We study the effects of the spacial structure of localized single-particle losses in weakly interacting fermionic quantum wires. We show that multiple dissipative impurities give rise to resonant effects visible in the transport properties and the particles' momentum distribution. These resonances can enhance or suppress the effective particle losses in the wire. Moreover, we investigate the interplay between interactions and the impurity shape and find that, differently from the coherent scatterer case, the impurity shape modifies the scaling of the scattering probabilities close to the Fermi momentum. We show that, while the fluctuation-induced quantum Zeno effect is robust against the shape of the impurities, the fluctuation-induced transparency is lifted continuously. This is reflected in the emergence of a continuous line of fixed points in the renormalization group flow of the scattering probabilities.

Inorganic metal halide perovskites are promising materials for next-generation technologies due to a plethora of unique physical properties, many of which cannot be observed in the oxide perovskites. On the other hand, the search for ferroelectricity and multiferroicity in lead-free inorganic halide perovskites remains a challenging research topic. Here, we experimentally show that cubic fluoroperovskites exhibit proximity to incipient ferroelectrics, which manifested in the softening of the low-frequency polar phonons in the Brillouin zone center at cooling. Furthermore, we reveal the coupling between harmonic and anharmonic force constants of the softening phonons and their correlation with the perovskite tolerance factor. Next, using first-principles calculations, we examine the lattice dynamics of the cubic fluoroperovskites and disclose the incipient lattice instability at which the harmonic force constants of low-lying phonons tend to decrease with a reduction of tolerance factor at all high-symmetry points of the Brillouin zone. The correlations with the tolerance factor indicate the geometric origin of observed incipient lattice instability in the cubic fluoroperovskites caused by the steric effect due to the volume filling of the unit cell by different radius ions. These results provide insights into the lattice dynamics and potential ferroelectric properties of inorganic lead-free metal halide perovskites, relevant to further design and synthesis of new multifunctional materials.

We use quantum kinetic theory to calculate the thermoelectric transport properties of the two-dimensional single-band Fermi-Hubbard model in the weak coupling limit. For generic filling, we find that the high-temperature limiting behaviors of the electrical $(\sim T)$ and thermal $(\sim T^2)$ resistivities persist down to temperatures of order the hopping matrix element $T\sim t$, almost an order of magnitude below the bandwidth. At half filling, perfect nesting leads to anomalous low-temperature scattering and nearly $T$-linear electrical resistivity at all temperatures. We hypothesize that the $T$-linear resistivity observed in recent cold atom experiments is continuously connected to this weak coupling physics and suggest avenues for experimental verification. We find a number of other novel thermoelectric results, such as a low-temperature Wiedemann-Franz law with Lorenz coefficient $5{\pi }^2/36$.

We present a detailed study of the bulk electronic structure of high-quality $\mathrm{V}{\mathrm{Se}}_2$ single crystals using optical spectroscopy. Upon entering the charge density wave phase below the critical temperature of 112 K, the optical conductivity of $\mathrm{V}{\mathrm{Se}}_2$ undergoes a significant rearrangement. A Drude response present above the critical temperature is suppressed while a new interband transition appears around 0.07 eV. From our analysis, we estimate that part of the spectral weight of the Drude response is transferred to a collective mode of the CDW phase. The remaining normal-state charge dynamics appears to become strongly damped by interactions with the lattice as evidenced by a mass enhancement factor $m^{*}/m\approx 3$. In addition to the changes taking place in the electronic structure, we observe the emergence of infrared active phonons below the critical temperature associated with the $4a\times 4a$ lattice reconstruction.

We report the low-temperature magnetic properties of ${\mathrm{Nd}}^{3+}$ pyrochlore ${\mathrm{Nd}}_2{\mathrm{ScNbO}}_7$. Susceptibility and magnetization show an easy-axis moment, and heat capacity reveals a phase transition to long-range order at $T_N=371\left(2\right)$ mK with a fully recovered $\mathrm{\Delta }S=Rln\left(2\right)$, 53% of it recovered for $T>T_N$. Elastic neutron scattering shows a long-range all-in all-out magnetic order with low-$Q$ diffuse elastic scattering. Inelastic neutron scattering shows a low-energy flat band, indicating a magnetic Hamiltonian similar to ${\mathrm{Nd}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$. Nuclear hyperfine excitations measured by ultra-high-resolution neutron backscattering indicate a distribution of static electronic moments below $T_N$, which may be due to $B$-site disorder influencing Nd crystal electric fields. Analysis of heat-capacity data shows an unexpected $T$-linear or $T^{3/2}$ term which is inconsistent with conventional magnon quasiparticles, but is consistent with fractionalized spinons or gapless local spin excitations. We use legacy data to show similar behavior in ${\mathrm{Nd}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$. Comparing local static moments also reveals a suppression of the nuclear Schottky anomaly in temperature, evidencing a fraction of Nd sites with nearly zero static moment, consistent with exchange-disorder-induced random singlet formation. Taken together, these measurements suggest an unusual fluctuating magnetic ground state which mimics a spin liquid, but may not actually be one.

We present a so-far undetected submonolayer phase of copper telluride on Cu(111) with ${(5\times \sqrt{3})}_{\text{rect}}$ periodicity and coverage of 0.40 ML tellurium (Te), which can be grown with perfect long-range order. It is structurally characterized by a combination of quantitative low-energy electron diffraction (LEED-IV), scanning tunneling microscopy (STM), and density functional theory (DFT). We find that Te induces the formation of two-atom-wide zig-zag troughs within the Cu(111) top layer via replacing 4 Cu atoms per unit cell by two Te atoms. Additionally, the interspace between the troughs is decorated by ${\mathrm{Cu}}_2{\mathrm{Te}}_2$ ad-chains sitting at hcp sites. All Te atoms exhibit the same local sixfold coordination: They occupy threefold hollow sites of the substrate and are one-sided attached to another three Cu atoms. The presented structural model is verified by a LEED-IV analysis with Pendry R factor of R = 0.174 and quantitative agreement of structural parameters with DFT predictions. It also has by far the lowest energy of all models tested by DFT and simulated STM images agree perfectly with experiment. It turns out that the new ${(5\times \sqrt{3})}_{\text{rect}}$ phase is the most dense surface telluride phase possible on Cu(111) before bulk-like copper telluride starts to grow.

Berezinskii-Kosterlitz-Thouless transition of the classical XY model is reinvestigated, combining the tensor network renormalization (TNR) and the level spectroscopy method based on the finite-size scaling of the conformal field theory. By systematically analyzing the spectrum of the transfer matrix of the systems of various moderate sizes, which can be accurately handled with a finite bond dimension, we determine the critical point removing the logarithmic corrections. This improves the accuracy by an order of magnitude over previous studies including those utilizing TNR. Our analysis also gives a visualization of the celebrated Kosterlitz renormalization group flow based on the numerical data.

In this paper we report the effect of uniaxial strain $\varepsilon$ applied along the crystalline $a$ axis on the newly discovered kagome superconductor ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. At ambient conditions, ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ shows a charge-density wave (CDW) transition at $T_{\mathrm{CDW}}=94.5\mathrm{K}$ and superconducts below $T_c=3.34\mathrm{K}$. In our paper, when the uniaxial strain $\varepsilon$ is varied from $-0.90\%$ to $0.90\%,T_c$ monotonically increases by $\sim 33\%$ from 3.0 to 4.0 K, giving rise to the empirical relation $T_c\left(\varepsilon \right)=3.4+0.56\varepsilon +0.12{\varepsilon }^2$. On the other hand, for $\varepsilon$ changing from $-0.76\%$ to $1.26\%,T_{\mathrm{CDW}}$ decreases monotonically by $\sim 10\%$ from 97.5 to 87.5 K with $T_{\mathrm{CDW}}\left(\varepsilon \right)=94.5-4.72\varepsilon -0.60{\varepsilon }^2$. The opposite response of $T_c$ and $T_{\mathrm{CDW}}$ to the uniaxial strain suggests strong competition between these two orders. Comparison with hydrostatic pressure measurements indicate that it is the change in the $c$ axis that is responsible for these behaviors of the CDW and superconducting transitions, and that the explicit breaking of the sixfold rotational symmetry by strain has a negligible effect. Combined with our first-principles calculations and phenomenological analysis, we conclude that the enhancement in $T_c$ with decreasing $c$ is caused primarily by the suppression of $T_{\mathrm{CDW}}$, rather than strain-induced modifications in the bare superconducting parameters. We propose that the sensitivity of $T_{\mathrm{CDW}}$ with respect to the changes in the $c$ axis arises from the impact of the latter on the trilinear coupling between the $M_1^{+}$ and the $L_2^{-}$ phonon modes associated with the CDW. Overall, our paper reveals that the $c$-axis lattice parameter, which can be controlled by both pressure and uniaxial strain, is a powerful tuning knob for the phase diagram of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$.

Magnetic structures are known to possess magnon excitations confined to their surfaces and interfaces, but these spatially localized modes are often not resolved in spectroscopy experiments. We develop a theory to calculate the confined magnon spectra and its associated spin scattering function, which is the physical observable in neutron and electron scattering, and a proxy for photon spectroscopy based on x-ray, Raman, and terahertz (THz) sources. We show that extra anisotropy at the surface or interface plays a key role in magnon confinement. We obtain analytical expressions for the confinement length scale and show that it is qualitatively similar for ferromagnets and antiferromagnets in dimension $d\ge 2$. For $d=1$, we find remarkable differences between ferromagnetic and antiferromagnetic models. The theory indicates the presence of several confined magnon resonances in addition to the usual magnons thought to explain the excitations of magnetic nanostructures. Detecting these modes may elucidate the impact of the interface on spin anisotropy and magnetic order.

The mechanism of unconventional superconductivity, such as high-temperature-cuprate, Fe-based, and heavy-fermion superconductors, has been studied as a central issue in condensed-matter physics. Spin fluctuations, instead of phonons, are considered to be responsible for the formation of Cooper pairs, and many efforts have been made to confirm this mechanism experimentally. Although a qualitative consensus seems to have been obtained, experimental confirmation has not yet been achieved. This is owing to a lack of the quantitative comparison between theory and experiments. Here, we show a semiquantitative comparison between the superconducting-transition temperature ($T_{\mathrm{SC}}$) and spin fluctuations derived from the NMR experiment on the ferromagnetic (FM) superconductor UCoGe in which the FM fluctuations and superconductivity are tunable by external fields. The enhancement and abrupt suppression of $T_{\mathrm{SC}}$ by applied fields, as well as the pressure variation of $T_{\mathrm{SC}}$ around the FM criticality are well understood by the change in the FM fluctuations on the basis of the single-band spin-triplet theoretical formalism. The present comparisons strongly support the theoretical formalism of spin-fluctuation-mediated superconductivity, particularly in UCoGe.

We study topological charge pumping in one-dimensional quasiperiodic systems. Since these systems lack periodicity, we cannot use the conventional approach based on the topological Chern number defined in the momentum space. Here, we develop a general formalism based on a real-space picture using the so-called Bott index. We extend the Bott index that was previously used to characterize quantum Hall effects in quasiperiodic systems, and apply it to topological charge pumping in quasiperiodic systems. The Bott index allows us to systematically compute topological indices of charge pumping, regardless of the detail of quasiperiodic models. We apply this formalism to the Fibonacci-Rice-Mele model which we made from a Fibonacci lattice, a well-known quasiperiodic system, and Rice-Mele model. We find that these quasiperiodic systems show topological charge pumping with a multilevel behavior due to the fractal nature of the Fibonacci lattice. Such multilevel pumping behaviors can be understood by a real-space renormalization group analysis.

Swedenborgite ${\mathrm{Ca}}_{0.5}{\mathrm{Y}}_{0.5}{\mathrm{BaCo}}_4{\mathrm{O}}_7$ (CYBCO) with geometrically frustrated kagome and triangular lattices exhibits a disordered ground state in spite of strong antiferromagnetic couplings between the Co spins. The character of the disordered state has been debated due to ambiguous Co states in the triangular layers. Here, we perform single-crystal diffuse neutron scattering experiments to fully characterize the short-range spin correlations in CYBCO. Through reverse Monte Carlo and self-consistent Gaussian approximation analyses, we confirm that the Co ions in both the kagome and triangular layers are magnetic, and the interlayer couplings, together with the lattice distortion, promote an incommensurate magnetic order that is suppressed possibly by exchange disorder. Our work clarifies the short-range spin correlations in CYBCO and establishes a general correspondence between unequal interactions and incommensurate spin correlations on the swedenborgite lattice.

We reveal the existence of a large in-plane heavy-hole $g$ factor in symmetric self-assembled (001) (In,Ga)As/GaAs quantum dots due to the warping of valence-band states. This warping dominates over the well-established mechanism associated with a reduced symmetry of the quantum dots and the corresponding mixing of heavy-hole and light-hole states. The effect of band warping is manifested in a unique angular dependence of the trion photon echo signal on the direction of the external magnetic field with respect to the sample axes. It results in a uniform magnetic-field-induced optical anisotropy for the entire quantum dot ensemble which is a prerequisite for the realization of spin quantum memories and spin-photon entanglement in the ensemble.

We characterize the magnetic properties for thulium ion energy levels in the ${\mathrm{Y}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ (Tm:YGG) lattice with the goal to improve decoherence and reduce linewidth broadening caused by local host spins and crystal imperfections. More precisely, we measure hyperfine tensors for the lowest level of ${}^3\mathrm{H}_6$ and excited ${}^3\mathrm{H}_4$ states using a combination of spectral hole burning, absorption spectroscopy, and optically detected nuclear magnetic resonance. By rotating the sample through a series of angles with an applied external magnetic field, we measure and analyze the orientation dependence of the ${\mathrm{Tm}}^{3+}$ ion's spin Hamiltonian. Using this spin Hamiltonian, we propose a set of orientations to improve material properties that are important for light-matter interaction and quantum information applications. Our results yield several important external field directions: some to extend optical coherence times, another to improve spin inhomogeneous broadening, and yet another that maximizes mixing of the spin states for specific sets of ions, which allows improving optical pumping and creation of lambda systems in this material.

Motivated by previous works on a Floquet version of the PXP model [B. Mukherjee et al., Phys. Rev. B 102, 075123 (2020) and B. Mukherjee et al., Phys. Rev. B 101, 245107 (2020)], we study a one-dimensional spin-$\frac{1}{2}$ lattice model with three-spin interactions in the same constrained Hilbert space (where all configurations with two adjacent $S^z=\uparrow$ spins are excluded). We show that this model possesses an extensive fragmentation of the Hilbert space which leads to a breakdown of thermalization upon unitary evolution starting from a large class of simple initial states. Despite the nonintegrable nature of the Hamiltonian, many of its high-energy eigenstates admit a quasiparticle description. A class of these, which we dub as “bubble eigenstates,” have integer eigenvalues (including mid-spectrum zero modes) and strictly localized quasiparticles while another class contains mobile quasiparticles leading to a dispersion in momentum space. Other anomalous eigenstates that arise due to a secondary fragmentation mechanism, including those that lead to flat bands in momentum space due to destructive quantum interference, are also discussed. The consequences of adding a (noncommuting) staggered magnetic field and a PXP term, respectively, to this model, where the former preserves the Hilbert space fragmentation while the latter destroys it, are discussed. Making the staggered magnetic field a periodic function of time defines an interacting Floquet system that also evades thermalization and has additional features like exact stroboscopic freezing of an exponentially large number of initial states at special drive frequencies. Finally, we map the model to a $\text{U}\left(1\right)$ lattice gauge theory coupled to dynamical fermions and discuss the interpretation of some of these anomalous states in this language. A class of gauge-invariant states show reduced mobility of the elementary charged excitations with only certain charge-neutral objects being mobile, suggesting a connection to fractons.

The $S=1$ Haldane state is constructed from a product of local singlet dimers in the bulk and topological states at the edges of a chain. It is a fundamental representative of topological quantum matter. Its well-known archetype, the quasi-one-dimensional ${\mathrm{SrNi}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ shows both conventional as well as unconventional magnetic Raman scattering. The former is observed as one- and two-triplet excitations with small linewidths and energies corresponding to the Haldane gap ${\mathrm{\Delta }}_H$ and the exchange coupling $J_c$ along the chain, respectively. Well-defined magnetic quasiparticles are assumed to be stabilized by interchain interactions and uniaxial single-ion anisotropy. Unconventional scattering exists as broad continua of scattering with an intensity $I\left(T\right)$ that shows fermionic statistics. Such statistics has also been observed in Kitaev spin liquids and could point to a non-Abelian symmetry. As the ground state in the bulk of ${\mathrm{SrNi}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ is topologically trivial, we suggest its fractionalization to be due to light-induced interchain exchange processes. These processes are supposed to be enhanced due to a proximity to an Ising ordered state with a quantum critical point. A comparison with ${\mathrm{SrCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$, the $S=1/2$ analog to our title compound, supports these statements.

Nonequilibrium dynamics of transverse-optical (TO) phonons and low-frequency interlayer shear and breathing modes are mapped in femtosecond midinfrared pump-probe experiments. Time-resolved changes of TO phonon absorption reveal a TO phonon lifetime of 1.2 ps, while low-frequency excitations decay with a time constant of 22 ps. The coupling of intralayer TO and interlayer motions manifests in a quasi-instantaneous redshift of the TO phonon resonance by some 10 ${\mathrm{cm}}^{-1}$. Theoretical calculations account for the coupling scenario and underline the relevance of interphonon interactions for the nonlinear phonon response.

We show that a class of $\mathcal{PT}$ symmetric non-Hermitian Hamiltonians realizing the Yang-Lee edge singularity exhibits an entanglement transition in the long-time steady state evolved under the Hamiltonian. Such a transition is induced by a level crossing triggered by the critical point associated with the Yang-Lee singularity and hence is first order in nature. At the transition, the entanglement entropy of the steady state jumps discontinuously from a volume-law to an area-law scaling. We exemplify this mechanism using a one-dimensional transverse field Ising model with additional imaginary fields, as well as the spin-1 Blume-Capel model and the three-state Potts model. We further make a connection to the forced-measurement induced entanglement transition in a Floquet nonunitary circuit subject to continuous measurements followed by post-selections. Our results demonstrate a new mechanism for entanglement transitions in non-Hermitian systems harboring a critical point.

Entanglement transitions in quantum dynamics present a novel class of phase transitions in nonequilibrium systems. When a many-body quantum system undergoes unitary evolution interspersed with monitored random measurements, the steady state can exhibit a phase transition between volume- and area-law entanglement. There is a correspondence between measurement-induced transitions in nonunitary quantum circuits in $d$ spatial dimensions and classical statistical mechanical models in $d+1$ dimensions. In certain limits these models map to percolation, but there is analytical and numerical evidence to suggest that away from these limits the universality class should generically be distinct from percolation. Intriguingly, despite these arguments, numerics on $1+1\mathrm{D}$ qubit circuits give bulk exponents which are nonetheless close to those of 2D percolation, with some possible differences in surface behavior. In the first part of this work we explore the critical properties of $2+1\mathrm{D}$ Clifford circuits. In the bulk, we find many properties suggested by the percolation picture, including several matching bulk exponents, and an inverse power law for the critical entanglement growth, $S(t,L)\sim L(1-a/t)$, which saturates to an area law. We then utilize a graph-state-based algorithm to analyze in $1+1\mathrm{D}$ and $2+1\mathrm{D}$ the critical properties of entanglement clusters in the steady state. We show that in a model with a simple geometric map to percolation—the projective transverse field Ising model—these entanglement clusters are governed by percolation surface exponents. However, in the Clifford models we find large deviations in the cluster exponents from those of surface percolation, highlighting the breakdown of any possible geometric map to percolation. Given the evidence for deviations from the percolation universality class, our results raise the question of why nonetheless many bulk properties behave similarly to those of percolation.

Recent studies have emphasized the importance of impurity scattering for the optical Higgs response of superconductors. In the dirty limit, an additional paramagnetic coupling of light to superconducting condensate arises, which drastically enhances excitation. So far, most work concentrated on the periodic driving with light, where the third-harmonic generation (THG) response of the Higgs mode was shown to be enhanced. In this paper, we extend this analysis by calculating full temperature and frequency dependence of THG to better compare the theory with current experimental setups. We additionally calculate the time-resolved optical conductivity of single- and two-band superconductors in a two-pulse quench-probe setup, where we find good agreement with existing experimental results. We use the Mattis-Bardeen approach to incorporate impurity scattering and calculate explicitly the time-evolution of the system. In contrast to previous work we calculate the response not only within a time-dependent density-matrix formalism but also in a diagrammatic picture derived from an effective action formalism, which gives a deeper insight into the microscopic processes.