arXiv subjects

John Schliemann

Publications and source records attributed to John Schliemann.

At least 19 recordsLinked to original sources

arXiv2025

Consecutive-gap ratio distribution for crossover ensembles

The study of spectrum statistics, such as the consecutive-gap ratio distribution, has revealed many interesting properties of many-body complex systems. Here we propose a two-parameter surmise expression for such distribution to describe the crossover between the Gaussian orthogonal ensemble (GOE) and Poisson statistics. This crossover is observed in the isotropic Heisenberg spin-$1/2$ chain with disordered local field, exhibiting the Many-Body Localization (MBL) transition. Inspired by the analysis of stability in dynamical systems, this crossover is presented as a flow pattern in the parameter space, with the Poisson statistics being the fixed point of the system, which represents the MBL phase. We also analyze an isotropic Heisenberg spin-$1/2$ chain with disordered local exchange coupling and a zero magnetic field. In this case, the system never achieves the MBL phase because of the spin rotation symmetry. This case is more sensitive to finite-size effects than the previous one, and thus the flow pattern resembles a two-dimensional random walk close to its fixed point. We propose a system of linearized stochastic differential equations to estimate this fixed point. We study the continuous-state Markov process that governs the probability of finding the system close to this fixed point as the disorder strength increases. In addition, we discuss the conditions under which the stationary probability distribution is given by a bivariate normal distribution.

cond-mat.dis-nn↗

arXiv2025

From Thermalization to Multifractality: Spin-Spin Correlation in Disordered $SU(2)$-Invariant 1D Heisenberg Spin Chains

We investigate spin correlations in one-dimensional $SU(2)$-invariant Heisenberg chains with exchange disorder for spin lengths $S=1/2$ and $S=1$. In the weak-disorder regime, the eigenmodes of the spin-spin correlation matrix are delocalized, consistent with ergodic behavior. Under strong disorder, the system enters a quasi-localized multifractal phase characterized by exponentially decaying, dimer-like spin correlations. Finite-size scaling of the inverse participation ratios of the correlation-matrix eigenmodes yields a correlation dimension, $D_2\approx 0.37-0.39$, confirming the stability of a multifractal regime that is distinct from both the ergodic limit ($D_2=1$) and the fully localized limit ($D_2=0$).

cond-mat.dis-nn↗

arXiv2025

Tunable plasmon modes and topological transitions in single- and bilayer semi-Dirac materials

We investigate the plasmonic response of single- and bilayer semi-Dirac materials under the influence of a tunable parameter $δ$ that governs topological transitions via Dirac cone generation/merging and incorporating band inversion terms. For single-layer systems, we demonstrate that the emergence of Dirac cones leads to an enhanced plasmon frequency range and that the plasmonic spectrum exhibits strong anisotropy, especially for finite $δ$ and vanishing inversion terms. In the bilayer configurations, we uncover a second plasmon mode whose relative phase, with respect to the first mode, can be actively controlled by rotating the upper layer which impacts the symmetry of the charge oscillations across the layers. This tunability enables switching between in- and out-of-phase plasmonic modes, offering a route toward phase-controlled collective excitations. Our results highlight the potential of semi-Dirac systems for topological plasmonics and interferometric applications in next-generation optoelectronic devices.

physics.optics↗

arXiv2025

Role of anisotropic confining potential and elliptical driving in dynamics of a Ge hole qubit

The squeezing of a Ge planar quantum dot enhances the Rabi frequency of electric dipole spin resonance by several orders of magnitude due to a strong Direct Rashba spin-orbit interaction in such geometries (Bosco et al 2021 Phys. Rev. B 104 115425). We investigate the geometric effect of an elliptical (squeezed) confinement and its interplay with the polarization of driving field in determining the Rabi frequency of a heavy-hole qubit in a planar Ge quantum dot. To calculate the Rabi frequency, we consider only the p-linear SOIs viz. electron-like Rashba, hole-like Rashba and hole-like Dresselhaus which are claimed to be the dominant ones by recent studies on planar Ge heterostructures. We derive approximate analytical expressions of the Rabi frequency using a Schrieffer-Wolff transformation for small SOI and driving strengths. Firstly, for an out-of-plane magnetic field with magnitude B, we get an operating region with respect to B, squeezing and polarization parameters where the qubit can be operated to obtain 'clean' Rabi flips. On and close to the boundaries of the region, the higher orbital levels strongly interfere with the two-level qubit subspace and destroy the Rabi oscillations, thereby putting a limitation on squeezing of the confinement. The Rabi frequency shows different behaviour for electron-like and hole-like Rashba SOIs. It vanishes for right (left) circular polarization in presence of purely electron-like (hole-like) Rashba SOI in a circular confinement. For both in- and out-of-plane magnetic fields, higher Rabi frequencies are achieved for squeezed configurations when the ellipses of polarization and the confinement equipotential have their major axes aligned but with different eccentricities. We also deduce a simple formula to calculate the effective heavy hole mass by measuring the Rabi frequencies using this setup.

cond-mat.mes-hall↗

arXiv2025

Friedel oscillations and chiral superconductivity in monolayer NbSe$_2$

In 1965 Kohn and Luttinger proposed a genuine electronic mechanism for superconductivity. Despite the bare electrostatic interaction between two electrons being repulsive, in a metal electron-hole fluctuations can give rise to Friedel oscillations of the screened Coulomb potential. Cooper pairing among the electrons then emerges when taking advantage of the attractive regions. The nature of the leading pairing mechanism in some two-dimensional transition metal dichalcogenides is still debated. Focusing on NbSe$_2$, we show that superconductivity can be induced by the Coulomb interaction when accounting for screening effects on the trigonal lattice with multiple orbitals. Using ab initio-based tight-binding parametrizations for the relevant low-energy d-bands, we evaluate the screened interaction microscopically, in a scheme including Bloch overlaps. In the direct space, we find long-range Friedel oscillations alternating in sign, a key to the Kohn-Luttinger mechanism. The momentum-resolved gap equations predict two degenerate solutions at the critical temperature Tc, signaling the unconventional nature of the pairing. Their complex linear combination, i.e., a chiral gap with p-like symmetry, provides the ground state of the system. Our prediction of a fully gapped chiral phase well below Tc is in excellent agreement with the spectral function extracted from tunneling spectroscopy measurements of single-layer NbSe$_2$.

cond-mat.supr-con↗

arXiv2024

Hyperplane-Symmetric Static Einstein-Dirac Spacetime

We derive the general solution to the coupled Einstein and Dirac field equations in static and hyperplane-symmetric spacetime of arbitrary dimension including a cosmological constant of either sign. As a result, only a massful Dirac field couples via the Einstein equations to spacetime, and in the massless case the Dirac field is required to fulfill appropriate constraints in order to eliminate off-diagonal components of the energy-momentum tensor. We also give explicit expressions for curvature invariants including the Ricci scalar and the Kretschmann scalar, indicating physical singularities. Moreover, we reduce the general solution of the geodesic equation to quadratures.

gr-qc↗

arXiv2024

Photodriven germanium hole qubit

Hole qubits in germanium quantum dots are promising candidates for coherent control and manipulation of the spin degree of freedom through electric dipole spin resonance. We theoretically study the time dynamics of a single heavy-hole qubit in a laser-driven planar germanium quantum dot confined laterally by a harmonic potential in presence of linear and cubic Rashba spin-orbit couplings and an out-of-plane magnetic field. We obtain an approximate analytical formula of the Rabi frequency using a Schrieffer-Wolff transformation and establish a connection of our model with the ESDR results obtained for this system. For stronger beams, we employ different methods such as unitary transformation and Floquet theory to study the time evolution numerically. We observe that high radiation intensity is not suitable for the qubit rotation due to the presence of high frequency noise superimposed on the Rabi oscillations. We display the Floquet spectrum and highlight the quasienergy levels responsible for the Rabi oscillations in the Floquet picture. We study the interplay of both the types of Rashba couplings and show that the Rabi oscillations, which are brought about by the linear Rashba coupling, vanish for typical values of the cubic Rashba coupling in this system.

cond-mat.mes-hall↗

arXiv2023

Imperfect Many-Body Localization in Exchange-Disordered Isotropic Spin Chains

We investigate many-body localization in isotropic Heisenberg spin chains with the local exchange parameters being subject to quenched disorder. Such systems incorporate a nonabelian symmetry in their Hamiltonian by an invariance under global SU(2)-rotations. Nonabelian symmetries are predicted to hinder the emergence of a many-body localized phase even in presence of strong disorder. We report on numerical studies using exact diagonalization for chains of common spin length 1/2 and 1. The averaged consecutive-gap ratios display a transition compatible with a crossover from an ergodic phase at small disorder strength to an incompletely localized phase at stronger disorder. The sample-to-sample variance of the averaged consecutive-gap ratio displays a maximum at the transition and distinguishes the incompletely localized phase from a fully many-body localized phase by its scaling behavior.

cond-mat.dis-nn↗

arXiv2022

Topological transitions in two-dimensional Floquet superconductors

We demonstrate the occurrence of a topological phase transition induced by an effective magnetic field in a two-dimensional electron gas with spin-orbit coupling and in proximity to an $s$-wave superconductor. The effective, perpendicular magnetic field is generated by an in plane, off-resonant ac-magnetic field or by circularly polarized light. The conditions for entering the topological phase do not rely on fine parameter tuning: For fixed frequency, one requires a minimal amplitude of the effective field which can be evaluated analytically. In this phase, chiral edge states generally emerge for a system in stripe geometry unless the Rashba and Dresselhaus coupling have the same magnitude. In this special case, for magnetic field driving the edge states become Majorana flat bands, due to the presence of a chiral symmetry; the light irradiated system is a trivial superconductor.

cond-mat.mes-hall↗

arXiv2020

Many-Body Localization: Transitions in Spin Models

We study the transitions between ergodic and many-body localized phases in spin systems, subject to quenched disorder, including the Heisenberg chain and the central spin model. In both cases systems with common spin lengths $1/2$ and $1$ are investigated via exact numerical diagonalization and random matrix techniques. Particular attention is paid to the sample-to-sample variance $(Δ_sr)^2$ of the averaged consecutive-gap ratio $\langle r\rangle$ for different disorder realizations. For both types of systems and spin lengths we find a maximum in $Δ_sr$ as a function of disorder strength, accompanied by an inflection point of $\langle r\rangle$, signaling the transition from ergodicity to many-body localization. The critical disorder strength is found to be somewhat smaller than the values reported in the recent literature. Further information about the transitions can be gained from the probability distribution of expectation values within a given disorder realization.

cond-mat.dis-nn↗

arXiv2020

Persistent spin textures and currents in wurtzite nanowire-based quantum structures

We explore the spin and charge properties of electrons in wurtzite semiconductor nanowires where radial and axial confinement leads to tubular or ring-shaped quantum structures. Accounting for spin-orbit interaction induced by the wurtzite lattice as well as a radial potential gradient, we analytically derive the corresponding low-dimensional Hamiltonians. It is demonstrated that the resulting tubular spin-orbit Hamiltonian allows to construct spin states that are persistent in time and robust against disorder. We find that these special scenarios are characterized by distinctive features in the optical conductivity spectrum, which enable an unambiguous experimental verification. In both types of quantum structures, we discuss the dependence of the occurring persistent charge and spin currents on an axial magnetic field and Fermi energy which show clear fingerprints of the electronic subband structure. Here, the spin-preserving symmetries become manifest in the vanishing of certain spin current tensor components. Our analytic description relates the distinctive features of the optical conductivity and persistent currents to bandstructure characteristics which allows to deduce spin-orbit coefficients and other band parameters from measurements.

cond-mat.mes-hall↗

arXiv2019

Floquet-Drude conductivity

This letter presents a generalization of the Drude conductivity for systems which are exposed to periodic driving. The probe bias is treated perturbatively by using the Kubo formula, whereas the external driving is included non-perturbatively using the Floquet theory. Using a new type of four-times Green's functions disorder is approached diagrammatically, yielding a fully analytical expression for the Floquet-Drude conductivity. Furthermore, the Floquet Fermi's golden rule is generalized to $tt'$-Floquet states, connecting the Floquet-Dyson series with scattering theory for Floquet states. Our formalism allows for a direct application to numerous systems e.g. graphene or spin-orbit systems.

cond-mat.mes-hall↗

arXiv2019

Holographic signatures of resolved cosmological singularities

The classical gravity approximation is often employed in AdS/CFT to study the dual field theory, as it allows for many computations. A drawback is however the generic presence of singularities in classical gravity, which limits the applicability of AdS/CFT to regimes where the singularities are avoided by bulk probes, or some other form of regularisation is applicable. At the same time, quantum gravity is expected to resolve those singularities and thus to extend the range of applicability of AdS/CFT also in classically singular regimes. This paper exemplifies such a computation. We use an effective quantum corrected Kasner-AdS metric inspired by results from non-perturbative canonical quantum gravity to compute the 2-point correlator in the geodesic approximation for a negative Kasner exponent. The correlator derived in the classical gravity approximation has previously been shown to contain a pole at finite distance as a signature of the singularity. Using the quantum corrected metric, we show explicitly how the pole is resolved and that a new subdominant long-distance contribution to the correlator emerges, caused by geodesics passing arbitrarily close to the resolved classical singularity. In order to compute analytically in this paper, two key simplifications in the quantum corrected metric are necessary. They are lifted in a companion paper using numerical techniques, leading to the same qualitative results.

hep-th↗

arXiv2019

Driven Hofstadter Butterflies and Related Topological Invariants

The properties of the Hofstadter butterfly, a fractal, self similar spectrum of a two dimensional electron gas, are studied in the case where the system is additionally illuminated with monochromatic light. This is accomplished by applying Floquet theory to a tight binding model on the honeycomb lattice subjected to a perpendicular magnetic field and either linearly or circularly polarized light. It is shown how the deformation of the fractal structure of the spectrum depends on intensity and polarization. Thereby, the topological properties of the Hofstadter butterfly in presence of the oscillating electric field are investigated. A thorough numerical analysis of not only the Chern numbers but also the $W_{3}$-invariants gives the appropriate insight into the topology of this driven system. This includes a comparison of a direct $W_3$-calculation to the method based on summing up Chern numbers of the truncated Floquet Hamiltonian.

cond-mat.mes-hall↗

arXiv2019

Photoinduced polarization enhancement in biased bilayer graphene in the Landau level regime

We investigate the charge carrier dynamics in bilayer graphene subject to monochromatic laser irradiation within the Landau level quantization regime. Even though the radiation field does not lift the energy degeneracy of the lowest Landau levels (LLs), it nevertheless has a strong effect on the photoinduced pseudospin polarization response for higher LLs ($n\ge2$). Our results show that the photoinduced bandgaps lead to a finite response of the averaged pseudospin polarization with nontrivial oscillating behavior. It is shown that the contribution from these higher LL transitions turns out to be crucial to achieve an enhanced photoinduced polarization in radiated bilayer graphene. The experimental feasibility of our findings is also discussed.

cond-mat.mes-hall↗

arXiv2019

Ultralong spin lifetimes in one-dimensional semiconductor nanowires

We experimentally demonstrate ultralong spin lifetimes of electrons in the one-dimensional (1D) quantum limit of semiconductor nanowires. Optically probing single wires of different diameters reveals an increase in the spin relaxation time by orders of magnitude as the electrons become increasingly confined until only a single 1D subband is populated. We find the observed spin lifetimes of more than $200\,\textrm{ns}$ to result from the robustness of 1D electrons against major spin relaxation mechanisms, highlighting the promising potential of these wires for long-range transport of coherent spin information.

cond-mat.mes-hall↗

arXiv2018

Spin relaxation in wurtzite nanowires

We theoretically investigate the D'yakonov-Perel' spin relaxation properties in diffusive wurtzite semiconductor nanowires and their impact on the quantum correction to the conductivity. Although the lifetime of the long-lived spin states is limited by the dominant $k$-linear spin-orbit contributions in the bulk, these terms show almost no effect in the finite-size nanowires. Here, the spin lifetime is essentially determined by the small $k$-cubic spin-orbit terms and nearly independent of the wire radius. At the same time, these states possess in general a complex helical structure in real space that is modulated by the spin precession length induced by the $k$-linear terms. For this reason, the experimentally detected spin relaxation largely depends on the ratio between the nanowire radius and the spin precession length as well as the type of measurement. In particular, it is shown that while a variation of the radius hardly affects the magnetoconductance correction, which is governed by the long-lived spin states, the change in the spin lifetime observed in optical experiments can be dramatic. We compare our results with recent experimental studies on wurtzite InAs nanowires.

cond-mat.mes-hall↗

arXiv2018

On the canonical structure of general relativity with a limiting curvature and its relation to loop quantum gravity

Chamseddine and Mukhanov recently proposed a modified version of general relativity that implements the idea of a limiting curvature. In the spatially flat, homogeneous, and isotropic sector, their theory turns out to agree with the effective dynamics of the simplest version of loop quantum gravity if one identifies their limiting curvature with a multiple of the Planck curvature. At the same time, it extends to full general relativity without any symmetry assumptions and thus provides an ideal toy model for full loop quantum gravity in the form of a generally covariant effective action known to all orders. In this paper, we study the canonical structure of this theory and point out some interesting lessons for loop quantum gravity. We also highlight in detail how the two theories are connected in the spatially flat, homogeneous, and isotropic sector.

gr-qc↗