As is known from visible-light experiments, silicon under femtosecond pulse irradiation can undergo so-called “nonthermal melting” if the density of electrons excited from the valence to the conduction band overcomes a certain critical value. Such ultrafast transition is induced by strong changes in the atomic potential energy surface, which trigger atomic relocation. However, heating of a material due to the electron-phonon coupling can also lead to a phase transition, called “thermal melting.” This thermal melting can occur even if the excited-electron density is much too low to induce nonthermal effects. To study phase transitions, and in particular, the interplay of the thermal and nonthermal effects in silicon under a femtosecond x-ray irradiation, we propose their unified treatment by going beyond the Born-Oppenheimer approximation within our hybrid model based on tight-binding molecular dynamics. With our extended model we identify damage thresholds for various phase transitions in irradiated silicon. We show that electron-phonon coupling triggers the phase transition of solid silicon into a low-density liquid phase if the energy deposited into the sample is above $\sim 0.65$ eV per atom. For the deposited doses of over $\sim 0.9$ eV per atom, solid silicon undergoes a phase transition into high-density liquid phase triggered by an interplay between electron-phonon heating and nonthermal effects. These thresholds are much lower than those predicted with the Born-Oppenheimer approximation ($\sim 2.1$ eV/atom), and indicate a significant contribution of electron-phonon coupling to the relaxation of the laser-excited silicon. We expect that these results will stimulate dedicated experimental studies, unveiling in detail various paths of structural relaxation within laser-irradiated silicon.

The study of dynamics in closed quantum systems has been revitalized by the emergence of experimental systems that are well-isolated from their environment. In this paper, we consider the closed-system dynamics of an archetypal model: spins driven across a second-order quantum critical point, which are traditionally described by the Kibble-Zurek mechanism. Imbuing the driving field with Newtonian dynamics, we find that the full closed system exhibits a robust new phenomenon—dynamic critical trapping—in which the system is self-trapped near the critical point due to efficient absorption of field kinetic energy by heating the quantum spins. We quantify limits in which this phenomenon can be observed and generalize these results by developing a Kibble-Zurek scaling theory that incorporates the dynamic field. Our findings can potentially be interesting in the context of early universe physics, where the role of the driving field is played by the inflaton or a modulus field.

Spin freezing in the $A$-site spinel ${\mathrm{FeAl}}_2{\mathrm{O}}_4$, which is a spin-liquid candidate, is studied using remnant magnetization and nonlinear magnetic susceptibility and isofield cooling and heating protocols. The remnant magnetization behavior of ${\mathrm{FeAl}}_2{\mathrm{O}}_4$ differs significantly from that of a canonical spin glass, which is also supported by analysis of the nonlinear magnetic susceptibility term ${\chi }_3\left(T\right)$. Through the power-law analysis of ${\chi }_3\left(T\right)$, a spin-freezing temperature $T_g=11.4\pm 0.9$ K and critical exponent $\gamma =1.48\pm 0.59$ are obtained. A Cole-Cole analysis of magnetic susceptibility shows the presence of broad spin relaxation times in ${\mathrm{FeAl}}_2{\mathrm{O}}_4$, however, the irreversible dc susceptibility plot discourages an interpretation based on conventional spin-glass features. The magnetization measured using the cooling-and-heating-in-unequal-fields protocol brings more insight into the magnetic nature of this frustrated magnet and reveals unconventional glassy behavior. Combining our results, we arrive at the conclusion that the present sample of ${\mathrm{FeAl}}_2{\mathrm{O}}_4$ consists of a majority spin-liquid phase with “glassy” regions embedded.

We describe why Ising spin chains with competing interactions in ${\mathrm{SrHo}}_2{\mathrm{O}}_4$ segregate into ordered and disordered ensembles at low temperatures $\left(T\right)$. Using elastic neutron scattering, magnetization, and specific heat measurements, the two distinct spin chains are inferred to have Néel $\left(\uparrow \downarrow \uparrow \downarrow \right)$ and double-Néel $\left(\uparrow \uparrow \downarrow \downarrow \right)$ ground states, respectively. Below $T_{\mathrm{N}}=0.68\left(2\right)\mathrm{K}$, the Néel chains develop three-dimensional long range order (LRO), which arrests further thermal equilibration of the double-Néel chains so they remain in a disordered incommensurate state for $T$ below $T_{\mathrm{S}}=0.52\left(2\right)\mathrm{K}$. ${\mathrm{SrHo}}_2{\mathrm{O}}_4$ distills an important feature of incommensurate low dimensional magnetism: kinetically trapped topological defects in a $\text{quasi}-d-\text{dimensional}$ spin system can preclude order in $d+1$ dimensions.

We study, both analytically and numerically, the phenomenon of energy dissipation in single-domain ferromagnetic nanoparticles driven by an alternating magnetic field. Our interest is focused on the power loss resulting from the Landau-Lifshitz-Gilbert equation, which describes the precessional motion of the nanoparticle magnetic moment. We determine the power loss as a function of the field amplitude and frequency and analyze its dependence on different regimes of forced precession induced by circularly and linearly polarized magnetic fields. The conditions to maximize the nanoparticle heating are also analyzed.

We introduce generic bosonic models exemplifying that chiral Meissner currents can persist in insulating phases of matter. We first consider interacting bosons on a two-leg ladder. The total density sector can be gapped in a bosonic Mott insulator at odd-integer filling, while the relative density sector remains superfluid due to interchain hopping. Coupling the relative density to gauge fields yields a pseudospin Meissner effect. We show that the same phase arises if the bosons are replaced by spinful fermions confined in Cooper pairs, and find a dual fermionic Mott insulator with spinon currents. We prove that, by tuning the mean density, the Mott insulator with Meissner currents turns into a low-dimensional bosonic $\nu =\frac{1}{2}$ Laughlin state for strong enough repulsive interactions across the ladder rungs. We finally discuss extensions to multileg ladders and bilayers in which spinon superfluids with Meissner currents become possible. We propose two experimental realizations, one with ultracold atoms in the setup of Atala et al. [Nat. Phys. 10, 588 (2014)] and another with Josephson junction arrays. We also address a Bose-Fermi mixture subject to a magnetic field in connection with the pseudogap phase of high-$T_c$ cuprates.

A long-standing puzzle regarding the disparity of local and long-range ${\text{CuO}}_6$ octahedral tilt correlations in the underdoped regime of ${\text{La}}_{2-x}{\text{Ba}}_x{\text{CuO}}_4$ is addressed by utilizing complementary neutron powder diffraction and inelastic neutron scattering (INS) approaches. This system is of interest because of the strong depression of the bulk superconducting transition at $x=1/8$ in association with charge and spin stripe order. The latter unidirectional order is tied to Cu-O bond-length anisotropy present in the so-called low-temperature tetragonal (LTT) phase. On warming, the lattice exhibits two sequential structural transitions, involving changes in the ${\text{CuO}}_6$ tilt pattern, first to the low-temperature orthorhombic (LTO) and then the high-temperature tetragonal (HTT) phase. Despite the changes in static order, inspection of the instantaneous local atomic structure suggests that the LTT-type tilts persist through the transitions. Analysis of the INS spectra for the $x=1/8$ composition reveals the dynamic nature of the LTT-like tilt fluctuations within the LTO and HTT phases. Within the low-temperature phase, the Cu-O bond-length splitting inferred from lattice symmetry and fitted atomic position parameters reaches a maximum of $0.3\%$ at $x=1/8$, suggesting that electron-phonon coupling may contribute to optimizing the structure to stabilize stripe order. This splitting is much too small to be resolved in the pair distribution function, and in fact we do not resolve any enhancement of the instantaneous bond-length distribution in association with stripe order. This study exemplifies the importance of a systematic approach using complementary techniques when investigating systems exhibiting a large degree of complexity and subtle structural responses.

We present neutron scattering measurements on low-energy phonons from a superconducting $\left(T_c=2.7\mathrm{K}\right)$ ${\mathrm{Sn}}_{0.8}{\mathrm{In}}_{0.2}\mathrm{Te}$ single-crystal sample. The longitudinal acoustic phonon mode and one transverse acoustic branch have been mapped out around the (002) Bragg peak for temperatures of 1.7 and 4.2 K. We observe a substantial energy width of the transverse phonons at energies comparable to twice the superconducting gap; however, there is no change in this width between the superconducting and normal states, and the precise origin of this energy width anomaly is not entirely clear. We also confirm that the compound is well ordered, with no indications of structural instability.

We report on the investigation of a superconducting anharmonic multilevel circuit that is coupled to a harmonic readout resonator. We observe multiphoton transitions via virtual energy levels of our system up to the fifth excited state. The back-action of these higher-order excitations on our readout device is analyzed quantitatively and demonstrated to be in accordance with theoretical expectation. By applying a strong microwave drive we achieve multiphoton dressing within our anharmonic circuit which is dynamically coupled by a weak probe tone. The emerging higher-order Rabi sidebands and associated Autler-Townes splittings involving up to five levels of the investigated anharmonic circuit are observed. Experimental results are in good agreement with master-equation simulations.

We investigate the in-plane field dependence of current-induced magnetic domain wall motion in asymmetric Co/Ni nanowires. The application of an in-plane field changed the direction of domain wall motion. In a systematic study, by varying the magnetic layer thickness, we show that the in-plane field induces a crossover from spin Hall torque to adiabatic spin-transfer torque for the magnetic domain wall motion. Furthermore, the dependence of the threshold current density on the in-plane longitudinal field provides a way for determining the strength of the Dzyaloshinskii-Moriya interaction.

In order to investigate physical properties around a ferromagnetic (FM) quantum transition point and a tricritical point (TCP) in the itinerant-electron metamagnetic compound UCoAl, we have performed the ${}^{59}\mathrm{Co}$ nuclear quadrupole resonance (NQR) measurement for the Fe-substituted ${\text{U(Co}}_{1-x}{\text{Fe}}_x\text{)Al}(x=0,0.5,1,\text{and}2\%)$ in zero external magnetic field. The Fe concentration dependence of ${}^{59}\mathrm{Co}-\text{NQR}$ spectra at low temperatures indicates that the first-order FM transition occurs at least above $x=1\%$. The magnetic fluctuations along the $c$ axis detected by the nuclear spin-spin relaxation rate $1/T_2$ exhibit an anomaly at $T_{\max }\sim 20\mathrm{K}$ and enhance with increasing $x$. These results are in good agreement with theoretical predictions and indicate the presence of prominent critical fluctuations at the TCP in this system.

We investigate the amplitude (Higgs) mode of the relativistic $O\left(N\right)$ model in the vicinity of the Wilson-Fisher quantum critical point in $D=4-\varepsilon$ space-time dimensions. We compute the universal part of the scalar spectral function near the transition, to leading nontrivial order in the ordered phase, and to next to leading order in both the disordered phase and the quantum critical regime. We find that, in the disordered phase, the spectral function has a threshold behavior with no Higgs-like peak, whereas in the ordered phase, the Higgs mode appears as a well defined resonance. The pole associated with this resonance is purely real in the $D\to 3+1$ limit, evolving smoothly with dimensionality to become purely imaginary at $D=2+1$ in the $N\to \infty$ limit. Our results complement previous studies of the scalar spectral function, and demonstrate that the resonance found in these studies can indeed be directly identified with the Higgs mode.

We report the crystal structure, magnetization, and neutron scattering measurements on the double perovskite ${\mathrm{Ba}}_2{\mathrm{YOsO}}_6$. The $Fm\overline{3}m$ space group is found both at 290 K and 3.5 K with cell constants $a_0=8.3541\left(4\right)$ Å and $8.3435\left(4\right)$ Å, respectively. ${\mathrm{Os}}^{5+} \left(5d^3\right)$ ions occupy a nondistorted, geometrically frustrated face-centered-cubic (fcc) lattice. A Curie-Weiss temperature $\theta \sim -700$ K suggests the presence of a large antiferromagnetic interaction and a high degree of magnetic frustration. A magnetic transition to long-range antiferromagnetic order, consistent with a type-I fcc state below $T_{\mathrm{N}}\sim 69$ K, is revealed by magnetization, Fisher heat capacity, and elastic neutron scattering, with an ordered moment of 1.65(6) ${\mu }_B$ on ${\mathrm{Os}}^{5+}$. The ordered moment is much reduced from either the expected spin-only value of $\sim 3 {\mu }_B$ or the value appropriate to $4d^3 {\mathrm{Ru}}^{5+}$ in isostructural ${\mathrm{Ba}}_2{\mathrm{YRuO}}_6$ of 2.2(1) ${\mu }_B$, suggesting a role for spin-orbit coupling (SOC). Triple-axis neutron scattering measurements of the order parameter suggest an additional first-order transition at $T=67.45$ K, and the existence of a second-ordered state. Time-of-flight inelastic neutron results reveal a large spin gap $\Delta \sim 17$ meV, unexpected for an orbitally quenched, $d^3$ electronic configuration. We discuss this in the context of the $\sim 5$ meV spin gap observed in the related ${\mathrm{Ru}}^{5+},4d^3$ cubic double perovskite ${\mathrm{Ba}}_2{\mathrm{YRuO}}_6$, and attribute the $\sim 3$ times larger gap to stronger SOC present in this heavier, $5d$, osmate system.

In this paper we present a theoretical model to describe the dynamics of paraexcitons in cuprous oxide at ultralow temperatures inside potential traps. A possible condensate is described by a generalized Gross-Pitaevskii equation. The noncondensed excitons evolve under a set of hydrodynamic equations, which were derived from a quantum Boltzmann equation. The model takes into account the finite lifetime of the excitons, the pump laser, the Auger-like two-body decay, as well as exciton-exciton and exciton-phonon scattering. The numerical results show the strong influence of the Auger effect on exciton temperatures and densities not only at high pump powers but also at ultralow temperatures. Furthermore, the excitons do not cool down to very low bath temperatures $(T_{\mathrm{B}}\ll 0.5\mathrm{K})$ under continuous wave excitation. We also compare the results of the theoretical model with experimental data.

Using the first-principles density functional theory method, we systematically investigate the structural and electronic properties of heterovalent interfaces of the lattice-matched II-VI/III-V semiconductors, i.e., ZnTe/GaSb, ZnSe/GaAs, ZnS/GaP, and ZnO/GaN. We find that, independent of the orientations, the heterovalent superlattices with period $n=6$ are energetically more favorable to form nonpolar interfaces. For the [001] interface, the stable nonpolar interfaces are formed by mixing 50% group-III with 50% group-II atoms or by mixing 50% group-V with 50% group-VI atoms; for the [111] nonpolar interfaces, the mixings are 25% group-III (II) and 75% group-II (III) atoms or 25% group-V (VI) and 75% group-VI (V) atoms. For all the nonpolar interfaces, the [110] interface has the lowest interfacial energy because it has the minimum number of II-V or III-VI “wrong bonds” per unit interfacial area. The interfacial energy increases when the atomic number of the elements decreases, except for the ZnO/GaN system. The band alignments between the II-VI and III-V compounds are drastically different depending on whether they have mixed-cation or mixed-anion interfaces, but the averaged values are nearly independent of the orientations. Similarly, other than ZnO/GaN, the valence-band offsets also increase as the atomic number of the elements decreases. The abnormal trends in interfacial energy and band alignment for ZnO/GaN are primarily attributed to the very short bond lengths in this system. The underlying physics behind these trends are explained.

In this work, we combine conductance and contact potential difference measurements in a consistent and systematic way, in steady-state and transient modes, both in the dark and under illumination. With this we obtain valuable information about the kinetics of charges at and close to the surface of GaN. We compare the processes involved in the accumulation and the decay of charge carriers generated via excitation with above and below band-gap light with varying light intensity. In particular, we probed the role played by localized defect states in the kinetics of photogenerated charges. These states are responsible for the trapping of photogenerated electrons in the space-charge region close to the surface, which explains the slow response of the photocurrent to illumination. These states are also involved in the transfer of electrons back to the surface after illumination, which results in the slow recovery of the photocurrent and the contact potential difference in the dark.

The production of self-organized nanostructures by ion beam sputtering has been of keen interest to researchers for many decades. Despite numerous experimental and theoretical efforts to understand ion-induced nanostructures, there are still many basic questions open to discussion, such as the role of erosion or curvature-dependent sputtering. In this work, a hybrid MD/kMC (molecular dynamics/kinetic Monte Carlo) multiscale atomistic model is developed to investigate these knowledge gaps, and its predictive ability is validated across the experimental parameter space. This model uses crater functions, which were obtained from MD simulations, to model the prompt mass redistribution due to single-ion impacts. Defect migration, which is missing from previous models that use crater functions, is treated by a kMC Arrhenius method. Using this model, a systematic study was performed for silicon bombarded by ${\mathrm{Ar}}^{+}$ ions of various energies (100 eV, 250 eV, 500 eV, 700 eV, and 1000 eV) at incidence angles of $0^{\circ }$ to ${80}^{\circ }.$ The simulation results were compared with experimental findings, showing good agreement in many aspects of surface evolution, such as the phase diagram. The underestimation of the ripple wavelength by the simulations suggests that surface diffusion is not the main smoothening mechanism for ion-induced pattern formation. Furthermore, the simulated results were compared with moment-description continuum theory and found to give better results, as the simulation did not suffer from the same mathematical inconsistencies as the continuum model. The key finding was that redistributive effects are dominant in the formation of flat surfaces and parallel-mode ripples, but erosive effects are dominant at high angles when perpendicular-mode ripples are formed. Ion irradiation with simultaneous sample rotation was also simulated, resulting in arrays of square-ordered dots. The patterns obtained from sample rotation were strongly correlated to the rotation speed and to the pattern types formed without sample rotation, and a critical value of about 5 rpm was found between disordered ripples and square-ordered dots. Finally, simulations of dual-beam sputtering were performed, with the resulting patterns determined by the flux ratio of the two beams and the pattern types resulting from single-beam sputtering under the same conditions.

Density functional computations of the electronic structure of small-sized nickel clusters $({\text{Ni}}_n,2\le n\le 12)$ have been carried out. We explored the potential energy surface at different spin multiplicities starting from guess structures obtained by statistical sampling. We focus this work on the search of the ground-state geometries and their magnetic properties. In some cases, close in energy geometries appear allowing a competition for the ground-state structure. For such cases, we investigated the energy profiles by searching for the transition states that connect structures belonging to different spin multiplicities.

We studied the structure of ultrathin Bi(110) films on $\mathrm{Si}\left(111\right)\sqrt{3}\times \sqrt{3}\text{−}\mathrm{B}$ substrates using scanning tunneling microscopy. Atomically flat Bi islands were nucleated on the substrates at room temperature. The edges of these islands were parallel to the short side of the Bi(110) rectangular unit cell. The islands extended along six specific orientations because the rectangular Bi(110) and rhombus $\sqrt{3}\times \sqrt{3}$ unit cells were commensurate at the interface. Bi(110) domains along different orientations coexisted and formed various domain boundary structures on the wide terraces of the islands. In particular, the domains along $\pm {87}^{\circ }$ from the $\left\{1\overline{1}0\right\}$ direction were connected perfectly on an atomic scale by changing the direction of the $p$-like bond of the in-plane zigzag chains locally at the straight domain boundary. No exclusive preference for the black phosphorus structure was observed for the Bi(110) ultrathin films, in contrast to the islands grown on the $\mathrm{Si}\left(111\right)7\times 7$ substrate.