arXiv subjects

Ronny Thomale

Publications and source records attributed to Ronny Thomale.

At least 91 records · Page 5Linked to original sources

arXiv2024

Theory of Eigenstate Thermalisation

If we prepare an isolated, interacting quantum system in an eigenstate and perturb a local observable at an initial time, its expectation value will relax towards a thermal expectation value, even though the time evolution of the system is deterministic. The eigenstate thermalization hypothesis (ETH) of Deutsch and Srednicki suggests that this is possible because each eigenstate of the full quantum system acts as a thermal bath to its subsystems, such that the reduced density matrices of the subsystems resemble thermal density matrices. Here, we use the observation that the eigenvalue distribution of interacting quantum systems is a Gaussian under very general circumstances, and Dyson Brownian motion random matrix theory, to derive the ETH and thereby elevate it from hypothesis to theory. Our analysis provides a derivation of statistical mechanics which neither requires the concepts of ergodicity or typicality, nor that of entropy. Thermodynamic equilibrium follows solely from the applicability of quantum mechanics to large systems and the absence of integrability.

quant-ph↗

arXiv2024

Candidate quantum spin liquids on the maple-leaf lattice

Motivated by recent numerical studies reporting putative quantum paramagnetic behavior in spin-$1/2$ Heisenberg models on the maple-leaf lattice, we classify Abrikosov fermion mean-field Ansätze of fully symmetric $U(1)$ and $\mathbb{Z}_{2}$ quantum spin liquids within the framework of projective symmetry groups. We obtain a total of $17$ $U(1)$ and $12$ $\mathbb{Z}_{2}$ algebraic PSGs, and, upon restricting their realization via mean-field Ansätze with nearest-neighbor amplitudes (relevant to the studied models), only 12 $U(1)$ and 8 $\mathbb{Z}_{2}$ distinct phases are obtained. We present both singlet and triplet fields for all Ansätze up to third nearest-neighbor bonds and discuss their spinon dispersions as well as their dynamical spin structure factors. We further assess the effects of Gutzwiller projection on the equal-time spin structure factors, and identify a $U(1)$ Fermi surface spin liquid whose structure factor most closely reproduces the one obtained from pseudo-fermion functional renormalization group calculations.

cond-mat.str-el↗

arXiv2024

Dynamic Phase Enabled Topological Mode Steering in Composite Su-Schrieffer-Heeger Waveguide Arrays

Topological boundary states localize at interfaces whenever the interface implies a change of the associated topological invariant encoded in the geometric phase. The generically present dynamic phase, however, which is energy and time dependent, has been known to be non-universal, and hence not to intertwine with any topological geometric phase. Using the example of topological zero modes in composite Su-Schrieffer-Heeger (c-SSH) waveguide arrays with a central defect, we report on the selective excitation and transition of topological boundary mode based on dynamic phase-steered interferences. Our work thus provides a new knob for the control and manipulation of topological states in composite photonic devices, indicating promising applications where topological modes and their bandwidth can be jointly controlled by the dynamic phase, geometric phase, and wavelength in on-chip topological devices.

physics.optics↗

arXiv2024

Strain-induced enhancement of the charge-density-wave in the kagome metal ScV$_6$Sn$_6$

The kagome geometry is an example of frustrated configuration in which rich physics takes place, including the emergence of superconductivity and charge density wave (CDW). Among the kagome metals, ScV$_6$Sn$_6$ hosts an unconventional CDW, with its electronic order showing a different periodicity than that of the phonon which generates it. In this material, a CDW-softened flat phonon band has a second-order collapse at the same time that the first order transition occurs. This phonon band originates from the out-of-plane vibrations of the Sc and Sn atoms, and it is at the base of the electron-phonon-coupling driven CDW phase of ScV$_6$Sn$_6$. Here, we use uniaxial strain to tune the frequency of the flat phonon band, tracking the strain evolution via time-resolved optical spectroscopy and first-principles calculations. Our findings emphasize the capability to induce an enhancement of the unconventional CDW properties in ScV$_6$Sn$_6$ kagome metal through control of strain.

cond-mat.str-el↗

arXiv2024

Circuit realisation of a two-orbital non-Hermitian tight-binding chain

We examine a non-Hermitian (NH) tight-binding system comprising of two orbitals per unit cell and their electrical circuit analogues. We distinguish the PT-symmetric and non-PT symmetric cases characterised by non-reciprocal nearest neighbour couplings and onsite gain/loss terms, respectively. The localisation of the edge modes or the emergence of the topological properties are determined via the maximum inverse participation ratio, which has distinct dependencies on the parameters that define the Hamiltonian. None of the above scenarios exhibits the non-Hermitian skin effect. We investigate the boundary modes corresponding to the topological phases in a suitably designed electrical circuit by analyzing the two-port impedance and retrieve the admittance band structure of the circuit via imposing periodic boundary conditions. The obtained results are benchmarked against the Hermitian version of the two-orbital model to compare and discriminate against those obtained for the NH variants.

cond-mat.mes-hall↗

arXiv2024

The kagome Hubbard model from a functional renormalization group perspective

The recent discovery of a variety of intricate electronic order in kagome metals has sprouted significant theoretical and experimental interest. From an electronic perspective on the potential microscopic origin of these phases, the most basic model is given by a Hubbard model on the kagome lattice. We employ functional renormalization group (FRG) to analyze the kagome Hubbard model. Through our methodological refinement of FRG both within its N-patch and truncated unity formulation, we resolve previous discrepancies of different FRG approaches (Wang et al., 2013 vs. Kiesel et al., 2013), and analyze both the pure ($p$-type) and mixed ($m$-type) van Hove fillings of the kagome lattice. We further study the RG flow into symmetry broken phases to identify the energetically preferred linear combination of the respective order parameter without any need for additional mean field analysis. Our findings suggest some consistency with recent experiments, and underline the richness of electronic phases already found in the kagome Hubbard model. We also provide a no-go theorem for a complex charge bond ordered phase in the single orbital kagome Hubbard model, suggesting that this model cannot capture aspects of orbital current phases.

cond-mat.str-el↗

arXiv2024

Two-dimensional Shiba lattices as possible platform for crystalline topological superconductivity

Localized or propagating Majorana boundary modes are the key feature of topological superconductors. They are rare in naturally-occurring compounds, but the tailored manipulation of quantum matter offers opportunities for their realization. Specifically, lattices of Yu-Shiba-Rusinov bound states $-$ Shiba lattices $-$ that arise when magnetic adatoms are placed on the surface of a conventional superconductor can be used to create topological bands within the superconducting gap of the substrate. Here, using scanning tunnelling microscopy to create and probe adatom lattices with single atom precision we reveal two signatures consistent with the realization of two types of mirror symmetry protected topological superconductors. The first has edge modes as well as higher-order corner states, and the second has symmetry-protected bulk nodal points. In principle, their topological character and boundary modes should be protected by the spatial symmetries of the adatom lattice. Our results highlight the potential of Shiba lattices as a platform to design the topology and sample geometry of 2D superconductors.

cond-mat.mes-hall↗

arXiv2024

Phase diagram of the $J$-$J_d$ Heisenberg Model on the Maple-Leaf Lattice: Neural networks and density matrix renormalization group

We microscopically analyze the nearest neighbor Heisenberg model on the maple-leaf lattice through neural quantum states (NQS) and infinite density matrix renormalization group (iDMRG). Embarking to parameter regimes beyond the exact dimer singlet ground state with a dimer bond spin exchange coupling $J_d$ varied against the exchange strength $J$ of all other bonds, iDMRG (NQS) finds a dimer state paramagnetic phase for $J_d/J > 1.464$ ($J_d/J > 1.39$) and a canted $120^\circ$ magnetic order for $J_d/J < 1.419$ ($J_d/J < 1.23$). Assessing training convergence inaccuracies of NQS and the influence of finite cylindric circumference for iDMRG, we discuss the possible existence of an intermediate phase between magnet and dimer paramagnet.

cond-mat.str-el↗

arXiv2023

$\require{mhchem}$Quantum paramagnetism in the decorated square-kagome antiferromagnet $\ce{Na6Cu7BiO4(PO4)4Cl3}$

$\require{mhchem}$The square-kagome lattice Heisenberg antiferromagnet is a highly frustrated Hamiltonian whose material realizations have been scarce. We theoretically investigate the recently synthesized $\ce{Na6Cu7BiO4(PO4)4Cl3}$ where a Cu$^{2+}$ spin-$1/2$ square-kagome lattice (with six site unit cell) is decorated by a seventh magnetic site alternatingly above and below the layers. The material does not show any sign of long-range magnetic order down to 50 mK despite a Curie-Weiss temperature of $-212$ K indicating a quantum paramagnetic phase. Our DFT energy mapping elicits a purely antiferromagnetic Hamiltonian that features longer range exchange interactions beyond the pure square-kagome model and, importantly, we find the seventh site to be strongly coupled to the plane. We combine two variational Monte Carlo approaches, pseudo-fermion/Majorana functional renormalization group and Schwinger-Boson mean field calculations to show that the complex Hamiltonian of $\ce{Na6Cu7BiO4(PO4)4Cl3}$ still features a nonmagnetic ground state. We explain how the seventh Cu$^{2+}$ site actually aids the stabilization of the disordered state. We predict static and dynamic spin structure factors to guide future neutron scattering experiments.

cond-mat.str-el↗

arXiv2023

Magnetic-coupled electronic landscape in bilayer-distorted titanium-based kagome metals

Quantum materials whose atoms are arranged on a lattice of corner-sharing triangles, $\textit{i.e.}$, the kagome lattice, have recently emerged as a captivating platform for investigating exotic correlated and topological electronic phenomena. Here, we combine ultra-low temperature angle-resolved photoemission spectroscopy (ARPES) with scanning tunneling microscopy and density functional theory calculations to reveal the fascinating electronic structure of the bilayer-distorted kagome material $\textit{Ln}$Ti${_3}$Bi${_4}$, where $\textit{Ln}$ stands for Nd and Yb. Distinct from other kagome materials, $\textit{Ln}$Ti${_3}$Bi${_4}$ exhibits two-fold, rather than six-fold, symmetries, stemming from the distorted kagome lattice, which leads to a unique electronic structure. Combining experiment and theory we map out the electronic structure and discover double flat bands as well as multiple van Hove singularities (VHSs), with one VHS exhibiting higher-order characteristics near the Fermi level. Notably, in the magnetic version NdTi${_3}$Bi${_4}$, the ultra-low base temperature ARPES measurements unveil an unconventional band splitting in the band dispersions which is induced by the ferromagnetic ordering. These findings reveal the potential of bilayer-distorted kagome metals $\textit{Ln}$Ti${_3}$Bi${_4}$ as a promising platform for exploring novel emergent phases of matter at the intersection of strong correlation and magnetism.

cond-mat.mtrl-sci↗

arXiv2023

Maple Leaf Antiferromagnet in a Magnetic Field

We analyze the quantum antiferromagnet on the maple leaf lattice in the presence of a magnetic field. Starting from its exact dimer ground state and for a magnetic field strength of the order of the local dimer spin exchange coupling, we perform a strong coupling expansion and extract an effective hardcore boson model. The interplay of effective many-body interactions, suppressed single-particle dynamics, and correlated hopping gives way to an intriguing series of superfluid to insulator transitions which correspond to magnetization plateaux in terms of the maple leaf spin degrees of freedom. While we find plateaux at intermediate magnetization to be dominated by bosonic density wave order, we conjecture plateau formation from multi-boson bound states due to correlated hopping for lower magnetization.

cond-mat.str-el↗

arXiv2023

Symmetry induced selective excitation of topological states in SSH waveguide arrays

The investigation of topological state transition in carefully designed photonic lattices is of high interest for fundamental research, as well as for applied studies such as manipulating light flow in on-chip photonic systems. Here, we report on topological phase transition between symmetric topological zero modes (TZM) and antisymmetric TZMs in Su-Schrieffer-Heeger (SSH) mirror symmetric waveguides. The transition of TZMs is realized by adjusting the coupling ratio between neighboring waveguide pairs, which is enabled by selective modulation of the refractive index in the waveguide gaps. Bi-directional topological transitions between symmetric and antisymmetric TZMs can be achieved with our proposed switching strategy. Selective excitation of topological edge mode is demonstrated owing to the symmetry characteristics of the TZMs. The flexible manipulation of topological states is promising for on-chip light flow control and may spark further investigations on symmetric/antisymmetric TZM transitions in other photonic topological frameworks.

physics.optics↗

arXiv2023

Hyperbolic fringe signal for twin impurity quasiparticle interference

We study the quasiparticle interference (QPI) pattern emanating from a pair of adjacent impurities on the surface of a gapped superconductor (SC). We find that hyperbolic fringes (HF) in the QPI signal can appear due to the loop contribution of the two-impurity scattering, where the location of the two impurities are the hyperbolic focus points. For a single pocket Fermiology, an HF pattern signals chiral SC order for non-magnetic impurities and requires magnetic impurities for non-chiral SC. For a multi-pocket scenario, a sign-changing order parameter such as $s_{\pm}$-wave likewise yields an HF signature. We discuss twin impurity QPI as a new tool to complement the analysis of superconducting order from local spectroscopy.

cond-mat.supr-con↗

arXiv2023

Sublattice Interference promotes Pair Density Wave order in Kagome Metals

Motivated by the observation of a pair density wave (PDW) in the kagome metal CsV${}_3$Sb${}_5$, we consider the fate of electrons near a p-type van Hove singularity (vHS) in the presence of local repulsive interactions. We study the effect of such interactions on Fermi surface "patches" at the vHS. We show how a feature unique to the Kagome lattice known as sublattice interference crucially affects the form of the interactions among the patches. The renormalization group (RG) flow of such interactions results in a regime where the nearest neighbor interaction $V$ exceed the onsite repulsion $U$. We identify this condition as being favorable for the formation of charge-density-wave (CDW) and PDW orders. In the weak coupling limit, we find a complex CDW order as the leading instability, which breaks time reversal symmetry. Beyond RG, we perform a Hartree-Fock study to a $V$-only model and find the pair-density-wave order indeed sets in at some intermediate coupling.

cond-mat.str-el↗

arXiv2023

Anomalous fractal scaling in two-dimensional electric networks

Much of the qualitative nature of physical systems can be predicted from the way it scales with system size. Contrary to the continuum expectation, we observe a profound deviation from logarithmic scaling in the impedance of a two-dimensional $LC$ circuit network. We find this anomalous impedance contribution to sensitively depend on the number of nodes $N$ in a curious erratic manner, and experimentally demonstrate its robustness against perturbations from the contact and parasitic impedance of individual components. This impedance anomaly is traced back to a generalized resonance condition reminiscent of the Harper's equation for electronic lattice transport in a magnetic field, even though our circuit network does not involve magnetic translation symmetry. It exhibits an emergent fractal parametric structure of anomalous impedance peaks for different $N$ that cannot be reconciled with continuum theory and does not correspond to regular waveguide resonant behavior. This anomalous fractal scaling extends to the transport properties of generic systems described by a network Laplacian whenever a resonance frequency scale is simultaneously present.

cond-mat.mes-hall↗

arXiv2023

Realizing efficient topological temporal pumping in electrical circuits

Quantized adiabatic transport can occur when a system is slowly modulated over time. In most realizations however, the efficiency of such transport is reduced by unwanted dissipation, back-scattering, and non-adiabatic effects. In this work, we realize a topological adiabatic pump in an electrical circuit network that supports remarkably stable and long-lasting pumping of a voltage signal. We further characterize the topology of our system by deducing the Chern number from the measured edge band structure. To achieve this, the experimental setup makes use of active circuit elements that act as time-variable voltage-controlled inductors.

cond-mat.other↗

arXiv2023

Interplay of spin and charge order in the electron-doped cuprates

We study magnetic and charge order in the electron-doped high-$T_c$ cuprates based on the one-band Hubbard model with onsite ($U$) and nearest-neighbor $(V)$ interactions. To investigate the interplay between the orders, we employ the Kotliar-Ruckenstein slave-boson method and analyze fluctuations descending from an antiferromagnetic parent state. Our analysis reveals incommensurate charge order whose ordering vector matches the doping-dependence of resonant inelastic x-ray scattering (RIXS) measurements in Nd$_{2-x}$Ce$_x$CuO$_4$ (NCCO). From our calculations of paramagnon dispersion as well as dynamical charge and spin structure factors, we reproduce all qualitative features of the RIXS signal.

cond-mat.str-el↗

arXiv2023

High Resolution Polar Kerr Effect Studies of CsV${}_3$Sb${}_5$: Tests for Time Reversal Symmetry Breaking Below the Charge Order Transition

We report high resolution polar Kerr effect measurements on CsV${}_3$Sb${}_5$ single crystals in search for signatures of spontaneous time reversal symmetry breaking below the charge order transition at $T^* \approx 94$ K. Utilizing two different versions of zero-area loop Sagnac interferometers operating at 1550 nm wavelength, each with the fundamental attribute that without a time reversal symmetry breaking sample at its path, the interferometer is perfectly reciprocal, we find no observable Kerr effect to within the noise floor limit of the apparatus at 30 nanoradians. Simultaneous coherent reflection ratio measurements confirm the sharpness of the charge order transition in the same optical volume as the Kerr measurements. At finite magnetic field we observe a sharp onset of a diamagnetic shift in the Kerr signal at $T^*$, which persists down to the lowest temperature without change in trend. Since 1550 nm is an energy that was shown to capture all features of the optical properties of the material that interact with the charge order transition, we are led to conclude that it is highly unlikely that time reversal symmetry is broken in the charge ordered state in CsV${}_3$Sb${}_5$.

cond-mat.str-el↗