We present a combined study from angle-resolved photoemission and density-functional-theory calculations of the temperature-dependent electronic structure in the excitonic insulator candidate ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$. Our experimental measurements unambiguously establish the normal state as a semimetal with a significant band overlap of $>100$ meV. Our temperature-dependent measurements indicate how these low-energy states hybridize when cooling through the well-known 327 K phase transition in this system. From our calculations and polarization-dependent photoemission measurements, we demonstrate the importance of a loss of mirror symmetry in enabling the band hybridization, driven by a shearlike structural distortion which reduces the crystal symmetry from orthorhombic to monoclinic. Our results thus point to the key role of the lattice distortion in enabling the phase transition of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$.

Recent theoretical progress has been achieved in demonstrating the existence of non-Abelian parafermion zero modes in domain walls on interfaces between two-dimensional Abelian topological phases of matter. Motivated by this ongoing search for non-Abelian defects, we investigate the properties of gapped interfaces of hierarchical fractional quantum Hall states, in the lowest Landau level, characterized by the Hall conductance ${\sigma }_{xy}(m,p)=\frac{p}{2mp+1}\frac{e^2}{h}$, for integer numbers $(m,p)$ with $m,p\ge 1$. The case $m=1$ corresponds to the experimentally well established sequence of fractional quantum Hall states with ${\sigma }_{xy}=\frac{1}{3}\frac{e^2}{h},\frac{2}{5}\frac{e^2}{h},\frac{3}{7}\frac{e^2}{h},\cdots$, which has been observed in many two-dimensional electron gases. Exploring the mechanism by which the $(m,p+1)$ hierarchical state is generated from the condensation of quasiparticles of the “parent” state $(m,p)$, we uncover a remarkably rich sequence of parafermions in hierarchical interfaces whose quantum dimension $d_{m,p}$ depends both upon the total quantum dimension ${\mathcal{D}}_{m,p}=\sqrt{2mp+1}$ of the bulk Abelian phase, as well as on the parity of the “hierarchy level” $p$, which we associate with the ${\mathbb{Z}}_2$ stability of Majorana zero modes in one-dimensional topological superconductors. We show that these parafermions reside on domain walls separating segments of the interface where the low-energy modes are gapped by two distinct mechanisms: (1) a charge neutral backscattering process or (2) an interaction that breaks $\text{U}\left(1\right)$ charge conservation symmetry and stabilizes a condensate whose charge depends on $p$. Remarkably, this charge condensate corresponds to clustering of quasiparticles of fractional charge $\frac{p}{2mp+1}e$, allowing us to draw a correspondence between these fractionalized condensates and Read-Rezayi non-Abelian fractional quantum Hall cluster states.

Achieving control over magnon spin currents in insulating magnets, where dissipation due to Joule heating is highly suppressed, is an active area of research that could lead to energy-efficient spintronics applications. However, magnon spin currents supported by conventional systems with uniform magnetic order have proven hard to control. An alternative approach that relies on topologically protected magnonic edge states of spatially periodic magnetic textures has recently emerged. A prime example of such textures is the ferromagnetic skyrmion crystal which hosts chiral edge states providing a platform for magnon spin currents. Here we uncover that an external magnetic field can drive a topological phase transition in the spin-wave spectrum of a ferromagnetic skyrmion crystal. The topological phase transition is signaled by the closing of a low-energy bulk magnon gap at a critical field. In the topological phase, below the critical field, two topologically protected chiral magnonic edge states lie within this gap, but they unravel in the trivial phase, above the critical field. Remarkably, the topological phase transition involves an inversion of two magnon bands that at the $\mathrm{\Gamma }$ point correspond to the breathing and counterclockwise modes of the skyrmions in the crystal. Our findings suggest that an external magnetic field could be used as a knob to switch on and off magnon spin currents carried by topologically protected chiral magnonic edge states.

We study superconductivity driven by screened Coulomb repulsion in three-dimensional Luttinger semimetals with a quadratic band touching and strong spin-orbit coupling. In these semimetals, the Cooper pairs are formed by spin-$\frac{3}{2}$ fermions with nontrivial wave functions. We numerically solve the linear Eliashberg equation to obtain the critical temperature of a singlet $s\text{−}\mathrm{wave}$ gap function as a function of doping, with account of spin-orbit and self-energy corrections. In order to understand the underlying mechanism of superconductivity, we compute the sensitivity of the critical temperature to changes in the dielectric function $\epsilon (i{\mathrm{\Omega }}_n,q)$. We relate our results to the plasmon and Kohn-Luttinger mechanisms. Finally, we discuss the validity of our approach and compare our results to the literature. We find good agreement with some bismuth-based half-Heuslers, such as YPtBi, and speculate on superconductivity in the iridate ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$.

We present an algorithm to determine topological invariants of inhomogeneous systems, such as alloys, disordered crystals, or amorphous systems. Based on the kernel polynomial method, our algorithm allows us to study samples with more than ${10}^7$ degrees of freedom. Our method enables the study of large complex compounds, where disorder is inherent to the system. We use it to analyze ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Te}$ and tighten the critical concentration for the phase transition. Moreover, we obtain the topological phase diagram for related alloys in the family of three-dimensional mirror Chern insulators.

We use dirty $d$-wave BCS theory to calculate absolute superfluid density, residual specific heat, Volovik effect, and thermal conductivity and compare to experiments on the cuprate superconductor ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$, showing that the theory provides a surprisingly good account of the data across the overdoped region. The starting point is an empirical angle-resolved photoemission spectroscopy–based parametrization of the electronic structure, including substantial Fermi-liquid renormalizations. Furthermore, a proper treatment of the less-explored weak out-of-plane dopant disorder limit is found to be essential. We then show that the same approach captures the low energy physics of another important overdoped cuprate, Tl-2201, thought to be much “cleaner” since it exhibits quantum oscillations, low residual resistivities, and small superconducting state Sommerfeld coefficients. We conclude that the low energy properties of cuprates are remarkably well described in the overdoped regime by dirty $d$-wave theory, without the need to introduce physics beyond the Landau-BCS paradigm.

Double well potentials offer the possibility of coherent state preparation and therefore constitute important building blocks in the analysis of quantum information and quantum engineering devices. Here we present a study of the coherent tunneling in a parabolic double well potential in the presence of different dissipative interactions. Specifically, we investigate the effects of an environmental coupling to the momentum and/or to the position of a particle in the potential. Using the semiclassical approximation to calculate instanton paths in Euclidean time, we find that momentum dissipation enhances the coherent tunnel splitting. In the presence of both types of dissipation, momentum dissipation shifts the critical coupling strength of the dissipative phase transition induced by the position dissipation.

Semimetals exhibiting nodal lines or nodal surfaces represent a novel class of topological states of matter. While conventional Weyl semimetals exhibit momentum-space Dirac monopoles, these more exotic semimetals can feature unusual topological defects that are analogous to extended monopoles. In this work, we describe a scheme by which nodal rings and nodal spheres can be realized in synthetic quantum matter through well-defined periodic-driving protocols. As a central result of our work, we characterize these nodal defects through the quantum metric, which is a gauge-invariant quantity associated with the geometry of quantum states. In the case of nodal rings, where the Berry curvature and conventional topological responses are absent, we show that the quantum metric provides an observable signature for these extended topological defects. Besides, we demonstrate that quantum-metric measurements could be exploited to directly detect the topological charge associated with a nodal sphere. We discuss possible experimental implementations of Floquet nodal defects in few-level atomic systems, paving the way for the exploration of Floquet-induced extended monopoles in quantum matter.

We compute the nominal magnetic penetration depth of $R{\mathrm{NiO}}_2$ ($R=\mathrm{La}$, Nd) from first-principles calculations and discuss the results in relation to the superconducting $T_c$. We find a marked discrepancy with the well-established phenomenology that correlates these two quantities in cuprates (Uemura plot). We also consider the two-dimensional ultrathin limit and estimate the maximum attainable $T_c$ to be $\sim 180$ K according to the Nelson-Kosterliz universal relation between the superfluid density and the transition temperature.

We explore a mechanism that allows to couple electrons with the transverse-optical (TO) phonon branch in a regime when the TO mode becomes highly anharmonic and drives the ferroelectric phase transition. We show that this anharmonicity, which leads to a collective motion of ions, is able to couple electronic and lattice displacement fields. An effective correlated electron-ion dynamics method is required to capture the effect of the onset of the local electric polarization due to this collective behavior close to the quantum critical point. We identify an intermediate temperature range where an emergent phonon drag may contribute substantially to thermoelectric conductivity in this regime. We find that, under optimal conditions, this extra contribution may be larger than values achieved so far in the benchmark material, PbTe. In the last part we make a case for the importance of our results in the generic problem of anharmonic electron-lattice dynamics.

We address the emergent quantum critical phenomena for (pseudo)spin-$\frac{3}{2}$ birefringent fermions, featuring two effective Fermi velocities, when they reside close to itinerant Mott transitions realized through spontaneous symmetry breaking and triggered by strong local or Hubbard-like repulsive interactions. Irrespective of the nature of the mass orderings that produce fully gapped quasiparticle spectra in the ordered phase, which otherwise can be grouped into three classes, the system always possesses a unique terminal velocity near the corresponding quantum critical point. The associated critical regime accommodates a relativistic non-Fermi liquid of strongly coupled collective bosonic and spin-$\frac{1}{2}$ Dirac excitations with vanishing weight of the quasiparticle pole. These conclusions are also operative near superconducting critical points. Therefore, relativistic non-Fermi liquid possibly constitutes a robust superuniversal description for the entire family of strongly correlated arbitrary half-integer spin Dirac materials.

Understanding the vibrational and thermal properties of amorphous solids is one of the most discussed and long-standing issues in condensed matter physics. Recent works have made significant steps toward understanding harmonic vibrational states. In particular, it has been established that quasilocalized vibrational modes emerge in addition to phononlike vibrational modes. In this work, we study the anharmonic properties of these vibrational modes. We find that vibrational modes exhibit anharmonicities that induce particle rearrangements and cause transitions to different states. These anharmonicities are distinct from those in (perfect) crystals, where particle rearrangements never occur. Remarkably, for both the phonon modes and quasilocalized modes, the vibrational modes exhibit strong anharmonicities, and the induced particle rearrangements are always localized in space and composed of 1 to 1000 particles. Our findings contribute to the understanding of low-temperature thermal properties, for which anharmonic vibrations are crucial.

We derive the effective single-band Hamiltonian in the flat ${\mathrm{NiO}}_2$ planes for nickelate compounds ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$. We implement the first-principles calculation to study electronic structures of nickelates using the Heyd-Scuseria-Ernzerhof hybrid density functional and derive a three-band Hubbard model for Ni-O $pd\sigma$ bands of ${\mathrm{Ni}}^{+}3d_{x^2-y^2}$ and ${\mathrm{O}}^{2-}2p_{x/y}$ orbitals in the ${\mathrm{NiO}}_2$ planes. To obtain the effective one-band $t-t^{\prime }-J$ model Hamiltonian, we perform the exact diagonalization of the three-band Hubbard model for the ${\mathrm{Ni}}_5{\mathrm{O}}_{16}$ cluster and map the low-energy spectra onto the effective one-band models. We find that the undoped ${\mathrm{NiO}}_2$ plane is a Hubbard Mott insulator and the doped holes are primarily located on Ni sites. The physics of the ${\mathrm{NiO}}_2$ plane is a doped Mott insulator, described by the one-band $t-t^{\prime }-J$ model with $t=265\mathrm{meV}, t^{\prime }=-21\mathrm{meV}$, and $J=28.6\mathrm{meV}$. We also discuss the electronic structure for the self-doping effect and heavy fermion behavior of electron pockets of ${\mathrm{Nd}}^{3+}5d$ character in ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$.

Edge states protected by bulk topology of photonic crystals show robustness against short-range disorder, making robust information transfer possible. Here, topological photonic crystals under long-range deformations are investigated. The vertices of each regular hexagon in a honeycomb crystalline structure are shifted randomly to establish a deformed system. By increasing the degree of random deformations, a transition from an ordered system to an amorphous system are investigated, where the close of the topological band gap is clearly shown. We further present comprehensive investigations into excitation methods of the proposed deformed system. Due to the lack of strict periodicity, excitation of topological edge modes becomes difficult. We discuss chiral and linearly polarized sources as two different methods respectively. It is found that chiral sources are sensitive and rely on the ordered lattice. Even a weak long-range deformation can bring fluctuations to transmission. We further designed and fabricated metal-dielectric-metal sandwichlike samples working in the microwave band. Using a linearly polarized source, we detected the existence of topological transport in the deformed system. This work investigates excitation and robustness of bulk topology against long-range deformations and may open the way for exploiting topological properties of materials with a deformed lattice.

We present a systematic investigation of the inverse spin-Hall effect (ISHE) in the $\pi$-conjugated polymer poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS). Using a number of reference experiments, we are able to identify and isolate side effects which obscure the small but finite inverse spin-Hall effect in the polymer. We employ a sample geometry in which the contact areas and the area of spin current injection are laterally separated, which allows us to distinguish the ISHE from thermovoltages induced by nonreciprocal magnetostatic spin waves (MSSW) and from the ISHE induced by spin pumping through the polymer into the contacts. With an additional control experiment, we can even quantify the Nernst effect, which also needs to be taken into account. With these results, we can unambiguously show that the ISHE is present in this material, albeit at a level which requires a dedicated sample design and careful consideration of various artifacts.

Frustrated quantum spin systems such as the Heisenberg and Kitaev models on various lattices have been known to exhibit various exotic properties not only at zero temperature but also for finite temperatures. Inspired by the remarkable development of the quantum frustrated spin systems in recent years, we investigate the finite-temperature properties of the $S=1/2$ Kitaev-Heisenberg models on kagome and triangular lattices by means of finite-temperature Lanczos methods with improved accuracy. In both lattices, multiple peaks are confirmed in the specific heat. To find the origin of the multiple peaks, we calculate the static spin structure factor. The origin of the high-temperature peak of the specific heat is attributed to a crossover from the paramagnetic state to a short-range ordered state whose static spin structure factor has zigzag or linear intensity distributions in momentum space. In the triangular Kitaev model, the “order by disorder” due to quantum fluctuation occurs. On the other hand, in the kagome Kitaev model it does not occur even with both quantum and thermal fluctuations.

High-order harmonic generation (HHG) in imperfect lattices with point defect vacancies is studied using a self-consistent time-dependent density-functional approach for a one-dimensional linear chain with a band gap. Compared with a perfect lattice with no vacancies, the HHG yield decreases by increasing the number of evenly distributed vacancies but remains almost unchanged in the case when vacancies are localized at neighboring lattice points. By introducing atomic-type point vacancies in the linear chain, it is effectively partitioned into several subsystems and the overall shape of HHG spectra of the resulting system can be modeled via the HHG of its subsystems provided each subsystem is large enough to behave like a bulk solid. For sufficiently small subsystems, the HHG spectra show detectable signatures of finite structure but maintains the overall structure of the spectrum of a bulk solids. A time-frequency profile of the emitted harmonics shows less regular sinusoidal patterns in the systems with vacancies compared with the perfect lattice. The role the vacancy-induced defect-state orbitals in the HHG process is investigated for different realizations of the linear chain and laser parameters.

We consider transport properties of a single edge of a two-dimensional topological insulator, in presence of Rashba spin-orbit coupling, driven by two external time-dependent voltages and connected to a thin superconductor. We focus on the case of a train of Lorentzian-shaped pulses, which are known to generate coherent single-electron excitations in two-dimensional electron gas, and prove that they are minimal excitations for charge transport also in helical edge states, even in the presence of spin-orbit interaction. Importantly, these properties of Lorentzian-shaped pulses can be tested computing charge noise generated by the scattering of particles at the thin superconductor. This represents a novel setup where electron quantum optics experiments with helical states can be implemented, with the superconducting contact as an effective beamsplitter. By elaborating on this configuration, we also evaluate charge noise in a collisional Hong-Ou-Mandel configuration, showing that due to the peculiar effects induced by Rashba interaction, a nonvanishing dip at zero delay appears.

A systematic framework to investigate the spin dynamics in a noncollinear antiferromagnet is proposed. Taking ${\mathrm{Mn}}_3\mathrm{Sn}$ as a representative example, we derive an effective low-energy model based on the multipole expansion of the magnetic structure, and investigate the uniform precession and the domain wall dynamics. We show that the solution for the effective model accurately reproduces the numerical calculation of the Landau-Lifshitz-Gilbert equations. Our results indicate that ${\mathrm{Mn}}_3\mathrm{Sn}$ has preferable properties for applications to a racetrack memory and a spin torque oscillator, and thus is a promising candidate for spintronics devices by using the multipole degrees of freedom.

Spin-boson Hamiltonians are an effective description for numerous quantum few- and many-body systems, such as atoms coupled to cavity modes, quantum electrodynamics in circuits, and trapped ion systems. While reaching the limit of strong coupling is possible in current experiments, the understanding of the physics in this parameter regime remains a challenge, especially when disorder and dissipation are taken into account. Here we investigate a regime where the spin dynamics can be related to an Ising energy function defined in terms of the spin-boson couplings. While in the coherent weak coupling regime it is known that an effective description in terms of spin Hamiltonian is possible, we show that a similar viewpoint can be adopted in the presence of dissipation and strong couplings. The resulting dynamics features approximately thermal regimes, separated by out-of-equilibrium ones in which detailed balance is broken. Moreover, we show that under appropriately chosen conditions one can even achieve cooling of the spin degrees of freedom. This points toward the possibility of using strongly coupled dissipative spin-boson systems for engineering complex energy landscapes together with an appropriate cooling dynamics.