arXiv subjects

Jukka I. Väyrynen

Publications and source records attributed to Jukka I. Väyrynen.

At least 19 recordsLinked to original sources

arXiv2026

Parafermions in polarized $ν=2/3$ Fractional Quantum (Anomalous) Hall Heterostructures

Motivated by recent experimental signatures of fractional Chern insulators in 2D quantum materials, we investigate the realization of $\mathbb Z_3$ parafermion zero modes (PZMs) on edges of spin-polarized $ν=2/3$ fractional quantum Hall states which are proximitized by superconductors. Unlike the spin-unpolarized Halperin state, the polarized $ν=2/3$ edge contains counterpropagating electron and fractional quasihole modes, giving rise to a plethora of competing tunneling and pairing processes. We focus on the vicinity of the fixed point where these modes reorganize into decoupled charge and neutral sectors, with the neutral sector describing an emergent dipole degree of freedom. Using bosonization and renormalization group analysis, we lay out the parameter space in which the interedge dipole interactions gap the neutral sector to stabilize one dominant tunneling and one dominant pairing channel resulting in a double sine-Gordon model in the charge sector which supports $\mathbb{Z}_3$ PZMs at domain walls. We discuss both clean and disordered systems. In clean systems this mechanism requires a certain degree of control of the interaction strength and range. In contrast, disorder can provide a more generic route: While short-ranged, edge-uncorrelated disorder % on the two edges suppresses the simultaneous pinning required for stable parafermion modes, when the disorder on the two edges is sufficiently correlated, as expected for smooth disorder potentials, the parameter regime hosting PZMs increases.

cond-mat.mes-hall↗

arXiv2026

Fractional Impurity Entropy from Potential Scattering in a Free Electron Gas

Fractional changes in entropy are often associated with strong electron correlations or non-Abelian anyons in topological systems. We show that neither condition is necessary -- a system of free electrons on a ring scattering off a repulsive point-like impurity undergoes a $k_B\ln\sqrt 2$ entropy change as the impurity strength is increased. This effect arises from a shift of allowed single-particle momenta from integer quantization to half-integer quantization, induced by the impurity. We show that this fractional entropy change is universal across a broad class of single-particle dispersions, demonstrating that such signatures can arise without strong correlations or topological order.

cond-mat.mes-hall↗

arXiv2026

Signatures of localization on the edge of a fractional topological insulator

Recent experiments in van der Waals moiré materials have reported realizations of two-dimensional fractional topological insulators (FTIs). We study transport properties of FTI edges at filling factors $ν_{\text{FTI}} = 2(1\pm 1/n)$ with integer $n \geq 2$ (even or odd), each of which supports four counter-propagating edge modes. The edge modes can undergo partial localization, leaving two of the four edge modes conducting. We identify three distinct partial-localization channels and, for each channel, determine the reduced theory describing the remaining conducting modes, from which we obtain the minimal quasiparticle charge. We further explore transport properties of each partially localized phase and in particular determine the corresponding edge conductances. We demonstrate that measurements of the conductance together with the minimal quasiparticle charge provide a fingerprint that uniquely distinguishes the three partially localized phases.

cond-mat.mes-hall↗

arXiv2026

Statistical Signatures of Majorana Zero Modes in Disordered Topological Superconductor Antidot Vortices

An antidot-pinned vortex in a three-dimensional topological insulator-superconductor platform hosts a Majorana zero mode (MZM). However, numerous Caroli-de Gennes-Matricon (CdGM) states coexist with it. We develop a general theory to study the effects of disorder on the system, emphasizing the difference between Majorana zero mode and CdGM states. Using both an analytical random matrix theory approach and numerical simulations, we derive the statistical distributions of these states. Our results demonstrate that the variance of the MZM probability density is twice that of the CdGM states, a difference due to the former having a real wave function as opposed to a complex one. This distinction can be measured by using scanning tunneling microscopy in a disordered antidot vortex, providing a signature of MZM beyond the zero-bias conductance peak.

cond-mat.supr-con↗

arXiv2026

Many-Body Entanglement Properties of Finite Interacting Fermionic Hamiltonians

We analyze many-body entanglement in interacting fermionic systems by using the $M$-body reduced density matrix. We demonstrate that if a particle number conserving fermionic Hamiltonian contains only up to $M$-body interaction terms, then its $N$-particle ground state cannot be maximally $M$-body entangled. As a key step in the proof, we show that the energy expectation value of a maximally $M$-body mixed state is equal to the spectral mean of the Hamiltonian on the corresponding $N$-particle subspace. We further demonstrate that the many-body entanglement structure of a ground state can place quantitative constraint on the interaction strength of its parent Hamiltonian. We illustrate the theorem and its implications in Hubbard and extended SYK models. Going beyond ground states, we analyze entanglement generation under unitary dynamics from Slater-determinant initial states in these models. We determine early-time growth and estimate entanglement saturation times. Finally, we derive explicit symmetry-refined saturation upper bounds for $M$-body entanglement in the presence of an Abelian symmetry.

quant-ph↗

arXiv2026

Quantum statistics and self-interference in extended colliders

Collision of quantum particles remains an effective way of probing their mutual statistics. Colliders based on quantum point contacts in quantum Hall edge states have been successfully used to probe the statistics of the underlying quantum particles. Notwithstanding the extensive theoretical work focusing on point-like colliders, when it comes to experiment, the colliders are rarely point-like objects and can support a resonant level or multiple tunneling points. We present a study of a paradigmatic extended (non-point-like) fermionic collider (and an extension to bosonic colliders). As with particle interferometers, in an extended collider there is an infinite number of trajectories for any single or multi-particle event. Self-interference of the former can lead to an apparent bunching of fermions when we compare the cross-current correlator with a classical benchmark representing two colliding beams. In view of this apparent bunching behavior of fermions, we identify an experimentally accessible current correlator which reveals the true mutual statistics of fermions.

cond-mat.mes-hall↗

arXiv2025

Wiedemann-Franz violation and thermal Hall effect in kagome metal TbCr6Ge6

The thermal Hall effect has emerged as a powerful probe of exotic excitations in correlated quantum materials, providing access to charge-neutral heat carriers that remain invisible to electrical transport. To directly examine how heat and charge respond in relation within a kagome metal, we investigate the ferrimagnetic rare-earth 1-6-6 compound TbCr6Ge6 using the Wiedemann-Franz (WF) framework. We observe a dramatic breakdown of the WF law across the ferrimagnetic transition, where both longitudinal and transverse Lorenz ratios, L_{xx,xy} = κ_{xx,xy} / (T σ_{xx,xy}), deviate strongly from the Sommerfeld value L_0. After a partial recovery toward L_0 near 5-7 K, the Lorenz ratios are sharply suppressed well below L_0 despite a metallic charge response. We further find a pronounced low-temperature suppression of both L_{xx} and L_{xy} and a sign-changing transverse Lorenz ratio, indicating a clear decoupling between heat and charge transport and signaling substantial contributions from charge-neutral excitations whose Berry-curvature-driven transverse response evolves with temperature and magnetic field. TbCr6Ge6 thus provides a tunable metallic platform in which exchange-driven ferrimagnetism governs both longitudinal and transverse thermal responses, enabling controlled departures from Wiedemann-Franz behavior over an experimentally accessible temperature and field range.

cond-mat.str-el↗

arXiv2025

High efficiency superconducting diode effect in a gate-tunable double-loop SQUID

In superconducting quantum interference devices (SQUIDs), the superconducting diode effect may be generated by interference of multiple harmonic components in the current-phase relationships (CPRs) of different branches forming SQUID loops. Through the inclusion of two gate-tunable Josephson junctions in series in each interference branch of a double-loop SQUID, we demonstrate independent control over both the harmonic content and the amplitude of three interfering CPRs, facilitating significant improvement in the maximum diode efficiency. Through optimized gate-controlled tuning of individual Josephson energies, diode efficiency exceeding 50% is demonstrated. Flux-dependent oscillations show quantitative agreement with a simple model of SQUID operation.

cond-mat.supr-con↗

arXiv2025

Native-oxide-passivated trilayer junctions for superconducting qubits

Superconducting qubits in today's quantum processing units are typically fabricated with angle-evaporated aluminum--aluminum-oxide--aluminum Josephson junctions. However, there is an urgent need to overcome the limited reproducibility of this approach when scaling up the number of qubits and junctions. Fabrication methods based on subtractive patterning of superconductor--insulator--superconductor trilayers, used for more classical large-scale Josephson junction circuits, could provide the solution but they in turn often suffer from lossy dielectrics incompatible with high qubit coherence. In this work, we utilize native aluminum oxide as a sidewall passivation layer for junctions based on aluminum--aluminum-oxide--niobium trilayers, and use such junctions in qubits. We design the fabrication process such that the few-nanometer-thin native oxide is not exposed to oxide removal steps that could increase its defect density or hinder its ability to prevent shorting between the leads of the junction. With these junctions, we design and fabricate transmon-like qubits and measure time-averaged coherence times up to 30 $μ$s at a qubit frequency of 5 GHz, corresponding to a qubit quality factor of one million. Our process uses subtractive patterning and optical lithography on wafer scale, enabling high throughput in patterning. This approach provides a scalable path toward fabrication of superconducting qubits on industry-standard platforms.

quant-ph↗

arXiv2025

Phases of Quasi-One-Dimensional Fractional Quantum (Anomalous) Hall - Superconductor Heterostructures

Motivated by recent observations of fractional Chern insulators (FCIs) in the vicinity of superconducting (SC) phases, we study fractional quantum (anomalous) Hall-superconductor heterostructures in the presence of $U(1)$ order-parameter fluctuations and particularly focus on the case of $ν= 2/3$ quantum Hall states leading to $\mathbb Z_3$ parafermions. We first employ a phenomenological field theory to qualitatively determine the phase diagram. Furthermore, we generalize a previously established alternating pattern of superconductor and tunneling regions, coupled to fractional quantum Hall edge states, to map the problem onto a topological Josephson junction chain involving lattice parafermions. Using density matrix renormalization group simulations, we establish a phase diagram composed of Mott insulating phases and two different Luttinger liquids whose fundamental excitations carry charges 2e and $2e/3$, respectively. In agreement with analytical considerations using conformal field theory, we numerically find transitions of Berezinskii-Kosterlitz-Thouless (BKT) type as well as a continuous $\mathbb Z_3 \times U(1)$ second-order phase transition characterized by central charge c = 9/5. We finally extract information about a possible ground state degeneracy and comment on the stability of parafermionic edge states in the presence of fluctuations. These theoretical foundations can be expected to be of practical importance for gate-defined FCI-SC heterostructures in moiré materials, in which broad superconducting transitions indicative of strong order parameter fluctuations were observed.

cond-mat.str-el↗

arXiv2025

Multichannel topological Kondo models and their low-temperature conductances

In the multichannel Kondo effect, overscreening of a magnetic impurity by conduction electrons leads to a frustrated exotic ground state. It has been proposed that multichannel topological Kondo (MCTK) model involving topological Cooper pair boxes with $M$ Majorana modes [SO($M$) "spin"] and $N$ spinless electron channels exhibits an exotic intermediate coupling fixed point. This intermediate fixed point has been analyzed through large-$N$ perturbative calculations, which gives a zero-temperature conductance decaying as $1/N^2$ in the large-$N$ limit. However, the conductance at this intermediate fixed point has not been calculated for generic $N$. Using representation theory, we verify the existence of this intermediate-coupling fixed point and find the strong-coupling effective Hamiltonian for the case $M=4$. Using conformal field theory techniques for SO($M$), we generalize the notion of overscreening and conclude that the MCTK model is an overscreened Kondo model. We find the fixed-point finite-size energy spectrum and the leading irrelevant operator (LIO). We express the fixed-point conductance in terms of the modular S-matrix of SO($M$) for general $N$, confirming the previous large-$N$ result. We describe the finite-temperature corrections to the conductance by the LIO and find that they are qualitatively different for the cases $N=1$ and $N\geq2$ due to the different fusion outcomes with the current operator. We also compare the multichannel topological Kondo model to the topological symplectic Kondo model.

cond-mat.str-el↗

arXiv2025

Design and benchmarks for emulating Kondo dynamics on a quantum chip

Motivated by recent advances in digital quantum simulation and the overall prospective of solving correlated many-electron problems using quantum algorithms, we design a gate-based quantum circuit that emulates the dynamics of the Kondo impurity model. We numerically determine the impurity magnetization, entanglement between impurity and fermionic sites and energy as a function of time (i.e.~circuit depth) for various initial states and find universal long-time dynamics. We complement the numerical simulations for moderate system size with an asymptotically exact analytical solution that is effective in the limit of large system sizes and for starting states corresponding to a filled Fermi sea. This work opens up the perspective of studying the dynamics of electronic quantum many-body states on quantum chips of the NISQ era.

cond-mat.str-el↗

arXiv2025

Characterizing maximally many-body entangled fermionic states by using $M$-body density matrix

Fermionic Hamiltonians play a critical role in quantum chemistry, one of the most promising use cases for near-term quantum computers. However, since encoding nonlocal fermionic statistics using conventional qubits results in significant computational overhead, fermionic quantum hardware, such as fermion atom arrays, were proposed as a more efficient platform. In this context, we here study the many-body entanglement structure of fermionic $N$-particle states by concentrating on $M$-body reduced density matrices (DMs) across various bipartitions in Fock space. The von Neumann entropy of the reduced DM is a basis independent entanglement measure which generalizes the traditional quantum chemistry concept of the one-particle DM entanglement, which characterizes how a single fermion is entangled with the rest. We carefully examine upper bounds on the $M$-body entanglement, which are analogous to the volume law of conventional entanglement measures. To this end we establish a connection between $M$-body reduced DM and the mathematical structure of hypergraphs. Specifically, we show that a special class of hypergraphs, known as $t$-designs, corresponds to maximally entangled fermionic states. Finally, we explore fermionic many-body entanglement in random states. We semianalytically demonstrate that the distribution of reduced DMs associated with random fermionic states corresponds to the trace-fixed Wishart-Laguerre random matrix ensemble. In the limit of large single-particle dimension $D$ and a non-zero filling fraction, random states asymptotically become absolutely maximally entangled.

quant-ph↗

arXiv2025

Appearance of Multiple Spectral Gaps in Voltage-Biased Josephson Junctions Without Floquet Hybridization

A time-periodic drive enables the engineering of non-equilibrium quantum systems by hybridizing Floquet sidebands. We investigated DC voltage-biased planar Josephson junctions built upon epitaxial Al/InAs heterostructures in which the intrinsic AC Josephson effect is theoretically expected to provide a time-periodic drive leading to Floquet hybridization. Tunneling spectroscopy is performed using probes positioned at the ends of the junction to study the evolution of the local density of states. With applied drive, we observe multiple coherence peaks which are studied as a function of DC voltage bias and in-plane magnetic field. Our analysis suggests that these spectral gaps arise from a direct mesoscopic coupling between the tunneling probe and the superconducting leads rather than from a Floquet-driven gap opening. Our numerical simulations indicate that an increase in the ratio of junction width to coherence length will enhance the contribution of Floquet hybridization. This work lays a foundation for the exploration of Floquet physics utilizing voltage-biased hybrid superconductor-semiconductor Josephson junctions and provides means for distinguishing direct couplings from genuine Floquet effects.

cond-mat.mes-hall↗

arXiv2024

Drag conductance induced by neutral-mode localization in fractional quantum Hall junctions

A junction of two 2/3 fractional quantum Hall (FQH) edges, with no charge tunneling between them, may exhibit Anderson localization of neutral modes. Manifestations of such localization in transport properties of the junction are explored. There are two competing localization channels, ``neutral-mode superconductivity'' and ``neutral-mode backscattering''. Localization in any of these channels leads to an effective theory of the junction that is characteristic for FQH effect of bosons, with a minimal integer excitation charge equal to two, and with elementary quasiparticle charge equal to 2/3. These values can be measured by studying shot noise in tunneling experiments. Under the assumption of ballistic transport in the arms connecting the junction to contacts, the two-terminal conductance of the junction is found to be 4/3 for the former localization channel and 1/3 for the latter. The four-terminal conductance matrix reveals in this regime a strong quantized drag between the edges induced by neutral-mode localization. The two localization channels lead to opposite signs of the drag conductance, equal to $\pm 1/4$, which can also be interpreted as a special type of Andreev scattering. Coherent random tunneling in arms of the device (which are segments of 2/3 edges) leads to strong mesoscopic fluctuations of the conductance matrix. In the case of fully equilibrated arms, transport via the junction is insensitive to neutral-mode localization: The two-terminal conductance is quantized to 2/3 and the drag is absent.

cond-mat.mes-hall↗

arXiv2024

Influence of disorder on antidot vortex Majorana states in 3D topological insulators

Topological insulator/superconductor two-dimensional heterostructures are promising candidates for realizing topological superconductivity and Majorana modes. In these systems, a vortex pinned by a pre-fabricated antidot in the superconductor can host Majorana zero-energy modes (MZMs), which are exotic quasiparticles that may enable quantum information processing. However, a major challenge is to design devices that can manipulate the information encoded in these MZMs. One of the key factors is to create small and clean antidots, so the MZMs, localized in the vortex core, have a large gap to other excitations. If the antidot is too large or too disordered, the level spacing for the subgap vortex states may become smaller than temperature. In this paper, we numerically investigate the effects of disorder, chemical potential, and antidot size on the subgap vortex spectrum, using a two-dimensional effective model of the topological insulator surface. Our model allows us to simulate large system sizes with vortices up to 1.8 $μ$m in diameter (with a 6 nm lattice constant). We also compare our disorder model with the transport data from existing experiments. We find that the spectral gap can exhibit a non-monotonic behavior as a function of disorder strength, and that it can be tuned by applying a gate voltage.

cond-mat.mes-hall↗

arXiv2024

Superconducting Diode Effect in Two-dimensional Topological Insulator Edges and Josephson Junctions

The superconducting diode effect -- the dependence of critical current on its direction -- can arise from the simultaneous breaking of inversion and time-reversal symmetry in a superconductor and has gained interest for its potential applications in superconducting electronics. In this letter, we study the effect in a two-dimensional topological insulator (2D TI) in both a uniform geometry as well as in a long Josephson junction. We show that in the presence of Zeeman fields, a circulating edge current enables a large non-reciprocity of the critical current. We find a maximum diode efficiency 1 for the uniform 2D TI and $(\sqrt{2} - 1)^2 \approx 0.17$ for the long Josephson junction.

cond-mat.supr-con↗

arXiv2024

Topological Kondo effect with spinful Majorana fermions

Motivated by the importance of studying topological superconductors beyond the mean-field approximation, we here investigate mesoscopic islands of time reversal invariant topological superconductors (TRITOPS). We characterize the spectrum in the presence of strong order parameter fluctuations in the presence of an arbitrary number of Kramers pairs of Majorana edge states and study the effect of coupling the Coulomb blockaded island to external leads. In the case of an odd fermionic parity on the island, we derive an unconventional Kondo Hamiltonian in which metallic leads couple to both topological Majorana degrees of freedom (which keep track of the parity in different leads) and the overall spin-1/2 in the island. For the simplest case of a single wire (two pairs of Majorana edge states), we demonstrate that anisotropies are irrelevant in the weak coupling renormalization group flow. This permits us to solve the Kondo problem in the vicinity of a Toulouse-like point using Abelian Bosonization. We demonstrate a residual ground state entropy of $\ln(2)$, which is protected by spin-rotation symmetry, but reduced to $\ln(\sqrt{2})$ (as in the spinless topological Kondo effect) by symmetry breaking perturbations. In the symmetric case, we further demonstrate the simultaneous presence of both Fermi liquid and non-Fermi liquid like thermodynamics (depending on the observable) and derive charge and spin transport signatures of the Coulomb blockaded island.

cond-mat.str-el↗