The structural phase transition path from the low-temperature polar structure up to the highest symmetric phase in the hybrid improper ferroelectric ${\mathrm{Ca}}_3{\mathrm{Mn}}_2{\mathrm{O}}_7$ compound is here investigated at atomic scale. Measurements using the perturbed angular correlation local probe technique are combined with ab initio electronic structure calculations to observe the evolution of the electric field gradient parameters at the $\mathrm{Ca}$ site within the 10–1200 K temperature range. The results show that polar-phase clusters persist at temperatures as high as 500 K. In addition, evidence is given for a structural phase transition occurring above 1150 K. The high-temperature symmetry is here confirmed to be $I4/mmm$.

We study the tunneling density of states (DOS) in an interacting disordered three-dimensional metal and calculate its energy dependence in the quasiballistic regime, for the deviation from the Fermi energy $E-E_F$, exceeding the elastic scattering rate. In this region, the DOS correction originates from the interplay of the interaction and single-impurity scattering. Depending on the distance between the interaction point and the impurity, one should distinguish (i) the smallest scales of the order of the Fermi wavelength and (ii) larger spatial scales of the order of $\hslash v_F/|E-E_F|$, where $v_F$ is the Fermi velocity. In two dimensions, the large-scale contribution prevails, resulting in a nearly universal DOS correction. The peculiarity of Friedel oscillations in three dimensions is that the contributions from small and large scales are typically comparable, making the DOS correction sensitive to the details of the interaction and demonstrating a significant particle-hole asymmetry. On the other hand, we show that the nonanalytic part of the DOS is determined by large scales and can be expressed in terms of the Fermi-surface characteristics only.

We provide evidence that a clean kicked Bose-Hubbard model exhibits a many-body dynamically localized phase. This phase shows ergodicity breaking up to the largest sizes we were able to consider. We argue that this property persists in the limit of large size. The Floquet states violate eigenstate thermalization and then the asymptotic value of local observables depends on the initial state and is not thermal. This implies that the system does not generically heat up to infinite temperature, for almost all the initial states. Differently from many-body localization here the entanglement entropy linearly increases in time. This increase corresponds to space-delocalized Floquet states which are nevertheless localized across specific subsectors of the Hilbert space: In this way the system is prevented from randomly exploring all the Hilbert space and does not thermalize.

Non-Hermitian lattice systems with unconventional transport phenomena and topological effects have attracted intensive attention recently. Non-Hermiticity is generally introduced by engineering on-site gain/loss distribution or inducing asymmetric couplings by applying an imaginary gauge field. Here, we extend the concept of non-Hermitian lattices from spatial to frequency dimension and emulate various non-Hermitian transport phenomena arising from asymmetric coupling in synthetic dimension. The non-Hermitian frequency lattice is created by introducing complex gauge potentials through appropriate complex modulations in a slab waveguide. This complex gauge potential can induce asymmetric couplings among spectral modes and give rise to various non-Hermitian transport phenomena such as amplified and decayed frequency diffraction, refraction and non-Hermitian Bloch oscillations. The latter manifest themselves as both power oscillation and asymmetric oscillation patterns, and can be exploited to probe in the bulk the non-Hermitian skin effect. Frequency-domain Bloch oscillations with both exponentially growing oscillation amplitude and energy are also predicted. Our results pave the way towards emulating non-Hermitian transport phenomena and topological effects in synthetic dimension on a photonic platform, with potential applications to spectral manipulation of optical signals and energy harvesting.

We study magnon-magnon interactions and their effects in a spiral magnet induced by combination of an antiferromagnetic Heisenberg interaction and a Dzyaloshinsky-Moriya interaction. We show that the main effect of magnon-magnon interactions on low-energy magnons is to renormalize the coefficient of energy dispersion. This could explain why some experiments are consistent with the noninteracting theory. We also show that although the magnon-magnon interactions induce the pair amplitude for low-energy magnons, its effect on the excitation energy is negligible. This suggests that for magnons the finite pair amplitude does not necessarily accompany the pair condensation.

We report the existence of a high-temperature magnetic anomaly in the three-dimensional Kitaev candidate material, $\beta \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$. Signatures of the anomaly appear in magnetization, heat capacity, and muon spin relaxation measurements. The onset coincides with a reordering of the principal axes of magnetization, which is thought to be connected to the onset of Kitaev-like correlations in the system. The anomaly also shows magnetic hysteresis with a spatially anisotropic magnitude that follows the spin-anisotropic exchange anisotropy of the underlying Kitaev Hamiltonian. We discuss possible scenarios for a bulk and impurity origin.

Diagrammatic expansions are a central tool for treating correlated electron systems. At thermal equilibrium, they are most naturally defined within the Matsubara formalism. However, extracting any dynamic response function from a Matsubara calculation ultimately requires the ill-defined analytical continuation from the imaginary- to the real-frequency domain. It was recently proposed [A. Taheridehkordi et al., Phys. Rev. B 99, 035120 (2019)] that the internal Matsubara summations of any interaction-expansion diagram can be performed analytically by using symbolic algebra algorithms. The result of the summations is then an analytical function of the complex frequency rather than Matsubara frequency. Here we apply this principle and develop a diagrammatic Monte Carlo technique which yields results directly on the real-frequency axis. We present results for the self-energy $\mathrm{\Sigma }\left(\omega \right)$ of the doped $32\times 32$ cyclic square-lattice Hubbard model in a nontrivial parameter regime, where signatures of the pseudogap appear close to the antinode. We discuss the behavior of the perturbation series on the real-frequency axis and in particular show that one must be very careful when using the maximum entropy method on truncated perturbation series. Our approach holds great promise for future application in cases when analytical continuation is difficult and moderate-order perturbation theory may be sufficient to converge the result.

As the surface Andreev bound state (ABS) forms at the open (1,1) edge of a $d_{x^2-y^2}$-wave superconductor, the local density of states increases. Therefore, a strong electron correlation and drastic phenomena may occur. However, a theoretical study on the effects of the ABS on the electron correlation has not been performed yet. To understand these effects, we study the large-cluster Hubbard model with an open (1,1) edge in the presence of a bulk $d$-wave gap. We calculate the site-dependent spin susceptibility by performing the random-phase approximation and modified fluctuation-exchange approximation in real space. We find that near the (1,1) edge, drastic ferromagnetic (FM) fluctuations occur owing to the ABS. In addition, as the temperature decreases, the system rapidly approaches a magnetic-order phase slightly below the transition temperature of the bulk $d$-wave superconductivity (SC). In this case, the FM fluctuations are expected to induce interesting phenomena such as edge-induced triplet SC and quantum critical phenomena.

We discuss the interplay between magnetic and structural degrees of freedom in elemental Mn. The equilibrium volume is shown to be sensitive to magnetic interactions between the Mn atoms. While the standard generalized gradient approximation underestimates the equilibrium volume, a more accurate treatment of the effects of electronic localization and magnetism is found to solve this longstanding problem. Our calculations also reveal the presence of a magnetic phase in strained $\alpha$-Mn that has been reported previously in experiments. This new phase of strained $\alpha$-Mn exhibits a noncollinear spin structure with large magnetic moments.

We investigate the thermoelectric transport properties of the half-filled lowest Landau level $v=1/2$ in a gated two-dimensional hole system in a strained Ge/SiGe heterostructure. The electron-diffusion dominated regime is achieved below 600 mK, where the diffusion thermopower ${S_{xx}}^d$ at $v=1/2$ shows a linear temperature dependence. In contrast, the diffusion-dominated Nernst signal ${S_{xy}}^d$ of $v=1/2$ is found to approach zero, which is independent of the measurement configuration (sweeping magnetic field at a fixed hole density or sweeping the density by a gate at a fixed magnetic field).

The current $I$ through nickelocene molecules and its noise are measured with a low-temperature scanning tunneling microscope on a Cu(100) substrate. Density functional theory calculations and many-body modeling are used to analyze the data. During contact formation, two types of current evolution are observed, namely an abrupt jump to contact and a smooth transition. These data along with conductance spectra $(dI/dV)$ recorded deep in the contact range are interpreted in terms of a transition from a spin-1 to a spin-$\frac{1}{2}$ state that is Kondo-screened. Many-body calculations show that the smooth transition is also consistent with a renormalization of spin excitations of a spin-1 molecule by Kondo exchange coupling. The shot noise is significantly reduced compared to the Schottky value of $2eI$. The noise can be described in the Landauer picture in terms of the spin polarization of the transmission of $\approx 35\%$ through two degenerate $d_{\pi }$-orbitals of the nickelocene molecule.

In this paper we show that graphene surface plasmons can be excited when an electromagnetic wave packet impinges on a single metal slit covered with graphene. The excitation of the plasmons localized over the slit is revealed by characteristic peaks in the absorption spectrum. It is shown that the position of the peaks can be tuned either by the graphene doping level or by the dielectric function of the material filling the slit. The whole system forms the basis for a plasmonic sensor when the slit is filled with an analyte.

 ${}^{115}\mathrm{In}$ nuclear quadrupole resonance (NQR) measurements have been carried out on ${\mathrm{Ce}}_3{\mathrm{PtIn}}_{11}$, a heavy fermion compound harboring two inequivalent Ce sites. This system exhibits magnetic transitions at $T_{1n}=2.2$ K and $T_{\mathrm{N}}=2$ K, prior to entering a superconducting state below $T_{\text{c}}=0.32$ K. Previous measurements imply that magnetism and superconductivity are spatially separated and only the Ce(2) sublattice orders magnetically, with a quantum critical point at pressure $p_{\mathrm{c}}=1.3$ GPa, while the Ce(1) sublattice is paramagnetic. However, the present nuclear relaxation rate behavior of $1/T_1\propto \sqrt{T}$ reveals the presence of critical spin fluctuations indicative of close proximity to another quantum critical point, presumably at a small negative pressure, which is associated solely with the Ce(1) sublattice. The ordered wave vector is found to be most likely $(1/2,1/2,h)$.

By means of first-principles calculations and a modeling analysis, we have predicted that traditional two-dimensional graphene hosts four types of topological phononic Dirac points (DPs) and a phononic nodal ring (PNR) in its phonon spectrum. In the phonon spectrum of graphene, there exist four types of DPs, DP1, DP2, DP3, and DP4, with both DP1 and DP2 located at the Brillouin zone (BZ) corners $K$ and $K^{\prime }$, DP3 located along the $\mathrm{\Gamma }\text{−}K$ line, and DP4 located along the $\mathrm{\Gamma }\text{−}M$ line, as well as the PNR surrounding the centered $\mathrm{\Gamma }$ point in the $q_{x,y}$ plane. The calculations further reveal that Berry curvatures are vanishingly zero throughout the whole BZ, except for the positions of these four Dirac phonons, at which the nonzero singular Berry curvatures appear with a Berry phase of $\pi$ or $-\pi$, confirming its topological nontrivial nature. The topologically protected nontrivial phononic edge states have been also evidenced along both the zigzag-edged and armchair-edged boundaries. These results may pave the way for further studies of the topological phononic properties of graphene, such as phononic destructive interference with a suppression of backscattering and intrinsic phononic quantum Hall-like effects.

In the theory of Anderson localization, a landscape function predicts where wave functions localize in a disordered medium, without requiring the solution of an eigenvalue problem. It is known how to construct the localization landscape for the scalar wave equation in a random potential, or equivalently for the Schrödinger equation of spinless electrons. Here, we generalize the concept to the Dirac equation, which includes the effects of spin-orbit coupling and allows us to study quantum localization in graphene or in topological insulators and superconductors. The landscape function $u\left(\mathit{r}\right)$ is defined on a lattice as a solution of the differential equation $\overbracket{H}u\left(\mathit{r}\right)=1$, where $\overbracket{H}$ is the Ostrowski comparison matrix of the Dirac Hamiltonian. Random Hamiltonians with the same (positive-definite) comparison matrix have localized states at the same positions, defining an equivalence class for Anderson localization. This provides for a mapping between the Hermitian and non-Hermitian Anderson model.

The recent observations of Majorana resonance increase our confidence in the realization of the first topological qubit. The nonlocal coherent nature of Majorana devices is the key factor for the topological protection of these qubits from decoherence. Direct observation of this coherent nature could provide a first-step benchmarking scheme to validate Majorana qubit quality, which could be the next experimental milestone. We propose a simple transport scheme with a Majorana island device along with a dissipative environment in the electrodes. We find that the dissipative environment renormalizes the quantum transport in significantly different ways: As the temperature is reduced, while the conductance for Majorana coherent teleportation increases, all other incoherent signals are strongly suppressed due to dissipation. This unique conductance scaling behavior is a clear benchmark to reveal the nonlocal coherent nature of Majorana devices.

Recently, a class of tensor networks called isometric tensor network states (isoTNS) was proposed which generalizes the canonical form of matrix product states to tensor networks in higher dimensions. While this ansatz allows for efficient numerical computations, it remained unclear which phases admit an isoTNS representation. In this work, we show that two-dimensional string-net liquids, which represent a wide variety of topological phases including discrete gauge theories, admit an exact isoTNS representation. We further show that the isometric form can be preserved after applying a finite-depth local quantum circuit. Taken together, these results show that long-range entanglement by itself is not an obstruction to isoTNS representation and suggest that all two-dimensional gapped phases with gappable edges admit an isoTNS representation.

The topological properties of electronic states in multivalley two-dimensional materials, such as mono- and bilayer graphene, or thin films of rhombohedral graphite give rise to various unusual magnetotransport regimes. Here, we investigate the tunability of the topological magnetic moment (related to the Berry curvature) of electronic states in bilayer graphene using strain and vertical bias. We show how one can controllably vary the valley $g$ factor of the band-edge electrons $g_v^{*}$ across the range $10<|g_v^{*}|<200$, and we discuss the manifestations of the topological magnetic moment in the anomalous contribution towards the Hall conductivity and in the Landau level spectrum.

$\mathrm{Cu}\left(\mathrm{In},\mathrm{Ga}\right){\mathrm{S}}_2$ is an emerging material for solar cells, reaching already efficiencies above 15%. For any solar cell material, it is essential to understand the recombination processes and doping behavior of its defects. Therefore, in this work, we present a detailed photoluminescence and admittance investigation of polycrystalline $\mathrm{CuIn}{\mathrm{S}}_2$ thin films to study the shallow intrinsic defects. We find evidence for two acceptors, 105 and 145 meV from the valence band, their predominance depending on the Cu excess, plus one donor, 30 meV from the conduction band. The high crystalline and electronic quality of our samples allows the detection of low intensity luminescence peaks. Based on these emissions we can analyze the phonon coupling of the observed defects and show that an emission previously attributed to a second donor is in fact a phonon replica of the first donor-acceptor transition.

Structural symmetry breaking and recovery in condensed-matter systems are closely related to exotic physical properties such as superconductivity (SC), magnetism, spin density waves, and charge density waves (CDWs). The interplay between different order parameters is intricate and often subject to intense debate, as in the case of CDW order and superconductivity. In ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{CuO}}_4$ (LBCO), the low-temperature structural domain walls are hypothesized as nanometer-scale pinning sites for the CDWs. Coherent x-ray diffraction techniques have been employed here to visualize the domain structures associated with these symmetry changes directly during phase transition. We have pushed Bragg coherent diffractive imaging (BCDI) into the cryogenic regime where most phase transitions in quantum materials reside. Utilizing BCDI, we image the structural evolution of LBCO microcrystal samples during the high-temperature tetragonal to low-temperature orthorhombic (LTO) phase transition. Our results show the formation of LTO domains close to the transition temperature and how the domain size decreases with temperature. The number of domains follows the secondary order parameter (or orthorhombic strain) measurement with a critical exponent that is consistent with the three-dimensional universality class.