Single crystals of ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$ were grown for $0\le x\le 0.41$ by a flux method and at $x=0.61$ by chemical vapor transport in order to study the Te substitution effects on the quality of the single crystals and on their superconducting characteristics. From the in-plane resistivity data of the grown crystals, we found that the superconducting transition temperature $T_{\mathrm{c}}$ slightly decreases with increasing the doping level of $x$ up to around $x=0.2\text{–}0.3$, then begins to increase with $x$ at around $x=0.4$, and reaches a maximum around $x=0.6$. These experimental facts are contrasted with the results of thin films where $T_{\mathrm{c}}$ increases up to 23 K around $x=0.2$. We also observed an anomaly in the temperature dependence of the in-plane resistivity due to the tetragonal-to-orthorhombic structural transition $T_{\mathrm{s}}$, which decreases nearly linearly with increasing $x$ until finally disappearing near $x=0.5$. These features were not reported in previous studies performed with polycrystalline and thin film samples.

We investigate ultrafast dynamics from photoinduced reflectivity of ${\mathrm{Sr}}_2{\mathrm{RhO}}_4$ by using femtosecond near-infrared pulses. We observe a clear temperature dependent anomaly in its electronic dynamics which slows down below $T_{\mathrm{S}}\sim 160$ K. In addition, coherent oscillations of the 5.3 THz $A_{1g}$ phonon exhibit a ${90}^{\circ }$ shift in its initial phase across $T_{\mathrm{S}}$, indicating a structural change in octahedral rotation distortions. We propose that octahedral structure in ${\mathrm{Sr}}_2{\mathrm{RhO}}_4$ evolves around $T_{\mathrm{S}}$, and it can influence the nonequilibrium dynamics of photo-induced carriers as well as realtime phonon responses.

The order parameter cumulants of infinite matrix product ground states are evaluated across a quantum phase transition. A scheme using the Binder cumulant, finite-entanglement scaling, and scaling functions to obtain the critical point and exponents of the correlation length and cumulants is presented. Analogous to the scaling relations that relate the exponents of various thermodynamic quantities, a cumulant exponent relation is derived and used to check the consistency and relationship between the cumulant exponents. This scheme gives a numerically economical way of accurately obtaining the critical exponents. Examples of this scheme are shown for four one-dimensional models—the transverse field Ising model, the topological Kondo insulator, the $S=1$ Heisenberg chain with single-ion anisotropy and the Bose-Hubbard model. A two-dimensional model is also exemplified in the square lattice transverse field Ising model on an infinite cylinder. These exemplary systems portray a variety of local and string order parameters as well as phase transition classes that can be studied with the scaling functions and infinite matrix product states.

Pd-intercalated ${\mathrm{ErTe}}_3$ is studied as a model system to explore the effect of increasing disorder on an incommensurate two-component charge density wave (CDW). The ordering vectors of the CDW components lie along the two in-plane principal axes of the nearly tetragonal crystal structure. Using scanning tunneling microscopy (STM), we show that introducing Pd intercalants (i.e., disorder) induces CDW dislocations, which appear to be associated with each CDW component separately. Increasing Pd concentration has a stronger effect on the secondary CDW order, manifested in a higher density of dislocations, and thus increases the anisotropy (nematic character) of the CDW. Suggestive evidence of Bragg glass phases at weak disorder is also discussed.

Scanning tunneling microscopy measurements on Ag(111)/$\mathrm{Mo}{\mathrm{S}}_2$ reveal atomically flat preferred, or “magic,” heights occurring at 6, 10, and 14 atomic layers. These results are consistent with Ag growth on a variety of semiconducting substrates and correlate with electronic energy savings in electronic structure calculations of freestanding Ag(111) films. Thus, under certain conditions, Ag will spontaneously form quantized structures independent of the substrate. To explain this, we have found Fermi surface nesting vectors in the bulk Ag band structure which account for these results and the fact Ag that is gapped along the surface normal. This model extends to a range of metallic systems which exhibit electronic confinement, epitaxial growth, and minimal strain. As with $\mathrm{Au}\text{/}\mathrm{Mo}{\mathrm{S}}_2$, the Ag/$\mathrm{Mo}{\mathrm{S}}_2$ system exhibits this behavior at unusually high temperatures so that these principles might be used for control over device features at the nanometer scale under standard fabrication conditions.

We simulate the near-field effects of strong coupling between molecular excitons and localized surface plasmons, supported by aluminum nanodisks. The simulations are done using a simple model of a two-level system, implemented in a commercial electromagnetic finite-difference time-domain solver. While the Rabi splitting is present in the near field, its spectral gap is seen to be smaller than the one obtained in the far field, although it follows a clear square root dependence on the molecular density as expected. Moreover, the energy exchange between the plasmonic mode and the excitonic material is evident in “beats” within the electromagnetic near field, which are out of phase with respect to the exciton population. Our results explicitly demonstrate the collective nature of strong coupling, which is expressed by the synchronized population oscillations at the collective Rabi frequency set by the number of molecules interacting with the plasmonic mode. This analysis sheds light on strong coupling effects in the near-field region using a versatile model, which provides a powerful tool to study strong coupling, near-field effects, and light-matter interactions in general.

The formation of novel two-dimensional electron gas (2DEG) with high mobility in metal/amorphous interfaces has motivated an ongoing debate regarding the formation and novel characteristics of these 2DEGs. Here we report an optical study, based on infrared spectroscopic ellipsometry, of nonmagnetic metal and amorphous semiconducting oxide ($\mathrm{Cu}/{\mathrm{Bi}}_2{\mathrm{O}}_3$) interfaces that confirms the formation of a 2DEG with spin orbit coupling (SOC). The 2DEG optical response was simulated with a uniaxial diagonal dielectric tensor within a subnanometer thin layer, where its $x$ and $z$ component line shapes resolved in both free-electron and peaklike contributions, resulting in very similar theoretical predictions [M. Xie et al., Phys. Rev. B 89, 245417 (2014)] of a 2DEG confined in the normal direction of a perovskite interface. In particular, the small but finite conducting character of the $z$ component provides an unambiguous signature of the presence of the 2DEG in the Cu/${\mathrm{Bi}}_2{\mathrm{O}}_3$ system. Although the original constituent materials do not possess SOC, the resulting interfacial hybridization of such states induce electronic asymmetric wave functions. This work demonstrates the detection of 2DEG in amorphous crystals, allowing one to study its challenging interfacial phenomena such as SOC and interface-bulk coupling, overcoming an experimental impediment which, for decades, has held back important advancements for the understanding of 2DEGs in amorphous materials.

We present first-principles calculations of the magnetic and orbital properties of ${\mathrm{Ba}}_2{\mathrm{NaOsO}}_6$ (BNOO), a $5d^1$ Mott insulator with strong spin-orbit coupling (SOC) in its low-temperature emergent quantum phases. Our computational method takes into direct consideration recent NMR results that established that BNOO develops a local octahedral distortion preceding the formation of long-range magnetic order. We found that the two-sublattice canted ferromagnetic ground state identified in Lu et al., [Nat. Commun. 8, 14407 (2017)] is accompanied by a two-sublattice staggered orbital ordering pattern in which the $t_{2g}$ orbitals are selectively occupied as a result of strong spin-orbit coupling. The staggered orbital order found here using first-principles calculations asserts the previous proposal of Chen et al., [Phys. Rev. B 82, 174440 (2010).] and Lu et al., [Nat. Commun. 8, 14407 (2017).] that a two-sublattice magnetic structure is the very manifestation of staggered quadrupolar order. Therefore, our results affirm the essential role of multipolar spin interactions in the microscopic description of magnetism in systems with locally entangled spin and orbital degrees of freedom.

We introduce a simple but efficient method for grand-canonical twist averaging in quantum Monte Carlo calculations. By evaluating the thermodynamic grand potential instead of the ground-state total energy, we greatly reduce the sampling errors caused by twist-dependent fluctuations in the particle number. We apply this method to the electron gas and to metallic lithium, aluminum, and solid atomic hydrogen. We show that, even when using a small number of twists, grand-canonical twist averaging of the grand potential produces better estimates of ground-state energies than the widely used canonical twist-averaging approach.

Evolution of the electronic state by Y doping was explored by optical conductivity covering a wide energy and compositional region deduced from systematic reflectivity measurements of ${\mathrm{Sm}}_{1-x}{\mathrm{Y}}_x\mathrm{S}$ single crystals. Although electrons doped onto the $5d$ band by Y dopants immediately realize a metallic state, the charge dynamics do not obey a simple Drude response and the spectral intensity is condensed in the mid-infrared region. Contrary to the pressure-induced case, a metallic phase appears in a certain compositional region before reaching the valence transition. The Fermi level is shifted upwards by electron doping. Therefore, the $5d$ band must overlap with the weakly dispersive $4f$ band more deeply than in the pressure-induced case to reach the valence transition, which can explain the stability of the divalent phase up to higher temperatures in the doped system than in the pressure-induced case. The present optical study provides us with a strategy that sulfur deficiency might increase operating temperature of negative thermal expansion in this system.

We report on the electronic structure of $\alpha$-Sn films in the very low thickness regime grown on InSb(111)A. High-resolution low photon energy angle-resolved photoemission spectroscopy allows for the direct observation of the linearly dispersing two-dimensional (2D) topological surface state (TSS) that exists between the second valence band and the conduction band. The Dirac point of this TSS was found to be 200 meV below the Fermi level in 10-nm-thick films, which enables the observation of the hybridization gap opening at the Dirac point of the TSS for thinner films. The crossover to a quasi-2D electronic structure is accompanied by a full gap opening at the Brillouin-zone center, in agreement with our density functional theory calculations. We further identify the thickness regime of $\alpha$-Sn films where the hybridization gap in the TSS coexists with the topologically nontrivial electronic structure and one can expect the presence of a one-dimensional helical edge state.

We report the temperature variation of the ${}^{119}\mathrm{Sn}$-Mössbauer spectra of the antiperovskite (inverse perovskite) oxide superconductor ${\mathrm{Sr}}_{3-x}\mathrm{SnO}$. Both superconductive (Sr deficient) and nonsuperconductive (nearly stoichiometric) samples exhibit major $\gamma$-ray absorption with an isomer shift similar to that of ${\mathrm{Mg}}_2\mathrm{Sn}$. This fact shows that ${\mathrm{Sr}}_{3-x}\mathrm{SnO}$ contains the metallic anion ${\mathrm{Sn}}^{4-}$, which is rare, especially among oxides. In both samples, we observed another $\gamma$-ray absorption with a larger isomer shift, indicating that there is another ionic state of Sn with a higher oxidation number. The temperature dependence of the absorption intensities reveals that the Sn ions exhibiting larger isomer shifts have a lower energy of the local vibration. The larger isomer shift and lower vibration energy are consistent with the values estimated from the first-principles calculations for hypothetical structures with various Sr-deficiency arrangements. Therefore, we conclude that the additional $\gamma$-ray absorptions originate from the Sn atoms neighboring the Sr deficiency.

The third-rank tensor of the static spin Hall conductivity is investigated for two-dimensional (2D) topological insulators by electronic structure calculations. For highly symmetric hexagonal systems its numerical values are close to the conductance quantum $e^2/h$, independent of the gap size. 2D crystals with a square Bravais lattice present similar effects, while rectangular translational symmetry yields conductivity values much below $e^2/h$, showing that a quantum spin Hall phase is not generally characterized by a quantized spin Hall conductivity. Vertical electric fields applied to hexagonal 2D crystals strongly reduce the conductivity, despite the conservation of the quantum spin Hall state up to a critical field strength. Weak symmetry-conserving biaxial but also symmetry-lowering uniaxial strain has a minor influence as long as inverted gaps dictate the topological character. The results are discussed in terms of the atomic geometry and the Rashba contribution to the spin-orbit interaction (SOI) using a tight-binding approximation. Translational and point-group symmetry as well as SOI rule the deviation from the quantization of the spin Hall conductance.

The relaxation process of the single-phonon modes excited in an argon crystal is simulated to investigate whether the perturbation theory still works when a phonon mode is subject to a large excitation. The excited modes are attained by adding the displacements and velocities according to the contribution of the modes to those of equilibrium states. The relaxation curves exhibit exponential decay, which agrees with the perturbation theory of phonon scattering. The scattering rates obtained from the relaxation curves increase with the temperature of the excited modes. However, the difference between the scattering rates is less than that of the excited mode temperature; the typical rate of increase is 20% even when the temperature of the excited mode is 50 times larger than that of the equilibrium state. This tendency is also found in a silicon crystal, which is a more realistic and complex material. From these results, it is revealed that the relaxation time approximation is also valid even under large nonequilibrium condition. The results also show that the scattering rates of the other phonon modes increase when one mode is excited to a high temperature. The relaxation at extremely low equilibrium state temperature is also investigated. The temporal variation of the energies shows exponential-like decay and oscillation that is lacking at high temperature. The influence of the temperature of the equilibrium state and the excited mode on the oscillation amplitude and frequency is evaluated. The results indicate that the amplitude of the oscillation depends on the temperature of both, whereas the frequency is only affected by the temperature of the excitation mode.

We address the textbook problem of dynamics of a spin placed in a dc magnetic field and subjected to an ac drive. If the drive is polarized in the plane perpendicular to the dc field, then the drive photons are resonantly absorbed when the spacing between the Zeeman levels is close to the photon energy. This is the only resonance when the drive is circularly polarized. For a linearly polarized drive, additional resonances corresponding to absorption of three, five, and multiple odd numbers of photons is possible. Interaction with the environment causes the broadening of the absorption lines. We demonstrate that the interaction with environment enables the forbidden two-photon absorption. We adopt a model of the environment in the form of random telegraph noise produced by a single fluctuator. As a result of the synchronous time fluctuations of different components of the random field, the shape of the two-photon absorption line is non-Lorentzian and depends dramatically on the drive amplitude. This shape is a monotonic curve at strong drive, while, at weak drive, it develops a two-peak structure reminiscent of an induced transparency on resonance.

Discrete time crystals (DTCs) are phases of matter characterized by the presence of an observable evolving with $nT$ periodicity under a $T$-periodic Hamiltonian, where $n>1$ is an integer insensitive to small parameter variations. In particular, DTCs with $n=2$ have been extensively studied in periodically quenched and kicked spin systems in recent years. In this paper, we study the emergence of DTCs in a many-body system whose semiclassical mean-field dynamics is nonintegrable, using a rather simple model depicting a harmonically driven spin chain. We advocate to first employ a semiclassical approximation to arrive at a mean-field Hamiltonian and then identify the parameter regime at which DTCs exist, with standard tools borrowed from studies of classical chaos. Specifically, we seek symmetric-breaking solutions by examining the stable islands on the Poincaré surface of section of the mean-field Hamiltonian. We then turn to the actual many-body quantum system, evaluate the stroboscopic dynamics of the total magnetization in the full quantum limit, and verify the existence of DTCs. Our effective and straightforward approach indicates that in general DTCs are one natural aspect of many-body quantum chaos with mixed classical phase space structure. Our approach can also be applied to general time-periodic systems, which is thus promising for finding DTCs with $n>2$ and opening possibilities for exploring DTCs properties beyond their time-translational breaking features.

Artificial spin ice (ASI) systems have emerged as promising hosts for magnonic applications due to a correspondence between their magnetic configuration and spin dynamics. Though it has been demonstrated that spin-wave spectra are influenced by the ASI microstate the precise nature of this relationship has remained unclear. Recent advances in controlling the magnetic configuration of ASI make harnessing the interplay between spin dynamics and the microstate achievable. This could allow diverse applications including reconfigurable magnonic crystals and programmable microwave filters. However, extracting any novel functionality requires a full understanding of the underlying spin-wave/microstate interaction. Here, we present a systematic analysis of how the microstate of a honeycomb ASI system affects its spin-wave spectrum through micromagnetic simulations. We find the spectrum to be highly tunable via the magnetic microstate, allowing the (de)activation of spin-wave modes and band-gap tuning via magnetic reversal of individual nanoislands. Symmetries of ASI systems and the chirality of “monopole” defects are found to play important roles in determining the high-frequency magnetic response.

The dissipative light-matter coupling can cause the attraction of two energy levels, i.e., level attraction, when competing with the coherent coupling that induces usual Rabi-level splitting. The level attraction shows attractive potential for topological information processing. However, the underlying microscopic quantum mechanism of dissipative coupling still remains unclear although the behavior has been understood to root in the non-Hermitian physics, which brings difficulties in quantifying and manipulating the competition between coherence and dissipation and thereby the flexible control of level attraction. Here, by coupling a magnon mode to a cavity supporting both standing and traveling waves, we identify the traveling-wave state to be responsible for magnon-photon dissipative coupling. By characterizing the radiative broadening of a magnon linewidth, we quantify the coherent and dissipative coupling strengths and their competition. The effective magnon-photon coupling strength, as a net result of competition, is analytically presented using quantum theory to show good agreement with measurements. In this manner, we extend the control dimension of level attraction by tuning field torque on magnetization or global cavity geometry. Our findings provide insights on engineered coupled harmonic oscillator systems.