The efficiency of the adiabatic demagnetization of the nuclear spin system (NSS) of a solid is limited, if quadrupole effects are present. Nevertheless, despite a considerable quadrupole interaction, recent experiments validated the thermodynamic description of the NSS in GaAs. This suggests that nuclear spin temperature can be used as a probe of nuclear magnetic resonances. We implement this idea by analyzing the modification of the NSS temperature in response to an oscillating magnetic field at various frequencies, an approach termed as the warm-up spectroscopy. It is tested in a $n$-GaAs sample where both mechanical strain and built-in electric field may contribute to the quadrupole splitting, yielding the parameters of electric field gradient tensors for ${}^{75}\mathrm{As}$ and both Ga isotopes, ${}^{69}\mathrm{Ga}$ and ${}^{71}\mathrm{Ga}$.

We theoretically demonstrate that the desired $p_{x,y}$-orbital honeycomb electron lattice can be readily realized by arranging CO molecules into a hexagonal lattice on a Cu(111) surface with scanning tunneling microscopy (STM). The electronic structure of the Cu surface states in the presence of CO molecules is calculated with various methods, i.e., density functional theory (DFT) simulations, the muffin-tin potential model, and the tight-binding model. Our calculations indicate that, by measuring the local density of states (LDOS) pattern using STM, the $p$-orbital surface bands can be immediately identified in experiment. We also give an analytic interpretation of the $p$-orbital LDOS pattern with the $k·p$ method. Meanwhile, different from the case of graphene, the $p$-orbital honeycomb lattice has two kinds of edge states, which can also be directly observed in STM experiment. Our work points out a feasible way to construct a $p_{x,y}$-orbital honeycomb electron lattice in a real system, which may have exotic properties, such as Wigner crystal, ferromagnetism, $f$-wave superconductivity, and the quantum anomalous Hall effect. Furthermore, we also propose a simple way to calculate and identify the modified Cu surface bands in the Cu/CO systems with the DFT simulations. Considering the recent works about a $p$-orbital square lattice in similar systems [M. R. Slot et al., Nat. Phys. 13, 672 (2017); L. Ma et al., Phys. Rev. B 99, 205403 (2019)], our work once again illustrates that the artificial electron lattice on a metal surface is an ideal platform to study the orbital physics in a controllable way.

In this work, we studied the bulk band structure of a topological insulator (TI) ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and determined the contributions of the Bi and Se orbital states to the valence bands using standing-wave excited hard x-ray photoemission spectroscopy (SW-HAXPES). This SW technique can provide the element-resolved information and extract individual Bi and Se contributions to the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ valence band. Comparisons with density-functional theory calculations (local density approximation and $\mathit{GW}$) reveal that the Bi $6s$, Bi $6p$, and Se $4p$ states are dominant in the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ HAXPES valence band. These findings pave a way for studying the element-resolved band structure and orbital contributions of this class of TIs.

We apply the dynamical mean-field approach to the four-component SU(4)-symmetric Fermi-Hubbard model to study transitions between different magnetically ordered phases as well as the hysteresis behavior in the unordered regime. At quarter filling (one particle per site) on the square lattice we identify both the two-sublattice and plaquette-ordered antiferromagnetic phases with the corresponding entropy-driven hierarchy for critical temperatures. We also analyze the behavior of thermodynamic characteristics: the local double occupancy, compressibility, and entropy per particle, which are relevant for experiments with ultracold alkaline-earth(-like) atoms in optical lattices.

We performed a comprehensive electron spin resonance, magnetization, and heat capacity study on the field-induced magnetic phase transitions in the kagome antiferromagnet $\alpha \text{−}{\mathrm{Cu}}_{3.26}{\mathrm{Mg}}_{0.74}{\left(\mathrm{OH}\right)}_6{\mathrm{Br}}_2$. With the successful preparation of single crystals, we mapped out the magnetic phase diagrams under the $c$ axis and $ab$-plane directional magnetic fields $B$. For $B\parallel c$, three-dimensional (3D) magnon Bose-Einstein condensation (BEC) is evidenced by the power law scaling of the transition temperature, $T_c\propto {(B_c-B)}^{2/3}$. For $B\parallel ab$, the transition from the canted-antiferromagnetic state to the fully polarized state is a crossover rather than a phase transition, and the characteristic temperature has a significant deviation from 3D BEC scaling. The different behaviors of the field-induced magnetic transitions for $B\parallel c$ and $B\parallel ab$ could result from the Dzyaloshinkii-Moriya (DM) interaction with the DM vector along the $c$ axis, which preserves the $c$-axis directional spin rotation symmetry and breaks the spin rotation symmetry when $B\parallel ab$. Our findings have the potential to shed light on the investigations of magnetic anisotropy on the field-induced magnon BEC in a quantum antiferromagnet.

In this work, we investigate collective modes in a nodal line semimetal with two nodal lines that have opposite spin polarization in the presence of spin population imbalance. We find the components of the polarization operator, taking into account the electron-electron exchange interaction, and obtain the dispersion relations of collective modes for the bulk and surface states. There exist four modes in the bulk, among which one is gapless and other three are gapped. The gapless surface mode is sensitive to the boundary conditions.

The evolution of quasiparticle electronic structure, exciton, and optical properties in the few-layer blue phosphorus has been studied with the many-body perturbation method. The quasiparticle band gap decreases gradually with increasing the layer number and the system will finally evolve to metallic state in the bulk form. In spite of the increasing thickness in these indirect-band-gap semiconductors, the exciton-dominated optical properties with large binding energies are still found in the few-layer systems. With increasing layer number, the overall dielectric screening gets enhanced. However, the persistent large joint density of states associated with the parallel band structure in blue phosphorus gives rise to a much larger binding energy than the value predicted in an effective screening model. Our studies have not only provided a deep understanding of excitons in 2D semiconductors but also provided solid data for further experimental exploration of the application of few-layer blue phosphorus in optoelectronics.

Topologically protected edge and junction states, previously predicted, have recently been observed in armchair graphene nanoribbon (AGNR) heterojunctions. Here, via tight-binding-based calculations, we explain the relation between the nature and number of the zero-mode edge states of finite-length AGNRs and their structure, topological invariants, and winding number. This allows us to rationalize the design of AGNR heterojunctions and superlattices with tailored phases. We show how the choice of widths, interface coupling geometry, and boundaries determines the emergence of topological states following patterns that depend on the structure and family of the constituent AGNRs. Furthermore, we prove that quantum-well-like states confined in one of the constituent ribbons develop in all the AGNR junctions irrespective of their trivial or topological character. The bipartite nature of the honeycomb lattice is determinant for the topological properties of the junctions: their electronic states can be topologically trivial or nontrivial depending on subtle differences at the boundaries of the ribbons.

Squeezing of phonons due to the nonlinear coupling to electrons is a way to enhance superconductivity as theoretically studied in a recent work [Kennes et al., Nat. Phys. 13, 479 (2017)]. We study quadratic electron-phonon interaction in the presence of phonon pumping and an additional external squeezing. Interference between these two driving sources induces a phase-sensitive enhancement of electron-electron attraction, which we find as a generic mechanism to enhance any boson-mediated interactions. The strongest enhancement of superconductivity is shown to be on the boundary with the dynamical lattice instabilities caused by driving. We propose several experimental platforms to realize our scheme.

We use a symmetry-motivated approach to analyze neutron pair distribution function data to investigate the character of the soft phonon modes in negative thermal expansion (NTE) material ${\mathrm{ReO}}_3$. This analysis shows that its local structure is dominated by an in-phase octahedral tilting mode and that the octahedral units are far less flexible to scissoring-type deformations than in the related NTE compound ${\mathrm{ScF}}_3$. The lack of flexibility in ${\mathrm{ReO}}_3$ restricts the NTE-driving phonons to a smaller region of reciprocal space, limiting the magnitude and temperature range of NTE. These results support the idea that structural flexibility is an important factor in NTE materials. Surprisingly, our results show that the local fluctuations, even at elevated temperatures, respect the symmetry and order parameter direction of the initial pressure-induced phase transition in ${\mathrm{ReO}}_3$. The result indicates that the dynamic motions associated with rigid unit modes are highly anisotropic in these systems.

We analyze the finite-size scaling of the average gap ratio and the entanglement entropy across the many-body localization (MBL) transition in one dimensional Heisenberg spin chain with quasiperiodic (QP) potential. By using the recently introduced cost-function approach, we compare different scenarios for the transition using exact diagonalization of systems up to 22 lattice sites. Our findings suggest that the MBL transition in the QP Heisenberg chain belongs to the class of the Berezinskii-Kosterlitz-Thouless transition, the same as in the case of uniformly disordered systems as advocated in recent studies. Moreover, we observe that the critical disorder strength shows a clear sublinear drift with the system size as compared to the linear drift seen in random disordered models, suggesting that the finite-size effects in the MBL transition for the QP systems are less severe than that in the random disordered scenario. Moreover, deep in the ergodic regime, we find an unexpected double-peak structure of distribution of on-site magnetizations that can be traced back to the strong correlations present in the QP potential.

We characterize the growth and spreading of operators and entanglement in two paradigmatic nonthermalizing phases—the many-body localized (MBL) phase and the random singlet phase (RSP)—using the entanglement contour and multipartite operator entanglement measures. The entanglement contour characterizes the spacetime spreading of entanglement and reveals logarithmically growing entanglement light cones in the MBL and RSP phases, sharply contrasting the linear light cones of clean, thermalizing systems. The operator entanglement characterizes scrambling, i.e., the delocalization of information. We find slow scrambling behavior in the MBL phase; the late-time value of the tripartite mutual information scales linearly with system size, but is submaximal. The tripartite logarithmic negativity is also negative and nonzero, but smaller in magnitude, revealing an intriguing distinction between classical and quantum information scrambling in the MBL phase. This is in contrast with the RSP, which, as a noninteracting model, is nonscrambling.

Recent tensor network techniques for simulating system-environment wave functions have provided profound insights into non-Markovian dissipation and decoherence in open quantum systems. Here, we propose a dynamically adaptive one-site time-dependent-variational-principle (A1TDVP) method for matrix product states in which local bond dimensions grow to capture developing system-bath entanglement. This avoids the need for multiple convergence runs with respect to bond dimensions and the unfavorable local Hilbert space scaling of two-site methods. A1TDVP is thus ideally suited for open quantum dynamics in finite-temperature bosonic environments, as the initial states typically have low bond dimension but require very large local physical dimensions. We demonstrate this with simulations of nonequilibrium heat flows through a qubit spin, finding a $30\times$ and $10\times$ speed-up over 2TDVP and 1TDVP, respectively.

Experiments in spintronics and magnonics operate with a macroscopically large number of localized spins within ferromagnetic (F) or antiferromagnetic (AF) materials, so that their nonequilibrium dynamics is standardly described by the Landau-Lifshitz (LL) equation treating localized spins as classical vectors of fixed length. However, spin is a genuine quantum degree of freedom, and even though quantum effects become progressively less important for spin value $S>1$, they exist for all $S<\infty$. While this has motivated exploration of limitations/breakdown of the LL equation—by using examples of F insulators to compare LL trajectories with quantum expectation values of localized spin operators—analogous comparison of fully quantum many-body vs quantum (for electrons)-classical (for localized spins) dynamics in systems where nonequilibrium conduction electrons are present is lacking. Here we employ quantum Heisenberg F or AF chains of $N=4$ sites, whose localized spins interact with conduction electrons via $sd$ exchange interaction, to perform such comparison by starting from unentangled pure (at zero temperature) or mixed (at finite temperature) quantum state of localized spins as the initial condition. This reveals that quantum-classical dynamics can faithfully reproduce fully quantum dynamics in the F metallic case, but only when spin $S$, Heisenberg exchange between localized spins, and $sd$ exchange are sufficiently small. Increasing any of these three parameters can lead to substantial deviations, which are explained by the dynamical buildup of entanglement between localized spins and/or between them and electrons. In the AF metallic case, substantial deviations appear even at early times, despite starting from an unentangled Néel state, which therefore poses a challenge on how to rigorously justify wide usage of the LL equation in phenomenological modeling of antiferromagnetic spintronics experiments. We also discuss finite temperature and finite size effects to demonstrate that: (i) including thermal fluctuations delays the onset of dynamical buildup of entanglement, but it does not suppress it; and (ii) no significant changes in the dynamics of this particular problem occur as we increase the chain length to $N>4$ sites, while even $N=4$ is sufficient to observe quantum-chaotic energy level statistics whose emergence in such a small quantum many-body system crucially relies on the interaction between localized spins and conduction electrons.

In the present study, the electronic, phonon, and electron-phonon interactions of titanium suboxide structures including TiO, ${\mathrm{Ti}}_2{\mathrm{O}}_3,$ ${\mathrm{Ti}}_3{\mathrm{O}}_5,$ ${\mathrm{Ti}}_4{\mathrm{O}}_7,$ and ${\mathrm{Ti}}_5{\mathrm{O}}_9$ were investigated in the framework of density-functional theory. The mechanism of superconductivity in these materials seems to be phonon mediated, probably with participation of paramagnetic spin fluctuations. The influence of pressure on the superconducting transition temperature $\left(T_C\right)$ of cubic TiO and ${\mathrm{Ti}}_4{\mathrm{O}}_7$ could be explained by this mechanism. Our calculations predict a low $T_{\mathrm{C}}$ of ∼3.5 K for ${\mathrm{Ti}}_5{\mathrm{O}}_9$. It implies that ${\mathrm{Ti}}_5{\mathrm{O}}_9$ would be a superconductor lying in the weak coupling regime.

The paradigm of Floquet engineering of topological states of matter can be generalized into the time-quasiperiodic scenario, where a lower-dimensional time-dependent system maps onto a higher-dimensional one by combining the physical dimensions with additional synthetic dimensions generated by multiple incommensurate driving frequencies. Differently from most previous works in which gapped topological phases were considered, we propose an experimentally realizable, one-dimensional chain driven by two frequencies, which maps onto a gapless Weyl semimetal in a synthetic dimension. Based on analytical reasoning and numerical simulations, we find that the nonadiabatic quantum dynamics of this system exhibit energy pumping behaviors characterized by universal functions. We also numerically find that such behaviors are robust against a considerable amount of spatial disorder.

In the rare-earth pyrochlore ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, there are strong interactions between crystal field and phonon excitations resulting in the hybridization of crystal field excitations with both acoustic and optical phonon excitations, which may be implicated in its evasion of the expected long-range ordered states. The magnetoelastic coupling mechanisms are thought to involve large quadrupolar matrix elements between the crystal field states that allow them to couple with intersecting phonons. The character of the hybridized modes can be determined by polarized neutron scattering, as is done here for the case of a crystal field-optical phonon coupling. The coupling mechanism can be further investigated by applying a magnetic field to modify the energies of the crystal field states relative to the phonon spectrum. For a weakly dispersive optical phonon and crystal field level, this has the effect of detuning the quasidegeneracy necessary for hybridization and suppressing the magnetoelastic signal. For a strongly dispersive acoustic phonon crossing a crystal field level, the magnetic field moves the crystal field level, changing the wave vector and energy at which the modes intersect. The field-driven modification of matrix elements for dipole and quadrupole operators involved in the formation of the coupled mode results in the suppression of the magnetoelastic coupling. As the crystal field states shift to higher energy and the magnetoelastic coupling is suppressed, the spin system is driven closer to classically anticipated ordered structures.

We investigate Nagaoka ferromagnetism in the two-dimensional Hubbard model with one hole using the spin-adapted [$\text{SU}\left(2\right)$ conserving] full configuration interaction quantum Monte Carlo method. This methodology gives us access to the ground-state energies of all possible spin states $S$ of finite Hubbard lattices, here obtained for lattices up to 26 sites for various interaction strengths ($U$). The critical interaction strength, $U_c$, at which the Nagaoka transition occurs is determined for each lattice and is found to be proportional to the lattice size for the larger lattices. Below $U_c$, the overall ground states are found to favour the minimal total spin ($S=\frac{1}{2}$), and no intermediate spin state is found to be the overall ground state on lattices larger than 16 sites. However, at $U_c$, the energies of all the spin states are found to be nearly degenerate, implying that large fluctuations in total spin can be expected in the vicinity of the Nagaoka transition.

In uranium-based heavy fermion systems, the $5f$ electrons display an intermediate character between partial localization and partial itinerancy, which makes the Kondo problem more complicated. Here we use scanning tunneling microscopy/spectroscopy to investigate the (001) surface of the Kondo lattice antiferromagnet ${\mathrm{USb}}_2$. Temperature-dependent $dI/dV$ spectra from 4.7 K up to 140 K reveal several peak structures around the Fermi level. Two pronounced peaks originate from the hybridization between the conduction and $5f$ electrons. We did not observe the mysteriously abrupt change of the electronic state at 45 K, which was previously reported by another scanning tunneling microscopy group and attributed to a novel first-order-like transition. Instead, we only observe continuous evolution of the $dI/dV$ spectra with temperature. Furthermore, in some scanning regions, we find significant Sb atoms missing from the top layer, which gives us the opportunity to investigate the electronic structure of the U-terminated surface. For the U-terminated surface, an additional sharp peak emerges, which is closely related to the magnetic order.

We discuss the annular electron confinement on the liquid helium surface induced by a submerged tubular electrode. For a shallow liquid layer the resulting potential has a minimum off the symmetry axis of the electrode. The ground-state angular momentum transitions that are driven by the external magnetic field can be resolved when the confinement radii in the first and second Rydberg subbands of the vertical quantization are different, e.g., for submersion depth of the tube comparable to its radius. Then, discontinuities in the main microwave absorption line appear with the period that corresponds to the subsequent magnetic flux quanta passing across the area within the ground-state confinement radius.