We investigate the nonequilibrium electronic structure and characteristic timescales in a candidate excitonic insulator, ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$, using time- and angle-resolved photoemission spectroscopy with a temporal resolution of 50 fs. Following a strong photoexcitation, the band gap closes transiently within 100 fs, i.e., on a timescale faster than the typical lattice vibrational period. Furthermore, we find that the characteristic time associated with the rise of the photoemission intensity above the Fermi energy decreases with increasing excitation strength, while the relaxation time of the electron population towards equilibrium shows an opposite behavior. We argue that these experimental observations can be consistently explained by an excitonic origin of the band gap in the material. The excitonic picture is supported by microscopic calculations based on the nonequilibrium Green's function formalism for an interacting two-band system. We interpret the speedup of the rise time with fluence in terms of an enhanced scattering probability between photoexcited electrons and excitons, leading to an initially faster decay of the order parameter. We show that the inclusion of electron-phonon coupling at a semiclassical level changes only the quantitative aspects of the proposed dynamics, while the qualitative features remain the same. The experimental observations and microscopic calculations allow us to develop a simple and intuitive phenomenological model that captures the main dynamics after photoexcitation in ${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$.

We investigate the ground state properties of one-dimensional hard-core bosons interacting via a variable long-range potential using the density matrix renormalization group. We show that restoring energy extensivity in the system, which is done by rescaling the interaction potential with a suitable size-dependent factor known as Kac's prescription, has a profound influence on the low-energy properties in the thermodynamic limit. While an insulating phase is found in the absence of Kac's rescaling, the latter leads to a new metallic phase that does not fall into the conventional Luttinger liquid paradigm. We discuss a scheme for the observation of this new phase using cavity-mediated long-range interactions with cold atoms. Our findings raise fundamental questions on how to study the thermodynamics of long-range interacting quantum systems.

Quantum dots are arguably one of the best platforms for optically accessible spin-based qubits. The paramount demand of extended qubit storage time can be met by using a quantum-dot-confined dark exciton: a long-lived electron-hole pair with parallel spins. Despite its name, the dark exciton reveals weak luminescence that can be directly measured. The origins of this optical activity remain largely unexplored. In this work, using the atomistic tight-binding method combined with the configuration-interaction approach, we demonstrate that atomic-scale randomness strongly affects the oscillator strength of dark excitons confined in self-assembled cylindrical InGaAs quantum dots with no need for faceting or shape-elongation. We show that this process is mediated by two mechanisms: mixing dark and bright configurations by exchange interaction, and the equally important appearance of nonvanishing optical transition matrix elements that otherwise correspond to nominally forbidden transitions in a nonalloyed case. The alloy randomness has an essential impact on both bright and dark exciton states, including their energy, emission intensity, and polarization angle. We conclude that, due to the atomic-scale alloy randomness, finding dots with the desired dark exciton properties may require exploration of a large ensemble, similarly to how dots with low bright exciton splitting are selected for entanglement generation.

$D{0}_3$-ordered ${\mathrm{Fe}}_3\mathrm{Ga}$ has been recently reported to show a giant anomalous Nernst effect due to a large Berry curvature along the interconnected nodal lines concentrated near the Fermi level. This peculiar band structure may be advantageous for spintronics device applications because one can expect a strong response to a gate-voltage application. This study evaluates the Gilbert damping constants, a key parameter governing the magnetization dynamics of epitaxial thin films of $D{0}_3\text{−}{\mathrm{Fe}}_3\mathrm{Ga}$ and bcc-Fe by ferromagnetic resonance measurements. We find that the Gilbert damping constant of $D{0}_3\text{−}{\mathrm{Fe}}_3\mathrm{Ga} [(6.0\pm 0.2)\times {10}^{-3}]$ is relatively low and comparable to that of Fe $[(2.3\pm 0.2)\times {10}^{-3}]$. The low Gilbert damping suggests that the interconnected nodal lines near the Fermi level do not hinder magnetization dynamics, making $D{0}_3\text{−}{\mathrm{Fe}}_3\mathrm{Ga}$ even more attractive as a building block of spintronics devices.

We present a comprehensive ultrasound study of the prototypical heavy-fermion material ${\mathrm{CeRhIn}}_5$, examining the origin of the enigmatic 30 T transition. For a field applied at $2^{\circ }$ from the $c$ axis, we observed two sharp anomalies in the sound velocity, at $B_m\approx 20\mathrm{T}$ and $B^{*}\approx 30\mathrm{T}$, in all the symmetry-breaking ultrasound modes at low temperatures. The lower-field anomaly corresponds to the well-known first-order metamagnetic incommensurate-to-commensurate transition. The higher-field anomaly takes place at 30 T, where an electronic-nematic transition was previously suggested to occur. Both anomalies, observed only within the antiferromagnetic state, are of similar shape, but the corresponding changes of the ultrasound velocity have opposite signs. Based on our experimental results, we suggest that a field-induced magnetic transition from a commensurate to another incommensurate antiferromagnetic state occurs at $B^{*}$. With further increasing the field angle from the $c$ axis, the anomaly at $B^{*}$ slowly shifts to higher fields, broadens, and becomes smaller in magnitude. Traced up to ${30}^{\circ }$ from the $c$ axis, it is no longer observed at ${40}^{\circ }$ below 36 T.

We synthesized two C-S-H compounds from a mixture of carbon and sulfur in hydrogen—$\mathrm{C}:{\left({\mathrm{H}}_2\mathrm{S}\right)}_2{\mathrm{H}}_2$ and from sulfur in mixed methane-hydrogen fluids—${\left(\mathrm{C}{\mathrm{H}}_4\right)}_x{\left({\mathrm{H}}_2\mathrm{S}\right)}_{\left(2-x\right)}{\mathrm{H}}_2$ at 4 GPa. X-ray synchrotron single-crystal diffraction and Raman spectroscopy have been applied to these samples up to 58 and 143 GPa, respectively. Both samples show a similar ${\mathrm{Al}}_2\mathrm{Cu}$-type $I4/mcm$ basic symmetry, while the hydrogen subsystem evolves with pressure via variously ordered molecular and extended modifications. The methane-bearing sample lowers symmetry to an orthorhombic Pnma structure after laser heating to 1400 K at 143 GPa. The results suggest that C-S-H compounds are structurally different from a common $Im\text{−}3m$ ${\mathrm{H}}_3\mathrm{S}$.

We develop a generic $\mathbf{k}·\mathbf{p}$ open momentum space method for calculating the Hofstadter butterfly of both continuum (moiré) models and tight-binding models, where the quasimomentum is directly substituted by the Landau level (LL) operators. By taking a LL cutoff (and a reciprocal lattice cutoff for continuum models), one obtains the Hofstadter butterfly with in-gap spectral flows. For continuum models such as the moiré model for twisted bilayer graphene, our method gives a sparse Hamiltonian, making it much more efficient than existing methods. The spectral flows in the Hofstadter gaps can be understood as edge states on a momentum space boundary, from which one can determine the two integers ($t_{\nu },s_{\nu }$) of a gap $\nu$ satisfying the Diophantine equation. The spectral flows can also be removed to obtain a clear Hofstadter butterfly. While $t_{\nu }$ is known as the Chern number, our theory identifies $s_{\nu }$ as a dual Chern number for the momentum space, which corresponds to a quantized Lorentz susceptibility ${\gamma }_{xy}=eB{s}_{\nu }$.

Highly nonlinear optical phenomena can provide access to properties of electronic systems which are otherwise difficult to access through conventional linear optical spectroscopies. In particular, high-harmonic generation (HHG) in crystalline solids is strikingly different from that in atomic gases, and it enables us to access electronic properties such as the band structure, Berry curvature, and valence electron density. Here, we show that polarization-resolved HHG measurements with band-gap resonant excitation can be used to probe the transition dipole moment (TDM) texture in momentum space in two-dimensional semiconductors. TDM is directly related to the internal structure of the electronic system and governs the optical properties. We study HHG in black phosphorus, which offers a simple two-band system. We observed a unique crystal-orientation dependence of the HHG yields and polarizations. Resonant excitation of band edge enables us to reconstruct the TDM texture related to the interatomic bonding structure. Our results demonstrate the potential of high-harmonic spectroscopy for probing electronic wave functions in crystalline solids.

Recently, quantum corrections to optical conductivity of disordered metals up to the UV region were observed. Although this increase of conductivity with frequency, also called anti-Drude behavior, should disappear at the electron collision frequency, such transition has never been observed, or described theoretically. Thus the knowledge of optical conductivity in a wide frequency range is of great interest. It is well known that the extrapolation of complex conductivity is ill-posed—a solution of the analytic continuation problem is not unique for data with finite accuracy. However, we show that assuming physically appropriate properties of the searched function $\sigma \left(\omega \right)$, such as symmetry, smoothness, and asymptotic solution for low and high frequencies, one can significantly restrict the set of solutions. We present a simple numerical method utilizing the radial basis function approximation and simulated annealing, which reasonably extrapolates the optical conductivity from visible frequency range down to the far-infrared and up to the ultraviolet region. The method was compared with two widely used analytic continuation techniques and resulting extrapolation obtained on MoC and NbN thin films was checked by transmission measurements across a wide frequency range.

Structural models of the prototypal phase change material Ge-Sb-Te are generated from first-principles molecular dynamics simulations with a particular attention to the ${\mathrm{Ge}}_x{\mathrm{Sb}}_x{\mathrm{Te}}_{100-2x}$ join. Constraint counting algorithms permit us to analyze the obtained amorphous networks within the framework of topological constraint (rigidity) theory. A flexible-to-rigid transition is found at $x\simeq 8.5$% which satisfies the Maxwell isostatic criterion and is characterized by an important topological disorder with large amounts of mixed geometries and miscoordinations. The angular constraint count furthermore leads to the identification of tetrahedral Ge sites whose population decreases with growing Ge/Sb content and is about 55% for ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$. Sb sites change from a dominant pyramidal geometry typical of Group V chalcogenides to a defect octahedral one which is reminiscent of the crystalline polymorph of ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$.

Fundamental block and staggered orders of magnetic Fe multipoles in ${\mathrm{Na}}_2\mathrm{Fe}{\mathrm{Se}}_2$ are classified by their symmetry and magnetoelectric properties. Our minimal structure models incorporate ferromagnetic or antiferromagnetic coupling between chains, and they are not compatible with axial ferromagnetism. The ferrate salt is valued in studies of highly correlated electrons as the only iron selenide known to possess chainlike structural units hosting ferrous cations. Axial and polar (Dirac) multipoles are compulsory in the electronic structure since Fe ions exhibit enantiomorphic symmetry in the parent ${\mathrm{K}}_2\mathrm{Zn}{\mathrm{O}}_2$-type compound. Calculated Bragg diffraction patterns for neutrons and x rays prove that the techniques can unlock individual contributions from both multipole types.

We report the results of a muon spin relaxation study of slow magnetic fluctuations in the pseudogap phase of underdoped single-crystalline ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y, y=6.77$ and 6.83. The dependence of the dynamic muon spin relaxation rate on applied magnetic field yields the rms magnitude $B_{\mathrm{loc}}^{\mathrm{rms}}$ and correlation time ${\tau }_c$ of fluctuating local fields at muon sites. The observed relaxation rates do not decrease with decreasing temperature below the pseudogap onset at $T^{*}$, as would be expected for a conventional magnetic transition; both $B_{\mathrm{loc}}^{\mathrm{rms}}$ and ${\tau }_c$ are roughly constant between $T^{*}$ and the superconducting transition. NMR relaxation rates due to putative loop-current fluctuations are estimated and found to be too small to be observed. Our results put strong constraints on the theories of the anomalous pseudogap magnetism in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$.

The Dzyaloshinskii-Moriya (DM) interaction plays an essential role in novel topological spintronics, and the ability to control this chiral interaction is of key importance. Developing a general microscopic framework to compute DM interactions using imaginary-time Green's function formalism, we theoretically show that the ac electric field component of a laser pulse induces nonequilibrium static DM interactions in an antiferromagnetic system in the presence of relativistic spin-orbit coupling. These induced DM interactions might even be anisotropic depending on the direction of magnetic moments and the laser pulse polarization. We further show that intense polarized laser pulses can in principle generate both classes of DM interactions, i.e., bulk-type and interfacial-type, in a magnetic system even though the crystal symmetry prohibits one of them in equilibrium. Our results reveal another aspect of rich behavior of periodically driven spin systems and out of equilibrium magnetic systems.

The phase transformation of noncrystalline solids is quite different from that of crystalline solids. To gain a better understanding, a kinetic model for pressure-induced phase transformation of noncrystalline solids is constructed by formulating activation- and free-energy distributions. The model is applied to permanent densification of $\mathrm{Si}{\mathrm{O}}_2$ glass, which occurs as a result of a transformation in the network structure. The model is found to be effective in reproducing interesting behavior, including the existence of an infinite number of intermediate states, each of which has a distinct compression curve like a phase, and a halt in the transformation before completion even at sufficiently high temperatures.

Controlling the anomalous Hall effect (AHE) in magnetic topological materials is an important property. Because of the close relationship between anomalous Hall conductance (AHC) and topological band (strong Berry curvature), AHC can be effectively tuned by magnetic field combined with strong spin-orbit interaction and special band structure. In this work, we observed a magnetic field driving the nonmonotonic magnetic field dependence of anomalous Hall resistivity and the sign change in magnetic-field-induced Weyl semimetal HoPtBi. The tunable ranges of the AHC and the anomalous Hall angle are $-75\sim 73{\mathrm{\Omega }}^{–1}\mathrm{c}{\mathrm{m}}^{–1}$ and $-12.3\sim 9.1\%$, respectively. Anisotropic measurements identified the magnetic field is the key factor in controlling the additional Hall term sign. Further analysis indicated that it originated from the field-induced shift of the Weyl points via exchange splitting of bands near the Fermi level. The large tunable effect of the magnetic field on the electronic band structure provides a path to tune the topological properties in this system. These findings suggest that HoPtBi is a good platform for tuning the Berry phase and AHC with the magnetic field.

We systematically study spectral characteristics of emission microwave signal by a spin current nano-oscillator (SCNO) based on $\mathrm{Ta}{\mathrm{O}}_x/\mathrm{Py}\left(3\right)/\mathrm{Pt}\left(2\right)$ trilayers as a function of current, in-plane magnetic field angle, electrostatic gating, and temperature. The current dependence of spectral characteristics shows that such SCNO exhibits a single coherent oscillation mode at low currents, and then transfers into a multimode coexistence regime with several oscillation peaks, related to spatially separated oscillation regions, at high currents. The linewidth of these modes shows an exponential temperature dependence, indicating thermally activated mode transitions or mode hopping behavior among these spatially separated oscillation regions due to the mode coupling caused by strong thermal-magnon-mediated scattering rate at high temperatures. Additionally, electrostatic gating on oscillation frequency shows a temperature-independent behavior, but gets enhanced in the strongly nonlinear oscillation regime. The enhanced phenomenon is caused by a combination of nonlinear frequency redshift and driving current shift due to electric-field modulation of current-induced spin-orbit torques. The demonstrated electric-field and current control of three-terminal SCNO provides an efficient approach to developing electrically tunable microwave generators in radio frequency integrated circuits and spin-wave-based logic gates in magnonic devices.

We study the three-band Hubbard model for the copper oxide plane of the high-temperature superconducting cuprates using determinant quantum Monte Carlo and the dynamical cluster approximation (DCA) and provide a comprehensive view of the pairing correlations in this model using these methods. Specifically, we compute the pair-field susceptibility and study its dependence on temperature, doping, interaction strength, and charge-transfer energy. Using the DCA, we also solve the Bethe-Salpeter equation for the two-particle Green's function in the particle-particle channel to determine the transition temperature to the superconducting phase on smaller clusters. Our calculations reproduce many aspects of the cuprate phase diagram and indicate that there is an “optimal” value of the charge-transfer energy for the model where $T_c$ is largest. These results have implications for our understanding of superconductivity in both the cuprates and other doped charge-transfer insulators.