Laser driving in systems with competing or coupled electronic orders can lead to the enhancement of orders or even to the appearance of hidden phases without an equilibrium analog. Here we consider a model for $A_3{\mathrm{C}}_{60}$ which exhibits a unique interplay between conventional and odd-frequency (or composite) orders. In particular, we show that photodoping of the antiferromagnetic Mott insulating phase, as realized in ${\mathrm{Cs}}_3{\mathrm{C}}_{60}$, results in a paramagnetic gapped state with broken orbital symmetry. This hidden phase, which does not exist under equilibrium conditions, can be interpreted as an odd-frequency orbital-ordered state and is conceptually related to the equilibrium Jahn-Teller metal in more weakly correlated compounds. Our study demonstrates the appearance of pure odd-frequency order via the nonthermal melting of magnetic order and provides an interesting example of nonequilibrium control of electronic orders in a multiorbital system.

The crystal structures, phonon spectra, and electronic properties of uranium trioxide (${\mathrm{UO}}_3$) under high pressure have been systematically explored using a particle swarm optimization structure prediction method in conjunction with first-principles calculations. Our calculated lattice constants and the transition pressure of the two experimentally reported phases of $\gamma$- and $\eta -{\mathrm{UO}}_3$ are consistent with previous experiments. At pressures of 13, 62, 220 GPa, three new structures of $P{6}_3$/$mmc$, $Pm\overline{3}n$, and $Fm\overline{3}m$ are predicted in sequence to be thermodynamically stable. Based on our calculated elastic constants and phonon spectra, we indicate that these three phases are mechanically and dynamically stable. Interestingly, upon phase transition from $P{6}_3$/$mmc$ to $Pm\overline{3}n$, ${\mathrm{UO}}_3$ undergoes a semiconductor-to-metal electronic transition. In addition, we report results of specific heat, entropy, bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and Debye temperature. Our results provide key insights into understanding the structural as well as the electronic behaviors of ${\mathrm{UO}}_3$ under the condition of external pressure.

We study dephasing enhanced transport in boundary-driven quasiperiodic systems. Specifically, we consider dephasing modeled by current-preserving Lindblad dissipators acting on the noninteracting Aubry-André-Harper and Fibonacci bulk systems. The former is known to undergo a critical localization transition with a suppression of ballistic transport above a critical value of the potential. At the critical point, the presence of nonergodic extended states yields anomalous subdiffusion. The Fibonacci model, on the other hand, yields anomalous transport with a continuously varying exponent depending on the potential strength. By computing the covariance matrix in the nonequilibrium steady state, we show that sufficiently strong dephasing always renders the transport diffusive. The interplay between dephasing and quasiperiodicity gives rise to a maximum of the diffusion coefficient for finite dephasing, which suggests that the combination of quasiperiodic geometries and dephasing can be used to control noise enhanced transport.

Non-Hermiticity from nonreciprocal hoppings has recently been shown to induce exotic non-Hermitian phenomena. Here, we study a class of nonreciprocal quasicrystal superconductors described by one-dimensional Kitaev and dimerized Kitaev chains with complex quasiperiodic potentials. We unveil that the quasiperiodic potentials lead to a localization transition and all states become localized, but the nonreciprocal hopping is favorable to the extension of these states. We also uncover the existence of correspondence between the localization transition and the unconventional real-complex transition of the energy spectrum. Moreover, the localization transition takes place accompanied by a topological phase transition, which can be determined by calculating the energy gap closing point and topological invariants. Especially in the dimerized Kitaev chain, three distinguishable regions of the wave function are observed, i.e., an extended region, a localized region, and an intermediate region with mobility edges, which originates from the interplay between the dimerized hopping and the non-Hermitian quasiperiodic potential. In this case, the topological phase transition point is consistent with the boundary between the intermediate and localized regions. Furthermore, the dimerized hopping can induce a second localization transition, where the extended bulk states become localized and then extended again. These unusual properties provide a distinctive paradigm compared with the diagonal quasiperiodic systems.

The search for magnetic hopfions has been the focus of intense research during the last years. In this direction, and using micromagnetic simulations, we studied the magnetization reversal mechanism in toroidal nanoparticles under the action of an Oersted magnetic field. Our results evidence the nucleation of four magnetic configurations as a function of geometry, two of them being hopfion-like textures. These mechanisms are preferred for large toroidal structures. The annihilation of such texture is indicated by strong changes in the energy, which characterizes a topological transition.

Formation of the magnetic structure and exciton condensate of local magnetic excitons in strongly correlated systems near the spin crossover under high pressure is considered in terms of the effective Hamiltonian obtained from a two-band Hubbard model in the regime of strong electron correlations. The coexistence of the long-range antiferromagnetic order and exciton condensate and the occurrence of the magnetization caused by the condensation of local magnetic excitons are demonstrated. Transformation of the electronic structure of the antiferromagnetic high-spin insulator into the paramagnetic two-band correlated metal via a narrow-gap antiferromagnetic excitonic semiconductor is obtained.

Spin-orbit coupling (SOC) is generally understood as a highly localized interaction within each atom, whereby core electrons holding large $J$ splittings transfer the SOC to the valence electrons of the same atom, while their direct impact on neighbor valence orbitals is usually small. Seivane and Ferrer [Phys. Rev. Lett. 99, 183401 (2007)] proposed an approach within a tight-binding type ab initio framework assuming that the transfer of SOC from core to valence orbitals only takes place when both are on the same atom, leading to the so-called on-site approximation, which then has been successfully applied to a variety of systems. In this work we thoroughly test its general validity by confronting SOC related properties such as spin splittings, spin textures, or magnetic anisotropies calculated under the on-site approximation versus the more general approach where all the contributions to the SOC, including three-center integrals, are explicitly included. After considering a variety of systems with different dimensionalities, all presenting a strong SOC, we conclude that although the on-site approximation often provides accurate results, it breaks down in some systems where $5d$ electrons are close to the Fermi level due to their strong SOC and moderately large spatial extension. Furthermore, there are a few examples where subtle inaccuracies lead to qualitatively wrong conclusions, the most clear case being the doping of the topological surface state in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$(0001). Finally, magnetic anisotropy energies calculated under this approximation tend to be underestimated.

We present a first-principles framework to extract deformation potentials in silicon based on density-functional theory (DFT) and density-functional perturbation theory (DFPT). We compute the electronic band structures, phonon dispersion relations, and electron-phonon matrix elements to extract deformation potentials for acoustic and optical phonons for all possible processes. The matrix elements clearly show the separation between intra- and intervalley scattering in the conduction band, and quantify the strength of the scattering events in the degenerate bands of the valence band. We then use an advanced numerical Boltzmann transport equation (BTE) simulator that couples DFT electronic structures and energy/momentum-dependent scattering rates to compute the transport properties for electrons and holes. By incorporating ionized impurity scattering as well, we calculate the $n$-type and $p$-type mobility versus carrier density and make comparisons to experiments, indicating excellent agreement. The fact that the method we present uses well-established theoretical tools and requires the extraction of only a limited number of matrix elements, makes it generally computationally very attractive, especially for semiconductors with a large unit cell and lower symmetry.

Discovery of topologically protected surface states, believed to be immune to weak disorder and thermal effects, opened up a new avenue to reveal exotic fundamental science and advanced technology. While time-reversal symmetry plays the key role in most such materials, the bulk crystalline symmetries such as mirror symmetry preserve the topological properties of topological crystalline insulators (TCIs). It is apparent that any structural change may alter the topological properties of TCIs. To investigate this relatively unexplored landscape, we study the temperature evolution of Dirac fermion states in an archetypical mirror-symmetry protected TCI, SnTe, employing high-resolution angle-resolved photoemission spectroscopy and density functional theory studies. Experimental results reveal a perplexing scenario: the bulk bands observed at 22 K move nearer to the Fermi level at 60 K and again shift back to higher binding energies at 120 K. The slope of the surface Dirac bands at 22 K becomes smaller at 60 K and changes back to a larger value at 120 K. Our results from the first-principles calculations suggest that these anomalies can be attributed to the evolution of the hybridization physics with complex structural changes induced by temperature. In addition, we discover drastically reduced intensity of the Dirac states at the Fermi level at high temperatures may be due to complex evolution of anharmonicity, strain, etc. These results address the robustness of the topologically protected surface states due to thermal effects and emphasize the importance of covalency and anharmonicity in the topological properties of such emerging quantum materials.

We investigated the $1S\text{−}2P$ transitions of the indirect excitons in diamond around 70 meV in the high-frequency terahertz region via time-resolved Lyman spectroscopy. We discovered significant splitting of the $2P$ levels and attributed it to the rotational-symmetry-breaking effect owing to the cubic crystal environment. The maximum energy separation of 14.9 meV reached 18% of the excitonic Rydberg constant—83.5 ± 2.9 meV—which deviated from the binding energy of the $1S$ ground state: 93.3 ± 2.0 meV. In addition to the anisotropy of the valence bands, we found the impact of the rotational symmetry breaking of the conduction band valleys, which leads to a significant deviation from the case with direct excitons. Detailed knowledge of the ground and excited states of long-lifetime excitons provides an unprecedented opportunity to develop the study of quantum many-body physics and quantum chaos.

This study investigates the quantum correction effect on electrical conductivity using a two-dimensional Wolff Hamiltonian, which is an effective model of the spin-orbit coupling (SOC) lattice system. The nonadiabatic transition processes in impurity scattering suppress the weak antilocalization (WAL) effect. The WAL effect in the SOC lattice system strongly depends on the spin relaxation length when compared with the Hikami-Larkin-Nagaoka (HLN) theory. The spin relaxation length in Bi thin film is discussed.

The phase singularity in open systems, known as the exceptional point (EP), has revealed exotic functionalities, especially in optics—as an illustration, ultrasensitive sensors and laser beam unidirectionality. The strong sensitivity to perturbations around the EP has been suggested for sensing applications. Nevertheless, the characteristics of such highly sensitive systems can be affected by unwanted perturbations during the fabrication process. However, if one can control perturbation, it can be considered as an additional degree of freedom to create and tune EPs, enabling fascinating phenomena. In this paper, we propose an analytical method to investigate such systems. We analytically derive the general conditions of EPs in the perturbed pair of coupled ring resonators, where both resonators can be perturbed by different scatterers. Several numerical examples are employed to verify the proposed analytical method. We propose a simple experimental scheme where the predicted effects can be confirmed. It is also shown that by changing the relative position of the scatterers with respect to each other, quite interesting states such as a chiral EP in one resonator or simultaneous chiral EPs in both resonators could be observed, making such a system a highly functional tunable device which can have several applications such as quantized reflection/transmission and $q$-bits.

The tunnel magnetoresistance (TMR) is one of the most important spintronic phenomena but its reduction at finite temperature is a severe drawback for applications. Here, we reveal a crucial determinant of the drawback, that is, the $s\text{−}d$ exchange interaction between conduction $s$ and localized $d$ electrons at interfacial ferromagnetic layers. By calculating the temperature dependence of the TMR ratio in Fe/MgO/Fe(001), we show that the obtained TMR ratio significantly decreases with increasing temperature owing to the spin-flip scattering in the ${\mathrm{\Delta }}_1$ state induced by the $s\text{−}d$ exchange interaction. The material dependence of the coupling constant $J_{sd}$ is also discussed on the basis of a nonempirical method.

We explore the interplay between topologies in the momentum and real spaces to formulate a thermodynamic description of nonequilibrium injection of topological charges under external bias. We show that the edge modes engendered by the momentum-space topology can play a functional role of connecting the external reservoirs to the bulk transport of topological charges in the real space. We illustrate our general results with two examples: the spin-torque injection of skyrmions in an electrically biased integer quantum Hall system, and the vortex injection in a topological $p+ip$ superconductor coupled to heat reservoirs. Based on the universal fractional entropy of the Majorana zero modes bound to the vortices, their controllable injection proposed in this work could provide a route for creating and manipulating Majorana fermions.

Random scattering of photons in disordered one-dimensional solids gives rise to an exponential suppression of transmission, which is known as Anderson localization. Here, we experimentally study Anderson localization in a superconducting waveguide quantum electrodynamics system comprising eight individually tunable qubits coupled to a photonic continuum of a waveguide. Employing the qubit frequency control, we artificially introduce frequency disorder to the system and observe an exponential suppression of the transmission coefficient in the vicinity of its subradiant dark modes. The localization length decreases with the disorder strength, which we control in situ by varying individual qubit frequencies. Employing a one-dimensional noninteracting model of coupled qubits and photons, we are able to support and complement the experimental results. The difference between our investigation and previous studies of localization in qubit arrays is the coupling via a common waveguide, allowing us to explore the localization of mediating photons in an intrinsically open system. The experiment opens the door to the study of various localization phenomena on a new platform, which offers a high degree of control and readout possibilities.

We report the coherent-incoherent crossover of the spin Hall effect in Pd. By measuring spin-orbit torques generated by oxygen-incorporated Pd films, we find two distinct regimes of the spin Hall effect; one in which the spin Hall conductivity is insensitive to the electric conductivity, and the other in which the spin Hall conductivity decreases with decreasing electric conductivity. The crossover between the two regimes is consistent with the prediction of the coherent-incoherent crossover of the spin Hall effect, which is analogous to that of the anomalous Hall effect. Unlike the anomalous Hall effect, for which the crossover has been observed for a wide class of ferromagnets, the crossover of the spin Hall effect has been confirmed only for Pt and Pt-based alloys. The observation of the crossover of the spin Hall effect for Pd supports the generality of this phenomenon, illustrating an important correspondence between the spin Hall effect and the anomalous Hall effect.

We develop a nonequilibrium Green's function formalism to study magnonic spin transport through a strongly anisotropic ferromagnetic insulator contacted by metallic leads. We model the ferromagnetic insulator as a finite-sized one-dimensional spin chain, with metallic contacts at the first and last sites that inject and detect spin in the form of magnons. In the presence of anisotropy, these ferromagnetic magnons become elliptically polarized, and spin conservation is broken. We show that this gives rise to a novel parasitic spin conductance, which becomes dominant at high anisotropy. Moreover, the spin state of the ferromagnet becomes squeezed in the high-anisotropy regime. We show that the squeezing may be globally reduced by the application of a local spin bias.

The concept of free fermion topology has been generalized to $d$-dimensional phases that exhibit $(d-n)$-dimensional boundary modes, such as zero-dimensional (0D) corner excitations. Motivated by recent extensions of these ideas to magnetic systems, we consider 2D quantum paramagnets formed by interacting spin dimers with dispersive triplet excitations. We propose two examples of such dimer models, where the spin-gapped bosonic triplon excitations are shown to host bands with nontrivial higher-order topology. We demonstrate this using real-space Bogoliubov–de Gennes calculations that reveal the existence of near-mid band gap corner triplon modes as a signature of higher-order bulk topology. We provide an understanding of the higher-order topology in these systems via computation of bulk topological invariants as well as the construction of edge theories and study their phase transitions as we tune parameters in the model Hamiltonians. We also discuss possible experimental approaches for detecting the emergent corner triplon modes.

We report on high-field electron spin resonance studies of two iridium hexahalide compounds ${\left({\mathrm{NH}}_4\right)}_2{\mathrm{IrCl}}_6$ and ${\mathrm{K}}_2{\mathrm{IrCl}}_6$. In the paramagnetic state, our measurements reveal isotropic $g$ factors $g=1.79\left(1\right)$ for the ${\mathrm{Ir}}^{4+}$ ions, in agreement with their cubic symmetries. Most importantly, in the magnetically ordered state, we observe two magnon modes with zero-field gaps of 11.3 and 14.2 K for ${\left({\mathrm{NH}}_4\right)}_2{\mathrm{IrCl}}_6$ and ${\mathrm{K}}_2{\mathrm{IrCl}}_6$, respectively. Based on that and using linear spin-wave theory, we estimate the nearest-neighbor exchange couplings and anisotropic Kitaev interactions $J_1/k_B=10.3\mathrm{K},$ $K/k_B=0.7\mathrm{K}$ for ${\left({\mathrm{NH}}_4\right)}_2{\mathrm{IrCl}}_6,$ and $J_1/k_B=13.8\mathrm{K},$ $K/k_B=0.9\mathrm{K}$ for ${\mathrm{K}}_2{\mathrm{IrCl}}_6$, revealing the nearest-neighbor Heisenberg coupling as the leading interaction term, with only a weak Kitaev anisotropy.

We report three-dimensional pressure-field-temperature phase diagrams of ${\mathrm{Yb}}_2{\mathrm{Pd}}_2\mathrm{Sn}$ and ${\mathrm{Yb}}_2{\left({\mathrm{Pd}}_{0.9}{\mathrm{Ni}}_{0.1}\right)}_2\mathrm{Sn}$ that are constructed from electrical resistivity measurements. In ${\mathrm{Yb}}_2{\mathrm{Pd}}_2\mathrm{Sn}$, which is known to be a pressure-induced antiferromagnet, we found an unexpected magnetic-field-induced phase at a higher field than the antiferromagnetic (AFM) phase under pressure. A non-Fermi-liquid (NFL) behavior characterized by $T^{3/2}$ dependence of the resistivity is observed along the AFM phase boundary, indicating the existence of the quantum critical line. As the AFM and field-induced phases approach each other with increasing pressure, the NFL behavior is more pronounced. ${\mathrm{Yb}}_2{\left({\mathrm{Pd}}_{0.9}{\mathrm{Ni}}_{0.1}\right)}_2\mathrm{Sn}$ is known to be an antiferromagnet at ambient pressure. We also found a field-induced phase under pressure. Although the temperature-pressure phase diagram of ${\mathrm{Yb}}_2{\left({\mathrm{Pd}}_{0.9}{\mathrm{Ni}}_{0.1}\right)}_2\mathrm{Sn}$ corresponds to that of ${\mathrm{Yb}}_2{\mathrm{Pd}}_2\mathrm{Sn}$ shifted by 1.7 GPa, the field-induced phase does not separate from the AFM phase. Furthermore, the NFL behavior is less pronounced than ${\mathrm{Yb}}_2{\mathrm{Pd}}_2\mathrm{Sn}$. The Ni substitution for Pd not only acts as a chemical pressure but also produces other effects for the magnetism and its criticality.