Studies of topological semimetals have revealed spectacular transport phenomena spanning extreme magnetoresistance effects and ultrahigh mobilities. As phonon dynamics and electron-phonon scattering play a critical role in the electrical and thermal transport, we pursue a fundamental understanding of these effects in type-I Weyl semimetals NbAs and TaAs. In the temperature-dependent Raman spectra of NbAs we reveal a previously unreported Fano line shape, a signature stemming from the electron-phonon interaction. Additionally, the temperature dependence of the $A_1$ phonon linewidths in both NbAs and TaAs strongly deviate from the standard model of anharmonic decay. To capture the mechanisms responsible for the observed Fano asymmetry and the atypical phonon linewidth, we present first-principles calculations of the phonon self-energy correction due to the electron-phonon interaction. Through this study of the phonon dynamics and electron-phonon interaction in these Weyl semimetals, we consider specific microscopic pathways which could contribute to the nature of their macroscopic transport.

Electromagnon is the spin wave in multiferroic materials and is known to accompany electric polarization due to the cross correlation between the charge and spin. Here, we theoretically show that the electromagnons also induce dc current upon their photoexcitations. The proposed dc current response originates from the shift current mechanism which is characterized by the so-called shift vector, a geometric quantity of the Bloch wave functions.

We present high-resolution thermal diffusivity measurements on several near optimally doped electron- and hole-doped cuprate systems in a temperature range that passes through the Mott-Ioffe-Regel limit, above which the quasiparticle picture fails. Our primary observations are that the inverse thermal diffusivity is linear in temperature and can be fitted to $D_Q^{-1}=aT+b$. The slope $a$ is interpreted through the Planckian relaxation time $\tau \approx \hslash /k_{B}T$ and a thermal diffusion velocity $v_B$, which is close, but larger than the sound velocity. The intercept $b$ represents a crossover diffusion constant that separates coherent from incoherent quasiparticles. These observations suggest that both phonons and electrons participate in the thermal transport, while reaching the Planckian limit for relaxation time.

Following recent experimental progress concerning Nagaoka ferromagnetism in finite-size quantum dot plaquettes, a general theoretical analysis is warranted in order to ascertain in rather generic terms which arrangements of a small number of quantum dots can produce saturated ferromagnetic ground states and under which constraints on interaction and interdot tunneling in the plaquette. This is particularly necessary since Nagaoka ferromagnetism is fragile and arises only under rather special conditions. We test the robustness of ground state ferromagnetism in the presence of a long-range Coulomb interaction and long-range as well as short-range interdot hopping by modeling a wide range of different plaquette geometries accessible by arranging a few ($\sim 4$) quantum dots in a controlled manner. We find that ferromagnetism is robust to the presence of long-range Coulomb interactions, and we develop conditions constraining the tunneling strength such that the ground state is ferromagnetic. Additionally, we predict the presence of a partially spin-polarized ferromagnetic state for 4 electrons in a Y-shaped 4-quantum-dot plaquette. Finally, we consider 4 electrons in a ring of 5 dots. This does not satisfy the Nagaoka condition; however, we show that the ground state is spin 1 for strong, but not infinite, on-site interaction. Thus, even though Nagaoka's theorem does not apply, the ground state for the finite system with one hole in a ring of 5 dots is partially ferromagnetic. We provide detailed fully analytical results for the existence or not of ferromagnetic ground states in several quantum dot geometries which can be studied in currently available coupled quantum dot systems.

Uniaxial pressure applied along a Ru-O-Ru bond direction induces an elliptical distortion of the largest Fermi surface of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, eventually causing a Fermi surface topological transition, also known as a Lifshitz transition, into an open Fermi surface. There are various anomalies in low-temperature properties associated with this transition, including maxima in the superconducting critical temperature and in resistivity. In the present paper, we report refined measurements of the strain at which this transition occurs, employing apparatus in which the stress on the sample is measured, and resonant ultrasound measurement of the low-temperature elastic moduli. The Lifshitz transition is found to occur at a longitudinal strain ${\varepsilon }_{xx}$ of $(-0.44\pm 0.06)\times {10}^{-2}$, which corresponds to a $B_{1g}$ strain ${\varepsilon }_{xx}-{\varepsilon }_{yy}$ of $(-0.66\pm 0.09)\times {10}^{-2}$. This is considerably smaller than the strain corresponding to a Lifshitz transition in density functional theory calculations, even if the spin-orbit coupling is taken into account. Using dynamical mean-field theory, we show that electronic correlations reduce the critical strain. It turns out that the orbital anisotropy of the local Coulomb interaction on the Ru site is, furthermore, important to bring this critical strain close to the experimental number and thus well into the experimentally accessible range of strains.

We present a study of the macroscopic dynamics of a polariton condensate formed by nonresonant optical excitation in a quasi-one-dimensional ring-shaped microcavity. The presence of a gradient in the cavity photon energy creates a macroscopic trap for the polaritons in which a coherent condensate is formed which evolves into a single-mode condensate at late times. With time- and energy-resolved imaging we show the role of interactions in the motion of the condensate as it undergoes equilibration in the ring. These experiments also give a direct measurement of the polariton-polariton interaction strength above the condensation threshold. Our observations are compared to the open-dissipative one-dimensional Gross-Pitaevskii equation which shows excellent qualitative agreement.

Interactions between the lattice and charge carriers can drive the formation of phases and ordering phenomena that give rise to conventional superconductivity, insulator-to-metal transitions, and charge-density waves. These couplings also play a determining role in properties that include electrical and thermal conductivity. Ultrafast electron diffuse scattering (UEDS) has recently become a viable laboratory-scale tool to track energy flow into and within the lattice system across the entire Brillouin zone, and to separate interactions in the time domain. Here, we present a detailed quantitative framework for the interpretation of UEDS signals, ultimately extracting the phonon-mode occupancies across the entire Brillouin zone. These transient populations are then used to extract momentum- and mode-dependent electron-phonon and phonon-phonon coupling constants. Results of this analysis are presented for graphite, which provides complete information on the phonon-branch occupations and a determination of the $A_1^{\prime }$ phonon-mode-projected electron-phonon coupling strength $\langle g_{e,A_1^{\prime }}^2\rangle =0.035\pm 0.001{\mathrm{eV}}^2$ that is in agreement with other measurement techniques and simulations.

Low-temperature electrical and thermal transport, magnetic penetration depth, and heat capacity measurements were performed on single crystals of the actinide superconductor ${\mathrm{UTe}}_2$ to determine the structure of the superconducting energy gap. Heat transport measurements performed with currents directed along both crystallographic $a$ and $b$ axes reveal a vanishingly small residual fermionic component of the thermal conductivity. The magnetic field dependence of the residual term follows a rapid, quasilinear increase consistent with the presence of nodal quasiparticles, rising toward the $a$-axis upper critical field where the Wiedemann-Franz law is recovered. Together with a quadratic temperature dependence of the magnetic penetration depth up to $T/T_c=0.3$, these measurements provide evidence for an unconventional spin-triplet superconducting order parameter with point nodes. Millikelvin specific heat measurements performed on the same crystals used for thermal transport reveal an upturn below 300 mK that is well described by a divergent quantum-critical contribution to the density of states (DOS). Modeling this contribution with a $T^{-1/3}$ power law allows restoration of the full entropy balance in the superconducting state and a resultant cubic power law for the electronic DOS below $T_c$, consistent with the point-node gap structure determined by thermal conductivity and penetration depth measurements.

We report the magnetic structure and electronic properties of the honeycomb antiferromagnet ${\mathrm{NaNi}}_2{\mathrm{BiO}}_{5.66}$. We find magnetic order with moments along the $c$ axis for temperatures below $T_{c1}=6.3\left(1\right)\mathrm{K}$ and then in the honeycomb plane for $T<T_{c2}=4.8\left(1\right)\mathrm{K}$ with a counterrotating pattern and an ordering wave vector $\mathbf{q}=(\frac{1}{3},\frac{1}{3},0.15\left(1\right))$. Density functional theory and electron spin resonance indicate this is high-spin ${\mathrm{Ni}}^{3+}$ magnetism near a high to low spin transition. The ordering wave vector, in-plane magnetic correlations, missing entropy, spin state, and superexchange pathways are all consistent with bond-dependent Kitaev-$\mathrm{\Gamma }$-Heisenberg exchange interactions in ${\mathrm{NaNi}}_2{\mathrm{BiO}}_{6-\delta }$.

The excitations in honeycomb $\alpha -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ have been investigated with high-resolution resonant inelastic x-ray scattering (RIXS) at the O $K$ edge. The low-energy response is dominated by a fully resolved ladder of excitations, which we interpret as being due to multiphonon processes in the presence of strong electron-phonon coupling (EPC). At higher energies, the orbital excitations are shown to be dressed by phonons. The high quality of the data permits a quantitative test of the analytical model for the RIXS cross section, which has been proposed to describe EPC in transition-metal oxides (TMOs). We find that the magnitude of the EPC is comparable to that found for a range of $3d$ TMOs. This indicates that EPC may be of equal importance in determining the phenomenology displayed by corresponding $5d$-based systems.

Spin-wave dynamics were studied in an extended thin film of single-crystalline yttrium iron garnet using time-resolved scanning transmission x-ray microscopy. A combination of mechanical grinding and focused ion beam milling has been utilized to achieve a soft x-ray transparent thickness of the underlying bulk gadolinium gallium garnet substrate. Damon-Eshbach type spin waves down to about 100 nm wavelength have been directly imaged in real space for varying frequencies and external magnetic fields. The dispersion relation extracted from the experimental data agreed well with theoretical predictions. A significant influence of the ion milling process on the local magnetic properties was not detected.

We study precursor states of fractional topological insulators (FTIs) in interacting fermionic ladders with spin-orbit coupling. Within a microscopically motivated bosonization approach, we investigate different competing phases depending on same-spin and interspin interactions at fractional effective filling $\nu =1/3$ per spin. In the spin-decoupled limit, we find that strong repulsive interactions of already moderate range may lead to a partially gapped state with two time-reversed copies of a quasi-one-dimensional Laughlin phase. This FTI precursor competes with an interleg partially gapped phase displaying quasi-long-range density wave order, however, it may be stabilized if interactions are $\mathrm{SU}\left(2\right)$ symmetric, or have suitable anisotropy, in leg space. When the FTI phase is present, it is moderately robust to small interspin interactions; these introduce competing partially gapped phases of orbital antiferromagnetic and bond density wave character. Performing a strong-coupling analysis of the FTI precursor regime, we find that the main effect of interspin interactions is to induce correlated quasiparticle backscattering between the precursor FTI edge modes. Although this process competes with the topological phase, we show, by considering an array of ladders, that its influence may disappear upon approaching the two-dimensional case. Considering time-reversal symmetry breaking perturbations, we also describe a protocol that adiabatically pumps $1/6$ charge per half-cycle, thus providing a quantized FTI signature arising already in the single ladder regime.

We propose a method to perform accurate and fast charge pumping in superconducting nanocircuits. Combining topological properties and quantum control techniques based on shortcuts to adiabaticity, we show that it is theoretically possible to achieve perfectly quantized charge pumping at any finite-speed driving. Model-specific errors may still arise due the difficulty of implementing the exact control. We thus assess this and other practical issues in a specific system comprised of three Josephson junctions. Using realistic system parameters, we show that our scheme can improve the pumping accuracy of this device by various orders of magnitude. Possible metrological perspectives are discussed.

We describe a 3d analog of the Jordan-Wigner transformation which maps an arbitrary fermionic system on a 3d spatial lattice to a 2-form ${\mathbb{Z}}_2$ gauge theory with an unusual Gauss law. An important property of this map is that it preserves the locality of the Hamiltonian. The map depends explicitly on the choice of a spin structure of the spatial manifold. We give examples of 3d bosonic systems dual to free fermions. We also describe the corresponding Euclidean lattice models, which is analogous to the Steenrod square term in ($3+1$)D [compared to the Chern-Simon term in ($2+1$)D].

Many-body localization (MBL), characterized by the absence of thermalization and the violation of conventional thermodynamics, has elicited much interest both as a fundamental physical phenomenon and for practical applications in quantum information. A phenomenological model which describes the system using a complete set of local integrals of motion (LIOMs) provides a powerful tool to understand MBL but can usually be computed only approximately. Here we explicitly compute a complete set of LIOMs with a nonperturbative approach by maximizing the overlap between LIOMs and physical spin operators in real space. The set of LIOMs satisfies the desired exponential decay of the weight of LIOMs in real space. This LIOM construction enables a direct mapping from the real-space Hamiltonian to the phenomenological model and thus enables studying the localized Hamiltonian and the system dynamics. We can thus study and compare the localization lengths extracted from the LIOM weights, their interactions, and dephasing dynamics, revealing interesting aspects of many-body localization. Our scheme is immune to accidental resonances and can be applied even at the phase transition point, providing a tool to study the microscopic features of the phenomenological model of MBL.

The crystal and magnetic properties of honeycomb ${\mathrm{BiMnTeO}}_6$ have been studied between 1.5 and 300 K using synchrotron x-rays and neutron-scattering experiments. Commensurate magnetic ordering is observed below $T_N=10\mathrm{K}$, and corresponds to a noncollinear spin arrangement, with spins tilted away from the anisotropy easy-axis set by the Jahn-Teller distortion of the ${\mathrm{MnO}}_6$ octahedra. Inelastic-scattering experiments show two main nearly localized magnetic excitations, which can be well described by an exchange Hamiltonian involving weak Mn-Mn magnetic interactions and a crystal-field Hamiltonian characterizing the strong easy-axis anisotropy associated with the $d_{z2}$ orbital ordering of ${\mathrm{Mn}}^{3+}$. The crystal-field levels can be accurately calculated, taking into account a transverse molecular field imposed by the magnetic ordering of the neighboring atoms below $T_N$. This makes ${\mathrm{BiMnTeO}}_6$ an interesting example of a multiaxis Ising system in a self-imposed transverse magnetic field.

While general quantum many-body systems require exponential resources to be simulated on a classical computer, systems of noninteracting fermions can be simulated exactly using polynomially scaling resources. Such systems may be of interest in their own right but also occur as effective models in numerical methods for interacting systems, such as Hartree-Fock, density functional theory, and many others. Often it is desirable to solve systems of many thousand constituent particles, rendering these simulations computationally costly despite their polynomial scaling. We demonstrate how this scaling can be improved by adapting methods based on matrix product states, which have been enormously successful for low-dimensional interacting quantum systems, to the case of free fermions. Compared to the case of interacting systems, our methods achieve an exponential speedup in the entanglement entropy of the state. We demonstrate their use to solve systems of up to one million sites with an effective matrix product state bond dimension of ${10}^{15}$.

The current-mirror circuit [A. Kitaev, arXiv:cond-mat/0609441] exhibits a robust ground-state degeneracy and wave functions with disjoint support for appropriate circuit parameters. In this protected regime, Cooper-pair excitons form the relevant low-energy excitations. Based on a full circuit analysis of the current-mirror device, we introduce an effective model that systematically captures the relevant low-energy degrees of freedom and is amenable to diagonalization using density matrix renormalization group (DMRG) methods. We find excellent agreement between DMRG and exact diagonalization, and can push DMRG simulations to much larger circuit sizes than feasible for exact diagonalization. We discuss the spectral properties of the current-mirror circuit and predict coherence times exceeding 1 ms in parameter regimes believed to be within reach of experiments.

We performed a point-contact spectroscopy study of intermediate valence compound YbPd, in which two Yb sites are ordered with the valence values of $3+$ and $2.6+$. Differential conductance spectra exhibit a dip structure at around zero bias with an asymmetric background, which can be reproduced by the summation of two Kondo resonances. Fitting with the Fano equation, the Kondo temperature of the $3+$ site is estimated to be $\sim 30\mathrm{K}$, while that of the $2.6+$ site is $\sim 800\mathrm{K}$. Additionally, the dip amplitude exhibits $-\ln T$ dependence below $\sim 25\mathrm{K}$ and saturates at $\sim 1\mathrm{K}$. These results provide direct evidence for the screening of magnetic moments at the valence ordering sites owing to the Kondo effect.

We report experimental evidence of a charge density wave (CDW) transition in a monolayer $1T-{\mathrm{VTe}}_2$ film. Low-energy electron diffraction measurements reveal $4\times 4$ reconstruction peaks below ${\mathit{T}}_{\mathrm{CDW}}=186$ K. Angle-resolved photoemission spectroscopy measurements reveal arclike pockets with anisotropic CDW gaps up to 50 meV. The anisotropic CDW gap is attributed to the imperfect nesting of the CDW wave vector, and first-principles calculations reveal phonon softening at the same vector. These results suggest the important roles of both Fermi-surface nesting and the electron-phonon interaction in the CDW mechanism of a monolayer ${\mathrm{VTe}}_2$ film.