arXiv subjects

Benedikt Scharf

Publications and source records attributed to Benedikt Scharf.

At least 19 recordsLinked to original sources

arXiv2024

Superconducting Diode Effect in Quantum Spin Hall Insulator-based Josephson Junctions

The superconducting diode effect (SDE) is a magneto-electric phenomenon where an external magnetic field imparts a non-zero center-of-mass momentum to Cooper pairs, either facilitating or hindering the flow of supercurrent depending on its direction. We propose that quantum spin Hall insulator (QSHI)-based Josephson junctions can serve as versatile platforms for non-dissipative electronics exhibiting the SDE when triggered by a phase bias and an out-of-plane magnetic field. By computing the contributions from Andreev bound states and the continuum of quasi-particle states, we provide both numerical and analytical results scrutinizing various aspects of the SDE, including its quality Q-factor. The maximum value of the $Q$-factor is found to be universal at low (zero) temperatures, which ties its origin to underlying topological properties that are independent of the junction's specific details. As the magnetic field increases, the SDE diminishes due to the closing of the induced superconducting gap caused by orbital effects. To observe the SDE, the QSHI-based Josephson junction must be designed so that its edges are transport-wise non-equivalent. Additionally, we explore the SDE in a more exotic yet realistic scenario, where the fermionic ground-state parity of the Josephson junction remains conserved while driving a current. In this 4$π$-periodic situation, we predict an enhancement of the SDE compared to its 2$π$-periodic, parity-unconstrained counterpart.

cond-mat.supr-con↗

arXiv2024

Phonon-assisted Auger decay of excitons in doped transition metal dichalcogenide monolayers

The competition between the radiative and nonradiative lifetimes determines the optical quantum yield and plays a crucial role in the potential optoelectronic applications of transition metal dichalcogenides (TMDC). Here, we show that, in the presence of free carriers, an additional nonradiative decay channel opens for excitons in TMDC monolayers. Although the usual Auger decay channel is suppressed at low doping levels by the simultaneous momentum and energy conservation laws, exciton-phonon coupling relaxes this suppression. By solving a Bethe-Salpeter Equation, we calculate the phonon-assisted Auger decay rates in four typical TMDCs as a function of doping, temperature, and dielectric environment. We find that even for a relatively low doping of 10$^{12}$ cm$^{-2}$, the nonradiative lifetime ranges from 16 ps to 165 ps in different TMDCs, offering competition to the radiative decay channel.

cond-mat.mes-hall↗

arXiv2024

Emergent bright excitons with Rashba spin-orbit coupling in atomic monolayers

Optical properties in van der Waals heterostructures based on monolayer transition-metal dichalcogenides (TMDs), are often dominated by excitonic transitions. While intrinsic spin-orbit coupling (SOC) and an isotropic band structure are typically studied in TMDs, in their heterostructures Rashba SOC and trigonal warping (TW), resulting in bands with threefold anisotropy, are also present. By considering a low-energy effective Hamiltonian and Bethe-Salpeter equation, we study the effect of Rashba SOC and TW on the band structure and absorption spectra. Rashba SOC is predicted to lead to emergent excitons, which are identified as an admixture between 1s and 2p symmetries. In contrast, for experimentally relevant values, TW has only a negligible effect on the absorption spectrum. These findings could guide experimental demonstrations of emergent bright excitons and further studies of the proximity effects in van der Waals heterostructure.

cond-mat.mes-hall↗

arXiv2022

Crystalline Weyl semimetal phase in Quantum Spin Hall systems under magnetic fields

We investigate an unconventional topological phase transition that occurs in quantum spin Hall (QSH) systems when applying an external in-plane magnetic field. We show that this transition between QSH and trivial insulator phases is separated by a stable topological gapless phase, which is protected by the combination of particle-hole and reflection symmetries, and thus, we dub it as crystalline Weyl semimetal. We explore the stability of this new phase when particle-hole symmetry breaking terms are present. Especially, we predict a robust unconventional topological phase transition to be visible for materials described by Kane and Mele model even if particle-hole symmetry is significantly broken.

cond-mat.mes-hall↗

arXiv2021

Lifting topological protection in a quantum spin Hall insulator by edge coupling

The scientific interest in two-dimensional topological insulators (2D TIs) is currently shifting from a more fundamental perspective to the exploration and design of novel functionalities. Key concepts for the use of 2D TIs in spintronics are based on the topological protection and spin-momentum locking of their helical edge states. In this study we present experimental evidence that topological protection can be (partially) lifted by pairwise coupling of 2D TI edges in close proximity. Using direct wave function mapping via scanning tunneling microscopy/spectroscopy (STM/STS) we compare isolated and coupled topological edges in the 2D TI bismuthene. The latter situation is realized by natural lattice line defects and reveals distinct quasi-particle interference (QPI) patterns, identified as electronic Fabry-Pérot resonator modes. In contrast, free edges show no sign of any single-particle backscattering. These results pave the way for novel device concepts based on active control of topological protection through inter-edge hybridization for, e.g., electronic Fabry-Pérot interferometry.

cond-mat.mes-hall↗

arXiv2021

Charge Density Wave Activated Excitons in TiSe$_2$-MoSe$_2$ Heterostructures

Layered materials enable the assembly of a new class of heterostructures where lattice-matching is no longer a requirement. Interfaces in these heterostructures therefore become a fertile ground for unexplored physics as dissimilar phenomena can be coupled via proximity effects. In this article we identify an unexpected photoluminescence (PL) peak when MoSe$_2$ interacts with TiSe$_2$. A series of temperature-dependent and spatially-resolved PL measurements reveal this peak is unique to the TiSe$_2$-MoSe$_2$ interface, higher in energy compared to the neutral exciton, and exhibits exciton-like characteristics. The feature disappears at the TiSe$_2$ charge density wave transition, suggesting that the density wave plays an important role in the formation of this new exciton. We present several plausible scenarios regarding the origin of this peak that individually capture some aspects of our observations, but cannot fully explain this feature. These results therefore represent a fresh challenge for the theoretical community and provide a fascinating way to engineer excitons through interactions with charge density waves.

cond-mat.mes-hall↗

arXiv2021

Thermodynamics in topological Josephson junctions

We study the thermodynamic properties of topological Josephson junctions using a quantum spin Hall (QSH) insulator-based junction as an example. In particular, we propose that phase-dependent measurements of the heat capacity offer an alternative to Josephson-current measurements to demonstrate key topological features. Even in an equilibrium situation, where the fermion parity is not conserved, the heat capacity exhibits a pronounced double peak in its phase dependence as a signature of the protected zero-energy crossing in the Andreev spectrum. This double-peak feature is robust against changes of the tunneling barrier and thus allows one to distinguish between topological and trivial junctions. At short time scales fermion parity is conserved and the heat capacity is $4π$-periodic in the superconducting phase difference. We propose a dispersive setup coupling the Josephson junction to a tank LC circuit to measure the heat capacity of the QSH-based Josephson junction sufficiently fast to detect the $4π$-periodicity. Although explicitly calculated for a short QSH-based Josephson junction, our results are also applicable to long as well as nanowire-based topological Josephson junctions.

cond-mat.mes-hall↗

arXiv2021

Planar Josephson Hall effect in topological Josephson junctions

Josephson junctions based on three-dimensional topological insulators offer intriguing possibilities to realize unconventional $p$-wave pairing and Majorana modes. Here, we provide a detailed study of the effect of a uniform magnetization in the normal region: We show how the interplay between the spin-momentum locking of the topological insulator and an in-plane magnetization parallel to the direction of phase bias leads to an asymmetry of the Andreev spectrum with respect to transverse momenta. If sufficiently large, this asymmetry induces a transition from a regime of gapless, counterpropagating Majorana modes to a regime with unprotected modes that are unidirectional at small transverse momenta. Intriguingly, the magnetization-induced asymmetry of the Andreev spectrum also gives rise to a Josephson Hall effect, that is, the appearance of a transverse Josephson current. The amplitude and current phase relation of the Josephson Hall current are studied in detail. In particular, we show how magnetic control and gating of the normal region can enable sizable Josephson Hall currents compared to the longitudinal Josephson current. Finally, we also propose in-plane magnetic fields as an alternative to the magnetization in the normal region and discuss how the planar Josephson Hall effect could be observed in experiments.

cond-mat.supr-con↗

arXiv2021

Crystalline Anisotropic Topological Superconductivity in Planar Josephson Junctions

We theoretically investigate the crystalline anisotropy of topological phase transitions in phase-controlled planar Josephson junctions (JJs) subject to spin-orbit coupling and in-plane magnetic fields. It is shown how topological superconductivity (TS) is affected by the interplay between the magnetic field and the orientation of the junction with respect to its crystallographic axes. This interplay can be used to electrically tune between different symmetry classes in a controlled fashion and thereby optimize the stability and localization of Majorana bound states in planar Josephson junctions. Our findings can be used as a guide for achieving the most favorable conditions when engineering TS in planar JJs and can be particularly relevant for setups containing non-collinear junctions which have been proposed for performing braiding operations on multiple Majorana pairs.

cond-mat.supr-con↗

arXiv2020

Topological Josephson Heat Engine

The promise of fault-tolerant quantum computing has made topological superconductors the focus of intense research during the past decade. In this context, topological Josephson junctions based on nanowires or on topological insulators provide an alternative route for probing topological superconductivity. As a hallmark of their topological nature, such junctions exhibit a ground-state fermion parity that is $4π$-periodic in the superconducting phase difference $ϕ$. Finding unambiguous experimental evidence for this $4π$-periodicity still proves a difficult task, however. Here we propose a topological Josephson heat engine implemented by a Josephson-Stirling cycle as an alternative thermodynamic approach to test the ground-state parity. Using a Josephson junction based on a quantum spin Hall (QSH) insulator, we show how the thermodynamic cycle can be used to test the $4π$-periodicity of the topological ground state and to distinguish between parity-conserving and non-parity-conserving engines. Interestingly, we find that parity conservation generally boosts both the efficiency and power of the topological heat engine with respect to its non-topological counterpart. Our results, applicable not only to QSH-based junctions but also to any topological Josephson junction, demonstrate the potential of the intriguing and fruitful marriage between topology and coherent thermodynamics.

cond-mat.supr-con↗

arXiv2020

Optically Probing Tunable Band Topology in Atomic Monolayers

In many atomically thin materials their optical absorption is dominated by excitonic transitions. It was recently found that optical selection rules in these materials are influenced by the band topology near the valleys. We propose that gate-controlled band ordering in a single atomic monolayer, through changes in the valley winding number and excitonic transitions, can be probed in helicity-resolved absorption and photoluminescence. This predicted tunable band topology is confirmed by combining an effective Hamiltonian and a Bethe-Salpeter equation for an accurate description of excitons, with first-principles calculations suggesting its realization in Sb-based monolayers.

cond-mat.mes-hall↗

arXiv2020

Resonant Tunneling Anisotropic Magnetoresistance Induced by Magnetic Proximity

We reveal that the interplay between Rashba spin-orbit coupling and proximity-induced magnetization in a two-dimensional electron gas leads to peculiar transport properties and large anisotropy of magnetoresistance. While the related tunneling anisotropic magnetoresistance (TAMR) has been extensively studied before, we predict an effect with a different origin arising from the evolution of a resonant condition with the in-plane rotation of magnetization and having a much larger magnitude. The resonances in the tunneling emerge from a spin-parity-time symmetry of the scattering states. However, such a symmetry is generally absent from the system itself and only appears for certain parameter values. Without resonant behavior in the topological surface states of a proximitized three-dimensional topological insulator (TI), TAMR measurements can readily distinguish them from often misinterpreted trivial Rashba-like states inherent to many TIs.

cond-mat.mes-hall↗

arXiv2019

Common nonlinear features and spin-orbit coupling effects in the Zeeman splitting of novel wurtzite materials

The response of semiconductor materials to external magnetic fields is a reliable approach to probe intrinsic electronic and spin-dependent properties. In this study, we investigate the common Zeeman splitting features of novel wurtzite materials, namely InP, InAs, and GaAs. We present values for the effective g-factors of different energy bands and show that spin-orbit coupling effects, responsible for the spin splittings, also have noticeable contributions to the g-factors. Within the Landau level picture, we show that the nonlinear Zeeman splitting recently explained in magneto photoluminescence experiments for InP nanowires by Tedeschi et al. [Phys. Rev. B 99, 161204 (2019)] are also present in InAs, GaAs and even in the conventional GaN. Such nonlinear features stem from the peculiar coupling of the A and B valence bands, as a consequence of the interplay between the wurtzite crystal symmetry and the breaking of time-reversal symmetry by the external magnetic field. Moreover, we develop an analytical model to describe the experimental nonlinear Zeeman splitting and apply it to InP and GaAs data. Extrapolating our fitted results, we found that the Zeeman splitting of InP reaches a maximum value, which is a prediction that could be probed at higher magnetic fields.

cond-mat.mes-hall↗

arXiv2019

Tuning Topological Superconductivity in Phase-Controlled Josephson Junctions with Rashba and Dresselhaus Spin-Orbit Coupling

Recently, topological superconductors based on Josephson junctions in two-dimensional electron gases with strong Rashba spin-orbit coupling have been proposed as attractive alternatives to wire-based setups. Here, we elucidate how phase-controlled Josephson junctions based on quantum wells with [001] growth direction and an arbitrary combination of Rashba and Dresselhaus spin-orbit coupling can also host Majorana bound states for a wide range of parameters as long as the magnetic field is oriented appropriately. Hence, Majorana bound states based on Josephson junctions can appear in a wide class of two-dimensional electron gases. We study the effect of spin-orbit coupling, the Zeeman energies, and the superconducting phase difference to create a full topological phase diagram and find the optimal stability region to observe Majorana bound states in narrow junctions. Surprisingly, for equal Rashba and Dresselhaus spin-orbit coupling, well localized Majorana bound states can appear only for phase differences $ϕ\neqπ$ as the topological gap protecting the Majorana bound states vanishes at $ϕ=π$. Our results show that the ratio between Rashba and Dresselhaus spin-orbit coupling or the choice of the in-plane crystallographic axis along which the superconducting phase bias is applied offer additional tunable knobs to test Majorana bound states in these systems. Finally, we discuss signatures of Majorana bound states that could be probed experimentally by tunneling conductance measurements at the edge of the junction.

cond-mat.supr-con↗

arXiv2019

Probing Many-Body Interactions in Monolayer Transition-Metal Dichalcogenides

Many-body interactions in monolayer transition-metal dichalcogenides are strongly affected by their unique band structure. We study these interactions by measuring the energy shift of neutral excitons (bound electron-hole pairs) in gated WSe$_2$ and MoSe$_2$. Surprisingly, while the blueshift of the neutral exciton, $X^0$, in electron-doped samples can be more than 10~meV, the blueshift in hole-doped samples is nearly absent. Taking into account dynamical screening and local-field effects, we present a transparent and analytical model that elucidates the crucial role played by intervalley plasmons in electron-doped conditions. The energy shift of $X^0$ as a function of charge density is computed showing agreement with experiment, where the renormalization of $X^0$ by intervalley plasmons yields a stronger blueshift in MoSe$_2$ than in WSe$_2$ due to differences in their band ordering.

cond-mat.mtrl-sci↗

arXiv2019

Dynamical screening in monolayer transition-metal dichalcogenides and its manifestations in the exciton spectrum

Monolayer transition-metal dichalcogenides (ML-TMDs) offer exciting opportunities to test the manifestations of many-body interactions through changes in the charge density. Tuning the charge density by a gate voltage leads to profound changes in the optical spectra of excitons in ML-TMDs. We review the band-gap renormalization and dynamical screening as a function of charge density, and then incorporate these effects through various approximations that model long-wavelength charge excitations in the Bethe-Salpeter Equation (BSE). We then show that coupling between excitons and shortwave charge excitations is essential to resolve several experimental puzzles. Unlike ubiquitous and well-studied plasmons, driven by collective oscillations of the background charge density in the long-wavelength limit, we discuss the emergence of shortwave plasmons that originate from the short-range Coulomb interaction through which electrons transition between the $\mathbf{K}$ and $-\mathbf{K}$ valleys. We study the coupling between the shortwave plasmons and the neutral exciton through the self-energy of the latter. We then elucidate how this coupling as well as the spin ordering in the conduction band give rise to an experimentally observed optical sideband in electron-doped W-based MLs, conspicuously absent in electron-doped Mo-based MLs or any hole-doped ML-TMDs. While the focus of this review is on the optical manifestations of many-body effects in ML-TMDs, a systematic description of the dynamical screening and its various approximations allow one to revisit other phenomena, such as nonequilibrium transport or superconducting pairing, where the use of the BSE or the emergence of shortwave plasmons can play an important role.

cond-mat.mtrl-sci↗

arXiv2019

Intervalley plasmons in crystals

Collective charge excitations in solids have been the subject of intense research ever since the pioneering works of Bohm and Pines in the 1950s. Most of these studies focused on long-wavelength plasmons that involve charge excitations with a small crystal-momentum transfer, $q \ll G$, where $G$ is the wavenumber of a reciprocal lattice vector. Less emphasis was given to collective charge excitations that lead to shortwave plasmons in multivalley electronic systems (i.e., when $q \sim G$). We present a theory of intervalley plasmons, taking into account local-field effects in the dynamical dielectric function. Focusing on monolayer transition-metal dichalcogenides where each of the valleys is further spin-split, we derive the energy dispersion of these plasmons and their interaction with external charges. Emphasis in this work is given to sum rules from which we derive the interaction between intervalley plasmons and a test charge, as well as a compact single-plasmon pole expression for the dynamical Coulomb potential.

cond-mat.mtrl-sci↗

arXiv2018

Testing Topological Protection of Edge States in Hexagonal Quantum Spin Hall Candidate Materials

We analyze the detailed structure of topological edge mode protection occuring in hexagonal quantum spin Hall (QSH) materials. We focus on bismuthene, antimonene, and arsenene on a SiC substrate, which, due to their large bulk gap, may offer new opportunities for room-temperature QSH applications. While time reversal symmetry is responsible for the principal symmetry protected character of QSH states, the hexagonal edge terminations yield further aspects of crystal symmetry which affect the topological protection. We show that armchair QSH edge states remain gapless under an in-plane magnetic field in the direction along the edge, a hallmark of their topological crystalline protection. In contrast, an out-of-plane magnetic field opens a gap of the order of a few meV within realistic ranges of parameters. We use these intriguing signatures of armchair QSH edge states to predict experimentally testable fingerprints of their additional topological crystalline character and their helicity emerging in tunneling spectroscopy and ballistic magnetotransport.

cond-mat.mes-hall↗