We develop a Hamiltonian-based microscopic description of laser pump induced displacive coherent phonons. The theory captures the feedback of the phonon excitation upon the electronic fluid, which is missing in the state-of-the-art phenomenological formulation. We show that this feedback leads to chirping at short timescales, even if the phonon motion is harmonic. At long times, this feedback appears as a finite phase in the oscillatory signal. We apply the theory to ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, explain the origin of the phase in the oscillatory signal reported in recent experiments, and we predict that the system will exhibit redshifted chirping at larger fluence. Our theory also opens the possibility to extract equilibrium information from coherent phonon dynamics.

We developed a tractable many-body theory in which quantum lattice fluctuations beyond the adiabatic approximation, as well as electronic fluctuations, can be described. A many-body wave function is constructed by the superposition of direct products of Slater determinants for the electrons and the coherent states of phonons. The method was applied to a one-dimensional electron system with Su-Schrieffer-Heeger electron-phonon coupling. We show that, in the heavily doped regime, the quantum lattice fluctuations due to the collective motion of charged solitons cause a power-law singularity in the wave-number dependence of the electron density $n_k$ at the Fermi wave number $k=k_F$. This indicates that a metallic state is induced by heavy doping, and it is a Tomonaga-Luttinger liquid. The current results can solve a long-standing problem for the electronic state of heavily doped polyacetylene, the coexistence of Pauli paramagnetism along with a strong infrared-active light absorption.

Optical properties of single-wall carbon nanotubes (SWCNTs) for light polarized parallel to the nanotube axis have been studied extensively, whereas their response to light polarized perpendicular to the nanotube axis has not been well explored. Here, by using a macroscopic film of highly aligned single-chirality (6,5) SWCNTs, we performed a systematic polarization-dependent optical absorption spectroscopy study. In addition to the commonly observed angular-momentum-conserving interband absorption of parallel-polarized light, which generates $E_{11}$ and $E_{22}$ excitons, we observed a small but unambiguous absorption peak whose intensity is maximum for perpendicular-polarized light. We attribute this feature to the lowest-energy cross-polarized interband absorption processes that change the angular momentum along the nanotube axis by $\pm 1$, generating $E_{12}$ and $E_{21}$ excitons. Unlike previous observations of cross-polarized excitons in polarization-dependent photoluminescence and circular dichroism spectroscopy experiments, our direct observation using absorption spectroscopy allowed us to quantitatively analyze this resonance. Specifically, we determined the energy and oscillator strength of this resonance to be 1.54 and 0.05, respectively, compared with the values for the $E_{11}$ exciton peak. These values, in combination with a comparison with theoretical calculations, in turn led to an assessment of the environmental effect on the strength of Coulomb interactions in this aligned single-chirality SWCNT film.

We have systematically measured the transport properties in the layered rhodium oxide ${\mathrm{K}}_x{\mathrm{RhO}}_2$ single crystals ($0.5\lesssim x\lesssim 0.67$), which is isostructural to the thermoelectric oxide ${\mathrm{Na}}_x{\mathrm{CoO}}_2$. We find that below $x=0.64$ the Seebeck coefficient is anomalously enhanced at low temperatures with increasing $x$, while it is proportional to the temperature as a conventional metal above $x=0.65$, suggesting the existence of a critical content $x^{*}\simeq 0.65$. For the origin of this anomalous behavior, we discuss a filling-induced Lifshitz transition, which is characterized by a sudden topological change in the cylindrical hole Fermi surfaces at the critical content $x^{*}$.

The Kondo-lattice compound ${\mathrm{CeRhIn}}_5$ displays a field-induced Fermi surface reconstruction at $B^{*}\approx 30$ T, which occurs within the antiferromagnetic state, prior to the quantum critical point at $B_{c0}\approx 50$ T. Here, in order to investigate the nature of the Fermi surface change, we measured the magnetostriction, specific heat, and magnetic torque of ${\mathrm{CeRhIn}}_5$ across a wide range of magnetic fields. Our observations uncover the field-induced itineracy of the $4f$ electrons, where above $B_{\mathrm{onset}}\approx 17$ T there is a significant enhancement of the Sommerfeld coefficient, and spin-dependent effective cyclotron masses determined from quantum oscillations. Upon crossing $B_{\mathrm{onset}}$, the temperature dependence of the specific heat also shows distinctly different behavior from that at low fields. Our results indicate that the Kondo coupling is remarkably robust upon increasing the magnetic field. This is ascribed to the delocalization of the $4f$ electrons at the Fermi surface reconstruction at $B^{*}$.

We study the nonequilibrium phase diagram and the dynamical phase transitions occurring during the prethermalization of nonintegrable quantum spin chains, subject to either quantum quenches or linear ramps of a relevant control parameter. We consider spin systems in which long-range ferromagnetic interactions compete with short-range, integrability-breaking terms. We capture the prethermal stages of the nonequilibrium evolution via a time-dependent spin-wave expansion at leading order in the spin-wave density. In order to access regimes with strong integrability breaking, instead, we perform numerical simulations based on the time-dependent variational principle with matrix product states. By investigating a large class of quantum spin models, we demonstrate that nonequilibrium fluctuations can significantly affect the dynamics near critical points of the phase diagram, resulting in a chaotic evolution of the collective order parameter, akin to the dynamics of a classical particle in a multiple-well potential subject to quantum friction. We also elucidate the signature of this novel dynamical phase on the time-dependent correlation functions of the local order parameter. We finally establish a connection with the notion of dynamical quantum phase transition associated with a possible nonanalytic behavior of the return probability amplitude, or Loschmidt echo, showing that the latter displays cusps whenever the order parameter vanishes during its real-time evolution.

For properties of interacting electron systems, Kohn-Sham (KS) theory is often favored over many-body perturbation theory (MBPT), owing to its low computational cost. However, the exact KS potential can be challenging to approximate, for example in the presence of localized subsystems where the exact potential is known to exhibit pathological features such as spatial steps. By modeling two electrons, each localized in a distinct potential well, we illustrate that the step feature has no counterpart in MBPTs (including Hartree-Fock and $GW)$ or hybrid methods involving Fock exchange because the spatial nonlocality of the self-energy renders such pathological behavior unnecessary. We present a quantitative illustration of the orbital-dependent nature of the nonlocal potential, and a numerical demonstration of Kohn's concept of the nearsightedness for self-energies, when two distant subsystems are combined, in contrast to the KS potential. These properties emphasize the value of self-energy-based approximations in developing future approaches within KS-like theories.

Based on first-principles calculations and symmetry analysis, we propose that a transition-metal rutile oxide, in particular ${\beta }^{\prime }\text{−}{\mathrm{PtO}}_2$, can host a three-dimensional topological Dirac semimetal phase. We find that ${\beta }^{\prime }\text{−}{\mathrm{PtO}}_2$ possesses an inner nodal chain structure when spin-orbit coupling is neglected. Incorporating spin-orbit coupling gaps the nodal chain while preserving a single pair of three-dimensional Dirac points protected by a screw rotation symmetry. These Dirac points are created by a band inversion of two $d$ bands, which is a realization of a Dirac semimetal phase in a correlated electron system. Moreover, a mirror plane in the momentum space carries a nontrivial mirror Chern number $n_M=-2$, which distinguishes ${\beta }^{\prime }\text{−}{\mathrm{PtO}}_2$ from the Dirac semimetals known so far, such as ${\mathrm{Na}}_3\mathrm{Bi}$ and ${\mathrm{Cd}}_3{\mathrm{As}}_2$. If we apply a perturbation that breaks the rotation symmetry and preserves the mirror symmetry, the Dirac points are gapped, and the system becomes a topological crystalline insulator.

We present analytical and numerical results on the formation and properties of the leaky stop band in one-dimensional photonic lattices. At the second stop band, one band edge mode suffers radiation loss generating guided-mode resonance whereas the other band edge mode becomes a non-leaky bound-state in the continuum. We show that the frequency location of the leaky band edge, and correspondingly the bound-state edge, is determined by superposition of Bragg processes generated by the first two Fourier harmonics of the spatial dielectric constant modulation. At the closed-band state, we discover an analytic condition for the exceptional point where frequency is fully degenerate.

The integer quantum Hall effect is a well-studied phenomenon at frequencies below about 100 Hz. The plateaus in high-frequency Hall conductivity were experimentally proven to retain up to 33 GHz, but the behavior at higher frequencies has remained largely unexplored. Using continuous-wave terahertz spectroscopy, the complex Hall conductivity of GaAs/AlGaAs heterojunctions was studied in the range of 69–1100 GHz. Above 100 GHz, the quantum plateaus are strongly smeared out and replaced by weak quantum oscillations in the real part of the conductivity. The amplitude of the oscillations decreases with increasing frequency. Near 1 THz, the Hall conductivity does not reveal any features related to the filling of Landau levels. Similar oscillations are observed in the imaginary part as well; this effect has no analogy at zero frequency. This experimental picture is in disagreement with existing theoretical considerations of the high-frequency quantum Hall effect.

Using a many-body interatomic potential for the Au-Pd system, we determine a bulk phase diagram which presents unexpected characteristics for a system with ordering tendency. Indeed, this system displays a miscibility gap between pure Pd and ${\mathrm{Au}}_c{\mathrm{Pd}}_{1-c}$ (with $c\approx 0.2$) beyond the order/disorder critical temperature of the ${\mathrm{AuPd}}_3$ compound. Enthalpic and entropic contributions of the permutation free energy are determined via Monte Carlo simulations, in particular the vibrational entropy, which is in good agreement with direct calculation. Finally, the effective site energy model, recently developed to describe the thermodynamical forces driving the bulk phase diagram, is used to demonstrate that the miscibility gap in the Au-Pd system comes from competition between elastic and chemical effects.

We study the transport properties of a one-dimensional spinful Fermi gas, after a junction of two semi-infinite subsystems held at different temperatures. The ensuing dynamics is studied by analyzing the space-time profiles of local observables emerging at large distances $x$ and times $t$, as a function of $\zeta =x/t$. At equilibrium, the system displays two distinct species of quasiparticles, naturally associated with different physical degrees of freedom. By employing the generalized hydrodynamic approach, we show that when the temperatures are finite no notion of separation can be attributed to the quasiparticles. In this case, the profiles can not be qualitatively distinguished by those associated to quasiparticles of a single species that can form bound states. On the contrary, signatures of separation emerge in the low-temperature regime, where two distinct characteristic velocities appear. In this regime, we analytically show that the profiles display a piecewise constant form and can be understood in terms of two decoupled Luttinger liquids.

The antisymmetric Dzyaloshinskii–Moriya interaction (DMI) plays a decisive role for the stabilization and control of chirality of skyrmion textures in various magnetic systems exhibiting a noncentrosymmetric crystal structure. A less studied aspect of the DMI is that this interaction is believed to be operative in the vicinity of lattice imperfections in crystalline magnetic materials, due to the local structural inversion symmetry breaking. If this scenario leads to an effect of sizable magnitude, it implies that the DMI introduces chirality into a very large class of magnetic materials—defect-rich systems such as polycrystalline magnets. Here, we show experimentally that the microstructural-defect-induced DMI gives rise to a polarization-dependent asymmetric term in the small-angle neutron scattering (SANS) cross section of polycrystalline ferromagnets with a centrosymmetric crystal structure. The results are supported by theoretical predictions using the continuum theory of micromagnetics. This effect, conjectured already by Arrott in 1963, is demonstrated for nanocrystalline terbium and holmium (with a large grain-boundary density), and for mechanically deformed microcrystalline cobalt (with a large dislocation density). Analysis of the scattering asymmetry allows one to determine the defect-induced DMI constant, $D=0.45\pm 0.07\mathrm{mJ}/{\mathrm{m}}^2$ for Tb at $100\mathrm{K}$. Our study proves the generic relevance of the DMI for the magnetic microstructure of defect-rich ferromagnets with vanishing intrinsic DMI. Polarized SANS is decisive for disclosing the signature of the defect-induced DMI, which is related to the unique dependence of the polarized SANS cross section on the chiral interactions. The findings open up the way to study defect-induced skyrmionic magnetization textures in disordered materials.

Many-body localization (MBL) addresses the absence of thermalization in interacting quantum systems, with nonergodic high-energy eigenstates behaving as ground states, only area-law entangled. However, computing highly excited many-body eigenstates using exact methods is very challenging. Instead, we show that one can address high-energy MBL physics using ground-state methods, which are much more amenable to many efficient algorithms. We find that a localized many-body ground state of a given interacting disordered Hamiltonian ${\mathcal{H}}_0$ is a very good approximation for a high-energy eigenstate of a parent Hamiltonian, close to ${\mathcal{H}}_0$ but more disordered. This construction relies on computing the covariance matrix, easily achieved using density-matrix renormalization group for disordered Heisenberg chains up to $L=256$ sites.

The feasibility of creating a Bose-Einstein condensate of magnons using a dense ensemble of nitrogen-vacancy spin defects in diamond is investigated. Through assessing a density-dependent spin-exchange interaction strength and the magnetic phase-transition temperature ($T_c$) using the Sherrington-Kirkpatrick model, the minimum temperature-dependent concentration for magnetic self-ordering is estimated. For a randomly dispersed spin ensemble, the calculated average exchange constant exceeds the average dipole interaction strengths for concentrations approximately greater than 70 ppm, while $T_c$ is estimated to exceed 10 mK beyond 90 ppm, reaching 300 K at a concentration of approximately 450 ppm. On this basis, the existence of dipole-exchange spin waves and their plane-wave dispersion is postulated and estimated using a semiclassical magnetostatic description. This is discussed along with a $T_c$-based estimate of the four-magnon scattering rate, which indicates magnons and their condensation may be detectable in thin films for concentrations greater than 90 ppm.

We report our combined experimental and theoretical study of magnetoelectric properties of an antiferromagnet $\mathrm{Sr}\left(\mathrm{TiO}\right){\mathrm{Cu}}_4{\left({\mathrm{PO}}_4\right)}_4$ in comparison with the isostructurals $\mathrm{Ba}\left(\mathrm{TiO}\right){\mathrm{Cu}}_4{\left({\mathrm{PO}}_4\right)}_4$ and $\mathrm{Pb}\left(\mathrm{TiO}\right){\mathrm{Cu}}_4{\left({\mathrm{PO}}_4\right)}_4$. The family of compounds commonly possesses a low-symmetric magnetic unit called the square cupola, which is a source of magnetoelectric responses associated with the magnetic multipoles activated under simultaneous breaking of spatial inversion and time-reversal symmetries. Measuring the full magnetization curves and the magnetic-field profiles of the dielectric constant for $\mathrm{Sr}\left(\mathrm{TiO}\right){\mathrm{Cu}}_4{\left({\mathrm{PO}}_4\right)}_4$ and comparing them with the theoretical analyses by the cluster mean-field theory, we find that the effective $S=1/2$ spin model, which was used for the previous studies for $\mathrm{Ba}\left(\mathrm{TiO}\right){\mathrm{Cu}}_4{\left({\mathrm{PO}}_4\right)}_4$ and $\mathrm{Pb}\left(\mathrm{TiO}\right){\mathrm{Cu}}_4{\left({\mathrm{PO}}_4\right)}_4$, well explains the experimental results by tuning the model parameters. Furthermore, elaborating on the phase diagram of the model, we find that the square cupolas could host a variety of magnetic multipoles, i.e., monopole, toroidal moment, and quadrupole tensor, depending on the parameters that could be modulated by deformations of the magnetic square cupolas. Our results not only provide a microscopic understanding of the series of the square cupola compounds, but also stimulate further exploration of the magnetoelectric behavior arising from cluster multipoles harboring in low-symmetric magnetic units.

Interfaces of ferroic oxides can show complex magnetic textures which have strong impact on spintronics devices. This has been demonstrated recently for interfaces with insulating antiferromagnets such as $\mathrm{BiFe}{\mathrm{O}}_3$. Here, noncollinear spin textures which can be switched in very low magnetic field are reported for conducting ferromagnetic bilayers of $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3\text{−}\mathrm{SrRu}{\mathrm{O}}_3$ (LSMO-SRO). The magnetic order and switching are fundamentally different for bilayers coherently grown in reversed stacking sequence. The SRO top layer forms a persistent exchange spring which is antiferromagnetically coupled to LSMO and drives switching in low fields of a few milliteslas. Density functional theory reveals the crucial impact of the interface termination on the strength of Mn-Ru exchange coupling across the interface. The observation of an exchange spring agrees with ultrastrong coupling for the $\mathrm{Mn}{\mathrm{O}}_2/\mathrm{SrO}$ termination. Our results demonstrate low-field switching of noncollinear spin textures at an interface between conducting oxides, opening a pathway for manipulating and utilizing electron transport phenomena in controlled spin textures at oxide interfaces.

A recent ferromagnetic resonance study [Jeon et al., Nat. Mater. 17, 499 (2018)] has reported that spin pumping into a singlet superconductor (Nb) can be greatly enhanced over the normal state when the Nb is coupled to a large spin-orbit-coupling (SOC) spin sink such as Pt. This behavior has been explained in terms of the generation of spin-polarized triplet supercurrents via SOC at the Nb/Pt interface, acting in conjunction with a nonlocally induced magnetic exchange field. Here we report the effect of adding a ferromagnet (Fe) to act as an internal source of an additional exchange field to the adjacent Pt spin sink. This dramatically enhances the spin pumping efficiency in the superconducting state compared with either Pt and Fe separately, demonstrating the critical role of the exchange field in generating superconducting spin currents in the Nb.

Ternary metal hydrides are convenient and valuable systems for investigating the metallization and superconductivity of metal hydrides because they can be synthesized under mild conditions and recovered under ambient pressure. In this study, the conducting behavior and structural phase transition of a hydrogen-rich metal hydride, $\mathrm{L}{\mathrm{i}}_5\mathrm{Mo}{\mathrm{H}}_{11}$, were investigated at pressures up to 210 GPa in a diamond anvil cell. The results showed that $\mathrm{L}{\mathrm{i}}_5\mathrm{Mo}{\mathrm{H}}_{11}$ transforms from an insulator to a poor metal at around 100 GPa. Superconductivity was observed at 100 GPa and retained until 210 GPa, and its maximum onset transition temperature was 6.5 K at 160 GPa. High-pressure synchrotron x-ray diffraction experiments revealed that the ambient-pressure hexagonal crystal structure is retained until at least 130 GPa. Furthermore, apart from the influence of pressure on the conducting behavior of $\mathrm{L}{\mathrm{i}}_5\mathrm{Mo}{\mathrm{H}}_{11}$, the effect of annealing time on the conducting and superconducting behaviors at room temperature and high pressure were also observed. We hypothesized that this time-dependent behavior is due to the restoration of the $\mathrm{Mo}{\mathrm{H}}_9$ cage structure after distortion or rotation caused by pressurization. These findings provide insight on the conducting and superconducting behaviors of ternary metal hydrides that, until recently, have been mostly studied by theoretical methods.