We examine the orbital-selective Mott transition in the non-hybridized two-band Hubbard model using the dynamical mean-field theory. We find that the orbital-selective Mott transition could be quantitatively depicted by the local two-qubit fidelity. Furthermore, within the orbital-selective Mott phase, the combined characteristics of the two orbitals lead to the presence of quantum entanglement, which is characterized by the non-semi-integer values of local two-qubit fidelity. It is demonstrated that the Hund's coupling results in the ground states of both wide and narrow bands exhibiting the specific superposition states, indicating the existence of quantum entanglement within orbital-selective Mott phase. Without Hund's coupling, there are no specific superposition states, nor does quantum entanglement occur within the orbital-selective Mott phase. The mechanisms underlying the orbital-selective Mott transition show prominent variations depending on the presence or absence of Hund's coupling and its transverse terms.

Erbium-doped solids are a promising platform for quantum technology due to the $\mathrm{E}{\mathrm{r}}^{3+}$ 1.5-µm optical transition in the telecom C band that can offer long coherence times. To characterize the intrinsic ${\mathrm{C}}_2$ site of $\mathrm{E}{\mathrm{r}}^{3+}$ in ${\mathrm{Y}}_2{\mathrm{O}}_3$ for quantum information and photonic signal-processing applications, absorption and site-selective fluorescence spectroscopy were used to assign the $\mathrm{E}{\mathrm{r}}^{3+}{}^{4}I_{15/2}$ $\leftrightarrow$ ${}^{4}I_{13/2}$ transitions. The lowest-to-lowest transition occurs at 1535.608 nm and laser absorption determined an inhomogeneous linewidth of 780 MHz in a 0.005%-doped crystal at 3.4 K. Time-resolved fluorescence established an upper limit for the excited-state lifetime of 8.5 ms. Angular-dependent Zeeman absorption was used to determine ground- and excited-state $g$ tensors. The ${\mathrm{C}}_2$-site $g$ tensor components for the ground state are $g_x=1.60$, $g_y=4.71$, and $g_z=11.93$ with a tipping angle of α = $2.{06}^{\circ }$, whereas for the excited state the components are $g_x=1.08$, $g_y=4.36$, and $g_z=10.07$ with a tipping angle of α = $-11.7^{\circ }$. Optical decoherence can be suppressed by the application of a magnetic field parallel to $\langle 111\rangle$ crystal axes where all ${\mathrm{C}}_2$ sites become nearly magnetically equivalent with large $g$ values so that spin flips of neighboring $\mathrm{E}{\mathrm{r}}^{3+}$ ions can be simultaneously suppressed. Using this magnetic field direction, two-pulse photon echoes gave a homogeneous linewidth of 5 kHz for an applied magnetic field of 0.5 T, narrowing and saturating at 2.5 kHz at 3.5 T. Decoherence over intermediate magnetic fields was described by spectral diffusion due to $\mathrm{E}{\mathrm{r}}^{3+}\text{−}\mathrm{E}{\mathrm{r}}^{3+}$ magnetic dipole interactions driven by the direct phonon process. With $\mathrm{E}{\mathrm{r}}^{3+}$:${\mathrm{Y}}_2{\mathrm{O}}_3$ absorption at a convenient telecom laser wavelength and the ability to fabricate waveguides, our study suggests that this is a promising material system for quantum information and spectral hole-burning signal-processing applications.

Motivated by significant discrepancies between experimental observations of electron-doped cuprates and numerical results of the Hubbard and $t\text{−}J$ models, we investigate the role of intersite interactions $V$ by studying the $t\text{−}J\text{−}V$ model on square lattices. Based on large-scale density matrix renormalization group simulations, we identify the ground-state phase diagram across varying intersite interactions $V$ and doping concentration $\delta$. We find that the phase diagram with finite intersite interactions $2\lesssim V/J\lesssim 3$ offers a more accurate description of electron-doped cuprates than the conventional Hubbard and $t\text{−}J$ models. Moreover, we reveal the role of intersite interactions $V$ at varying doping levels: at light doping, intersite interactions favor Néel antiferromagnetic order, and suppress both superconductivity and charge density wave; around optimal doping, these interactions support a pseudogap-like phase while suppressing superconductivity, and we further perform the slave-boson mean-field analysis to understand the numerical results microscopically; at higher doping, the effects of intersite interactions become insignificant, with our numerical predictions suggesting the emergence of an incommensurate spin density wave phase. Our specific focus around optimal doping with various intersite interactions identifies successive phases including phase separation, uniform $d$-wave SC, and a pseudogap-like phase, and reveals a relative insensitivity of the charge density wave to superconductivity. Our study suggests the $t\text{−}J\text{−}V$ model as the minimal model to capture the essential physics of electron-doped cuprates.

For decades, the thermal broadening of zero-phonon lines of single organic molecules embedded in amorphous solids was associated with their weak electron-phonon coupling to quasilocal vibrational modes. In this paper we show that the generally accepted perturbative approach for the pure dephasing problem is not applicable to the studied chromophore-doped glass, just as there is no evidence of weak coupling to individual quasilocal modes. Experimental and theoretical analysis of the temperature-dependent fluorescence excitation spectra of single tetra-$tert$-butylterrylene molecules in a polymer matrix of polyisobutylene is used as an example. Analysis of wide-range temperature measurements (9–60 K) with the nonperturbative theory of electron-phonon coupling demonstrates that the observed zero-phonon line broadening is due to the appearance of a wide spectrum of resonant modes induced by the impurity molecules themselves. This spectrum and the associated thermal broadening are obtained in the long-wave approximation for propagating waves using the density of the vibrational states for the host matrix and two quadratic coupling constants describing the interaction between the host and guest molecules. We also test the model considering the resonant modes arising from the host normal modes of a quasilocal character and find it completely incompatible with the experimental results.

Advent of new-generation materials, with flat topological bands and ultranarrow band gaps (in the order of 1 meV), poses challenges on their precise characterization. We uncover a useful connection between the integrated current noise $S\left(\omega \right)$ and the topological band gap in dispersionless quantum states, $\int d\omega [{\mathcal{S}}_{xx}^{\text{flat}}+{\mathcal{S}}_{yy}^{\text{flat}}]=C{e}^2{\mathrm{\Delta }}^2$ (in units $\hslash =1$), where $C$ is the Chern number, $e$ is electric charge, and $\mathrm{\Delta }$ is the topological band gap. This relationship may serve as a working principle for experimental probe of topological band gaps in flat band materials. Possible applications include moiré systems, such as twisted bilayer graphene and twisted transition metal dichalcogenides, where a band gap measurement in meV regime presents an experimental challenge.

The combination of the non-Hermitian skin effect (NHSE) and topological effects plays a crucial role in both fundamental and applied physics. Here, we show the interplay between the NHSE and topological effects in two- and three-dimensional quasicrystals. Unlike the delocalization of higher-order topological states (HOTSs) in crystals, we find that in non-Hermitian quasicrystals, HOTSs delocalize along the boundaries rather than in the bulk regions. Through effective analysis of the quasicrystal boundaries, we establish a physical connection between the unique lattice arrangement of quasicrystals and the delocalization of HOTSs. Our results offer some perspectives for investigating intriguing physical phenomena emerging from the combination of higher-dimensional quasicrystals, the NHSE, and topological effects, as well as for broadening their applications.

We use molecular dynamics simulations with machine-learned atomistic force fields to simulate photoexcitation of ${\mathrm{BaTiO}}_3$ by a femtosecond laser pulse whose photon energy exceeds the optical gap. We demonstrate selective displacive excitation of coherent zone-center ferroelectric mode phonons and of the strongly anharmonic central mode. We show that the direction of $\mathbf{P}$ can either be reversed by a pulse in hundreds of femtoseconds or, on a longer timescale and when combined with a weak field, switched to any one of its symmetry-equivalent directions.

The conventional wisdom suggests that transports of conserved quantities in nonintegrable quantum many-body systems at high temperatures are diffusive. Recent study suggests a potential counterexample of this paradigm. We independently complement this counterexample by directly uncovering anomalous infinite-temperature hydrodynamics in a spin-1/2 XXZ chain with power-law couplings. This model, classified as nonintegrable due to its Wigner-Dyson level-spacing statistics in the random matrix theory, exhibits a surprising superdiffusive-ballistic-superdiffusive transport transition by varying the power-law exponent of couplings for a fixed anisotropy. Our findings are verified by multiple observables, including the spin-spin autocorrelator, mean-square displacement, and spin conductivity. Interestingly, we further quantify the degree of quantum chaos using the Kullback-Leibler divergence between the entanglement entropy distributions of the model's eigenstates and a random state. Remarkably, an observed local maximum in the divergence near the transition boundary suggests a link between anomalous hydrodynamics and a suppression of quantum chaos. This work offers another deep understanding of emergent anomalous transport phenomena in a wider range of nonintegrable quantum many-body systems.

The nature of the skyrmion-crystal (SkX) formation and various multiple-$q$ phases encompassing the SkX phase are investigated by extensive Monte Carlo simulations on the frustrated $J_1-J_3$ triangular-lattice Heisenberg model with the weak easy-axis magnetic anisotropy. Phase diagrams in the temperature $T$ vs magnetic-field $H$ plane are constructed, leading to a rich variety of multiple-$q$ phases. The anisotropy stabilizes the SkX state down to $T=0$ at intermediate fields, while in the lower-field range the SkX state becomes only metastable, and new multiple-$q$ states with a broken $C_3$ symmetry are instead stabilized. Implications to experiments are discussed.

We develop an efficient real-time simulation method for the spin-charge coupled system in the velocity gauge. This method enables us to compute the real-time simulation for the two-dimensional system with a complex spin texture. We focus on the effect of the collective excitation of the localized spins on the electronic transport properties of the nontrivial topological state in real space. To investigate this effect, we calculate the linear optical conductivity by calculating the real-time evolution of the Kondo lattice model on the triangular lattice, which hosts an all-in/all-out (triple-$Q$) magnetic structure. In the linear conductivity spectra, we observe multiple peaks below the band gap regime, attributed to the resonant contributions of collective modes similar to the skyrmionic system, alongside broadband modifications resulting from off-resonant spin dynamics. This result shows that the collective excitation, similar to the skyrmionic system, influences the optical response of the electron systems based on symmetry analysis. We elucidate the interference between the contributions from the different spin excitations to the optical conductivity in the multiple-spin texture, pointing out the mode-dependent electrical activity. We show the complex interplay between the complex spin texture and the itinerant electrons in the two-dimensional spin-charge coupled system.

Quantum spin liquids are exotic many-body states featured with long-range entanglement and fractional anyon quasiparticles. Quantum phase transitions of spin liquids are particularly interesting problems related with novel phenomena of anyon condensation and anyon confinement. Here we study a quantum dimer model, which implements a transition between a ${\mathbb{Z}}_2$ spin liquid (${\mathbb{Z}}_2\mathrm{SL}$) and a valence bond solid (VBS) on the kagome lattice. The transition is driven by the condensation of vison excitation of the ${\mathbb{Z}}_2$ spin liquid, which impacts on other anyon excitations especially leading to the confinement of spinon excitations. By numerical exact diagonalization of the dimer model, we directly measure the vison condensation using vison string operators, and explicitly check a confining potential acting on spinon excitations in the VBS state. It is observed that topological degeneracy of the spin-liquid state is lifted concomitantly with the vison condensation. The dimer ordering pattern of the VBS state is identified by investigating dimer structure factor. Furthermore, we find an interesting state that exhibits features of spin liquid and VBS simultaneously. We discuss the origin of the mixed behaviors and possible scenarios expected in thermodynamic limit. This work complements the previous analytical studies on the dimer model [Phys. Rev. B 87, 104408 (2013) and Phys. Rev. B 92, 205131 (2015)] by providing numerical evidences on the vison condensation and the spinon confinement in the ${\mathbb{Z}}_2\mathrm{SL}$-to-VBS transition.

By combining first-principles path integral Monte Carlo methods and mean-field techniques, we explore the properties of cylindrically trapped doubly dipolar Bose gases. We first verify the emergence of a pancake quantum droplet at low temperatures, validating previous mean-field calculations. In a regime of small doubly dipolar interactions, first-principles calculations agree with the generalized Gross-Pitaevskii equation. Such an accordance disappears in a large interaction limit. Here the path integral Monte Carlo method estimates the strong doubly dipolar regime with accuracy. In contrast, the Gross-Pitaevskii equation does not seize quantum fluctuations in full. We also provide a complete description of the system's quantum behavior in a wide range of parameters. When the system forms a droplet, the superfluid fraction exhibits an anisotropic behavior if compared to the usual Bose gas regime. Interestingly, we observe that the transition temperature from thermal gas to droplet is higher than that of the thermal gas to a Bose-Einstein condensate, indicating the robustness of the droplet against thermal fluctuations. Further, we investigate the anisotropic behavior of the superfluid fraction during the structural transition from a pancake to a cigar-shaped droplet by varying the ratio between electric and magnetic dipole interaction strengths. Our findings furnish evidence that the stability of doubly dipolar Bose-Einstein condensates can be detected in experiments by means of dysprosium atoms.

Coulomb blockade occurs for electrons tunneling into nanoislands because of the quantization of charge. Here, using spectroscopy measurements of nonmagnetic islands grown on a high-$T_{\mathrm{c}}$ superconductor [one-unit-cell (1-UC) FeSe], we systematically investigate the dynamical Coulomb blockade (DCB), which is found to reflect the Cooper pairing potential in the superconducting substrate. The tunneling spectra are acquired on single-crystalline Pb nanoislands and show a clear suppression of the tunnel current around zero-bias voltage with a gaplike structure. The observed spectral gaps can be attributed to DCB based on our comprehensive investigations, including experiments with finely varying island sizes and calculations of the spectra using the $P\left(E\right)$ theory of DCB. Our detailed analysis suggests that the observed DCB can be related to the sign-reversing pairing potential in the 1-UC FeSe substrate below the islands. The sign reversal is furthermore revealed in a transition of the superconducting gap of FeSe from a $\mathsf{U}$-like to a $\mathsf{V}$-like line shape as the distance between neighboring doublet islands is decreased, indicating the presence of a nodal-like gap as expected for a sign-reversing superconductor. Our configuration of nonmagnetic nanoislands on a high-$T_{\mathrm{c}}$ superconductor for spectroscopy measurements may serve as a local, spatially sensitive, and tunable probe for detecting the sign-reversing order parameter in unconventional superconductors.

The structural behavior of lanthanum metal has been investigated up to 227 GPa using synchrotron x-ray diffraction. Rather than having the rhombohedral or orthorhombic structures previously reported to be stable above 80 GPa, we find that La adopts the tetragonal $tI2$ structure seen previously in Ce and Pr. We show that the recent claims of a rhombohedral structure arose from the misidentification of lanthanum hydride peaks. The revised pressure dependence of the atomic volume of La above 100 GPa further constrains the stoichiometry of the superconducting ${\mathrm{LaH}}_{10}$ superhydride, which is synthesized from La above 160 GPa.

We study the one-dimensional non-Hermitian lattices with staggered on-site modulations and nonreciprocal hopping up to the next-nearest-neighboring (NNN) sites. Due to the NNN nonreciprocity, the non-Hermitian skin effect (NHSE) in the system under open boundary conditions (OBC) can be energy dependent and there will be NHSE edges in the eigenenergy spectrum, which separates the eigenstates localized at the opposite ends of the lattice. We find that the interplay between the nonreciprocal hopping and on-site modulations can reverse the direction of the skin effect and modify the position of the NHSE edge. Moreover, by tuning the system parameters, some of the eigenstates under OBC will become fully extended with the corresponding eigenenergies being imaginary under both open and periodic boundary conditions. Thus the extended states can coexist with the NHSE in the same system. The NHSE can even be completely dissolved with all the eigenstates being extended when the modulation is imaginary. Our work unveils the intricate interplay between on-site modulations and nonreciprocal hopping in non-Hermitian systems.

We investigate the critical behavior and the dynamics of the amplitude (Higgs) mode close to the superfluid-insulator quantum phase transition in an amorphous system (i.e., a system subject to topological randomness). In particular, we map the two-dimensional Bose-Hubbard Hamiltonian defined on a random Voronoi-Delaunay lattice onto a ($2+1$)-dimensional layered classical XY model with correlated topological disorder. We study the resulting model by laying recourse to classical Monte Carlo simulations. We specifically focus on the scalar susceptibility of the order parameter to study the dynamics of the amplitude mode. To do so, we harness the maximum entropy method to perform the analytic continuation of the scalar susceptibility to real frequencies. Our analysis shows that the amplitude mode remains delocalized in the presence of such topological disorder, quite at odds with its behavior in generic disordered systems, where the randomness localizes the Higgs mode. Furthermore, we show that the critical behavior of the topologically disordered system is identical to that of its translationally invariant counterpart, consistent with a modified Harris criterion. This suggests that the localization of the collective excitations in the presence of disorder is tied to the critical behavior of the quantum phase transition rather than a simple Anderson-localization-type interference mechanism.

We revisit the nature and impact of both the chiral damping (CD) and Dzyaloshinskii-Moriya interaction (DMI) in uniaxial chiral ferromagnetic nanowires with broken inversion symmetry. We propose that CD, akin to its chiral energy counterpart (DMI), can be described in terms of the Lifshitz invariants permissible by the underlying symmetry of the system. This representation offers a clearer foundation for integrating CD into the dynamics of various chiral magnetic textures. We theoretically investigate the current-induced motion of chiral domain walls (DWs), driven by both spin-transfer torque and the spin Hall effect in the presence of CD. We demonstrate that it is possible to unambiguously separate the influence of CD from that of DMI by analyzing the current-induced dynamics. In particular, below the Walker breakdown (WB), the DMI does not affect the DW velocity, whereas increases in the strength of CD result in a decrease in the DW velocity. Moreover, for the spin-orbit torque driven motion, while the DMI enhances both the WB current density and the maximum attainable velocity below the WB, the CD only enhances the WB without affecting the maximum attainable velocity below the WB. Our findings open up intriguing opportunities for exploitation in exotic magnetic textures.

We revisit critical phenomena in isotropic ferromagnets with strong dipolar interactions. The corresponding renormalization group fixed point, dipolar fixed point, was first studied in 1973 by Aharony and Fisher [Phys. Rev. Lett. 30, 559 (1973]. It is distinct from the Heisenberg fixed point, although the critical exponents are close. On the theoretical side, we discuss scale invariance without conformal invariance realized by this fixed point. We elucidate the nonrenormalization of the virial current due to a shift symmetry, and show that the same mechanism is at work in all other known local fixed points which are scale but not conformal invariant. On the phenomenological side, we discuss the relative strength of dipolar and short-range interactions. In some materials, like the europium compounds, dipolar interactions are strong, and the critical behavior is dipolar. In others, like Fe or Ni, dipolar interactions are weaker, and the Heisenberg critical behavior in a range of temperatures is followed by the dipolar behavior closer to the critical point. Some of these effects have been seen experimentally.

Electric field control of magnetization is expected to be utilized for energy-efficient nonvolatile magnetic memory application. $\mathrm{C}{\mathrm{o}}^{3+}$-substituted $\mathrm{BiFe}{\mathrm{O}}_3$ (BFCO) exhibits both ferroelectric polarization ($\sim 100\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2$) and the spin canting induced weakly ferromagnetic moment ($\sim 0.03{\mu }_B/\mathrm{f}.\mathrm{u}.$) perpendicular to each other at room temperature. Magnetization reversal by electric field was observed in a BFCO thin film. In our previous theoretical calculation, spin canted collinear structure having a weak ferromagnetic moment was stabilized when high-spin $\mathrm{C}{\mathrm{o}}^{3+}$ was substituted in $\mathrm{BiFe}{\mathrm{O}}_3$. To confirm the presence of high-spin $\mathrm{C}{\mathrm{o}}^{3+}$ in BFCO, the pressured-induced spin-state transition of $\mathrm{C}{\mathrm{o}}^{3+}$ was investigated by first-principles calculations and Co $K\beta$ synchrotron x-ray emission spectroscopy. We investigated the spin state of $\mathrm{C}{\mathrm{o}}^{3+}$ during the pressure-induced structural transition from $R3c$ to Pnma. As a result, a pressure-induced transition from high-spin to low-spin $\mathrm{C}{\mathrm{o}}^{3+}$ was predicted. Moreover, the presence of high-spin $\mathrm{C}{\mathrm{o}}^{3+}$ at ambient pressure was confirmed in $\mathrm{BiF}{\mathrm{e}}_{0.8}\mathrm{C}{\mathrm{o}}_{0.2}{\mathrm{O}}_3$ by observing the decrease of $K{\beta }^{\prime }$ peaks of Co $K\beta$ x-ray emission spectra under pressure. This study provides evidence that the weak ferromagnetism of BFCO is attributed to high-spin $\mathrm{C}{\mathrm{o}}^{3+}$ and provides insight that may help to enhance weak magnetization by substitution of other elements.