations around a deterministic base path obey Gaussian statistics, where we assume that the quantum adiabatic theorem is also satisfied for fluctuating paths. We show that universal non-Abelian geometric dephasing (NAGD) contributions are contained in the fluctuation-averaged evolution operator. This operator plays a key role in all experimental protocols proposed in this work for the detection of NAGD. In particular, we formulate interference measurements providing full access to NAGD. Apart from a factor due to the dynamic phase and its fluctuations, the averaged evolution operator contains the averaged Berry matrix. A polar decomposition of this non-unitary matrix into the product of a unitary, $V$, and a positive semi-definite Hermitian matrix, $R$, reveals the physics of the NAGD contributions. Unlike the conventional dynamic dephasing rate, the NAGD eigenrates encoded by $R$ depend on the loop orientation sense and, in particular, change sign under a reversal of the direction. A negative rate then implies amplification of coherences as compared to the ones when only dynamic dephasing is present. The non-Abelian character of geometric dephasing is rooted in the non-commutativity of $V$ and $R$ and causes smoking-gun signatures in interference experiments. We also clarify why systems subject to topological protection do not show geometric dephasing. Without full protection, however, geometric dephasing can arise and has to be taken into account. As concrete application, we propose spin-echo NAGD detection protocols for modified Majorana braiding setups.

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arXiv2020

Non-Abelian Geometric Dephasing

We study the adiabatic dynamics of degenerate quantum states induced by loop paths in a control parameter space. The latter correspond to noisy trajectories if the system is weakly coupled to environmental modes. On top of conventional dynamic dephasing, we find a universal non-Abelian geometric dephasing (NAGD) contribution and express it in terms of the non-Abelian Berry connection and curvature. We show that NAGD implies either decay or amplification of coherences as compared to the coherences when only dynamic dephasing is present. The full NAGD matrix structure can be probed through interference experiments. We outline such a detection scheme for modified Majorana braiding setups.

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arXiv2020

Parity-to-charge conversion in Majorana qubit readout

We study the time-dependent effect of Markovian readout processes on Majorana qubits whose parity degrees of freedom are converted into the charge of a tunnel-coupled quantum dot. By applying a recently established effective Lindbladian approximation [1-3], we obtain a completely positive and trace preserving Lindblad master equation for the combined dot-qubit dynamics, describing relaxation and decoherence processes beyond the rotating-wave approximation. This approach is applicable to a wide range of weakly coupled environments representing experimentally relevant readout devices. We study in detail the case of thermal decay in the presence of a generic Ohmic bosonic bath, in particular for potential fluctuations in an electromagnetic circuit. In addition, we consider the nonequilibrium measurement environment for a parity readout using a quantum point contact capacitively coupled to the dot charge.

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arXiv2020

Weak measurement protocols for Majorana bound state identification

We propose a continuous weak measurement protocol testing the nonlocality of Majorana bound states through current shot noise correlations. The experimental setup contains a topological superconductor island with three normal-conducting leads weakly coupled to different Majorana states. Putting one lead at finite voltage and measuring the shot noise correlations between the other two (grounded) leads, devices with true Majorana states are distinguished from those without by strong current correlations. The presence of true Majoranas manifests itself in unusually high noise levels or the near absence of noise, depending on the chosen device configuration. Monitoring the noise statistics amounts to a weak continuous measurement of the Majorana qubit and yields information similar to that of a full braiding protocol, but at much lower experimental effort. Our theory can be adapted to different platforms and should allow for clear identification of Majorana states.

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arXiv2020

Phase diagram and phonon-induced backscattering in topological insulator nanowires

We present an effective low-energy theory of electron-phonon coupling effects for clean cylindrical topological insulator nanowires. Acoustic phonons are modelled by isotropic elastic continuum theory with stress-free boundary conditions. We take into account the deformation potential coupling between phonons and helical surface Dirac fermions, and also include electron-electron interactions within the bosonization approach. For half-integer values of the magnetic flux $Φ_B$ along the wire, the low-energy theory admits an exact solution since a topological protection mechanism then rules out phonon-induced $2k_F$-backscattering processes. We determine the zero-temperature phase diagram and identify a regime dominated by superconducting pairing of surface states. As example, we consider the phase diagram of HgTe nanowires. We also determine the phonon-induced electrical resistivity, where we find a quadratic dependence on the flux deviation $δΦ_B$ from the nearest half-integer value.

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arXiv2019

Spin chain network construction of chiral spin liquids

We show that a honeycomb lattice of Heisenberg spin-$1/2$ chains with three-spin junction interactions allows for controlled analytical studies of chiral spin liquids (CSLs). Tuning these interactions to a chiral fixed point, we find a Kalmeyer-Laughlin CSL phase which here is connected to the critical point of a boundary conformal field theory. Our construction directly yields a quantized spin Hall conductance and localized spinons with semionic statistics as elementary excitations. We also outline the phase diagram away from the chiral point where spinons may condense. Generalizations of our approach can provide microscopic realizations for many other CSLs.

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arXiv2019

Superconductivity from piezoelectric interactions in Weyl semimetals

We present an analytical low-energy theory of piezoelectric electron-phonon interactions in undoped Weyl semimetals, taking into account also Coulomb interactions. We show that piezoelectric interactions generate a long-range attractive potential between Weyl fermions. This potential comes with a characteristic angular anisotropy. From the one-loop renormalization group approach and a mean-field analysis, we predict that superconducting phases with either conventional s-wave singlet pairing or nodal-line triplet pairing could be realized for sufficiently strong piezoelectric coupling. For small couplings, we show that the quasi-particle decay rate exhibits a linear temperature dependence where the prefactor vanishes only in a logarithmic manner as the quasi-particle energy approaches the Weyl point. For practical estimates, we consider the Weyl semimetal TaAs.

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arXiv2019

Fidelity and visibility reduction in Majorana qubits by entanglement with environmental modes

We study the dynamics and readout of topological qubits encoded by zero-energy Majorana bound states in a topological superconductor. We take into account bosonic modes due to the electromagnetic environment which couple the Majorana manifold to above-gap continuum quasi-particles. This coupling causes the degenerate ground state of the topological superconductor to be dressed in a polaron-like manner by quasi-particle states and bosons, and the system to become gapless. Topological protection and hence full coherence is only maintained if the qubit is operated and read out within the low-energy spectrum of the dressed states. We discuss reduction of fidelity and/or visibility if this condition is violated by a quantum-dot readout that couples to the bare (undressed) Majorana modes. For a projective measurement of the bare Majorana basis, we formulate a Bloch-Redfield approach that is valid for weak Majorana-environment coupling and takes into account constraints imposed by fermion-number-parity conservation. Within the Markovian approximation, our results essentially confirm earlier theories of finite-temperature decoherence based on Fermi's golden rule. However, the full non-Markovian dynamics reveals, in addition, the fidelity reduction by a projective measurement. Using a spinless nanowire model with $p$-wave pairing, we provide quantitative results characterizing these effects.

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arXiv2018

Non-Topological Majorana Zero Modes in Inhomogeneous Spin Ladders

We show that the coupling of homogeneous Heisenberg spin-1/2 ladders in different phases leads to the formation of interfacial zero energy Majorana bound states. Unlike Majorana bound states at the interfaces of topological quantum wires, these states are void of topological protection and generally susceptible to local perturbations of the host spin system. However, a key message of our work is that in practice they show a high degree of resilience over wide parameter ranges which may make them interesting candidates for applications.

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arXiv2018

Simulating dynamically assisted production of Dirac pairs in gapped graphene monolayers

In a vicinity of the Fermi surface, graphene layers with bandgaps allow for closely simulating the vacuum of quantum electrodynamics and, thus, its yet unverified strong-field phenomenology with accessible field strengths. This striking feature is exploited to investigate a plausible materialization of dynamically assisted pair production through the analog production of light but massive pairs of Dirac quasiparticles. The process is considered in a field configuration combining a weak high-frequency electric mode and a strong low-frequency electric field oscillating in time. Its theoretical study is carried out from a quantum kinetic approach, similar to the one governing the spontaneous production of pairs in QED. We show that the presence of the weak assisting mode can strongly increase the number of produced massive Dirac pairs as compared with a setup driven by the strong field only. The efficiency of the process is contrasted, moreover, with the case of gapless graphene to highlight the role played by the quasiparticle mass.

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arXiv2018

Parafermionic generalization of the topological Kondo effect

We propose and study a parafermionic generalization of the topological Kondo effect. The latter has been predicted to arise for a Coulomb-blockaded mesoscopic topological superconductor (Majorana box), where at least three normal leads are tunnel-coupled to different Majorana zero modes on the box. The Majorana states represent a quantum impurity spin that is partially screened due to cotunneling processes between leads, with a stable non-Fermi liquid ground state. Our theory studies a generalization where (i) Majorana states are replaced by topologically protected parafermionic zero modes, (ii) charging effects again define a spin-like quantum impurity on the resulting parafermion box, and (iii) normal leads are substituted by fractional edge states. In this multi-terminal problem, different fractional edge leads couple only via the parafermion box. We show that although the linear conductance tensor exhibits similar behavior as in the Majorana case, both at weak and strong coupling, our parafermionic generalization is actually not a Kondo problem but defines a rich new class of quantum impurity problems. At the strong-coupling fixed point, a current injected through a reference lead will be isotropically partitioned into outgoing currents in all other leads, together with a universal negative current scattered into the reference lead. The device can thus be operated as current extractor, where the current partitioning is noiseless at the fixed point. We describe a fractional quantum Hall setup proximitized by superconductors and ferromagnets, which could allow for an experimental realization in the near future.

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arXiv2018

Quantum transport in coupled Majorana box systems

We present a theoretical analysis of low-energy quantum transport in coupled Majorana box devices. A single Majorana box represents a Coulomb-blockaded mesoscopic superconductor proximitizing two or more long topological nanowires. The box thus harbors at least four Majorana zero modes (MZMs). Setups with several Majorana boxes, where MZMs on different boxes are tunnel-coupled via short nanowire segments, are key ingredients to recent Majorana qubit and code network proposals. We construct and study the low-energy theory for multi-terminal junctions with normal leads connected to the coupled box device by lead-MZM tunnel contacts. Transport experiments in such setups can test the nonlocality of Majorana-based systems and the integrity of the underlying Majorana qubits. For a single box, we recover the previously described topological Kondo effect which can be captured by a purely bosonic theory. For several coupled boxes, however, non-conserved local fermion parities require the inclusion of additional local sets of Pauli operators. We present a renormalization group analysis and develop a nonperturbative strong-coupling approach to quantum transport in such systems. Our findings are illustrated for several examples, including a loop qubit device and different two-box setups.

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