Motivated by experimental observations of time-symmetry breaking behavior in a periodically driven (Floquet) system, we study a one-dimensional spin model to explore the stability of such Floquet discrete time crystals (DTCs) under the interplay between interaction and the microwave driving. For intermediate interactions and high drivings, from the time evolution of both stroboscopic spin polarization and mutual information between two ends, we show that Floquet DTCs can exist in a prethermal time regime without the tuning of strong disorder. For much weak interactions the system is a symmetry-unbroken phase, while for strong interactions it gives its way to a thermal phase. Through analyzing the entanglement dynamics, we show that large driving fields protect the prethermal DTCs from many-body localization and thermalization. Our results suggest that by increasing the spin interaction, one can drive the experimental system into optimal regime for observing a robust prethermal DTC phase.

An atomic-scale theory of the viscoelastic response of metallic glasses is derived from first principles, using a Zwanzig-Caldeira-Leggett system-bath Hamiltonian as a starting point within the framework of nonaffine linear response to mechanical deformation. This approach provides a generalized Langevin equation (GLE) as the average equation of motion for an atom or ion in the material, from which non-Markovian nonaffine viscoelastic moduli are extracted. These can be evaluated using the vibrational density of states (DOS) as input, where the boson peak plays a prominent role in the mechanics. To compare with experimental data for binary ZrCu alloys, a numerical DOS was obtained from simulations of this system, which also take electronic degrees of freedom into account via the embedded-atom method for the interatomic potential. It is shown that the viscoelastic $\alpha$-relaxation, including the $\alpha$-wing asymmetry in the loss modulus, can be very well described by the theory if the memory kernel (the non-Markovian friction) in the GLE is taken to be a stretched-exponential decaying function of time. This finding directly implies strong memory effects in the atomic-scale dynamics and suggests that the $\alpha$-relaxation time is related to the characteristic time scale over which atoms retain memory of their previous collision history. This memory time grows dramatically below the glass transition.

Acoustic sensors operating in lossy environments, such as water, require significant sensitivity to overcome the sound attenuation in the environment and thus see farther. We show here that a surprisingly large class of passive fluids has the ability to enhance the sound pressure propagating inside them without employing active actuation. Specifically, the general requirements for this remarkable property are fluid impedance higher than the impedance of the environment and negligible insertion loss as sound propagates from the environment into the high impedance fluid. We demonstrate the pressure enhancing effect by designing a broadband isotropic metafluid that increases the pressure of sound waves impinging from water. We validate the design in numerical simulations showing that significant sound pressure level increases are achievable in realistic metafluid structures in large bandwidths covering several octaves. Our approach opens up unexplored avenues towards improving acoustic transducer sensitivity, which is critical in applications, such as medical ultrasound imaging, sonar, and acoustic communications.

Kinks and antikinks of the classical ${\phi }^4$ field model are topological solutions connecting its two distinct ground states. Here we establish an analogy between the excitations of a long graphene nanoribbon buckled in the transverse direction and ${\phi }^4$ model results. Using molecular dynamics simulations, we investigated the dynamics of a buckled graphene nanoribbon with a single kink and with a kink-antikink pair. Several features of the ${\phi }^4$ model have been observed including the kink-antikink capture at low energies, kink-antikink reflection at high energies, and a bounce resonance. Our results pave the way towards the experimental observation of a rich variety of ${\phi }^4$ model predictions based on graphene.

The occurrence of bcc-fcc ($\alpha -\gamma$) and fcc-bcc ($\gamma -\delta$) phase transitions in magnetic iron stems from the interplay between magnetic excitations and lattice vibrations. However, this fact has never been confirmed by a direct dynamic simulation, treating noncollinear magnetic fluctuations and dynamics of atoms, and their coupling at a finite temperature. Starting from a large set of data generated by ab initio simulations, we derive noncollinear magnetic many-body potentials for bcc and fcc iron, describing fluctuations of atomic coordinates in the vicinity of near perfect lattice positions. We then use spin-lattice dynamic simulations to evaluate the difference between the free energies of bcc and fcc phases, assessing their relative stability within a unified dynamic picture. We find two intersections between the bcc and fcc free energy curves, which correspond to the $\alpha -\gamma$ bcc-fcc and $\gamma -\delta$ fcc-bcc phase transitions. The maximum bcc-fcc free energy difference over the temperature interval between the two phase transitions is 2 meV per atom, in agreement with other experimental and theoretical estimates.

We study the quantum phase transition between a paramagnetic and ferromagnetic metal in the presence of Rashba spin-orbit coupling in one dimension. Using bosonization, we analyze the transition by means of renormalization group, controlled by an $\varepsilon$ expansion around the upper critical dimension of two. We show that the presence of Rashba spin-orbit coupling allows for a new nonlinear term in the bosonized action, which generically leads to a fluctuation driven first-order transition. We further demonstrate that the Euclidean action of this system maps onto a classical smectic-A–C phase transition in a magnetic field in two dimensions. We show that the smectic transition is second order and is controlled by a new critical point.

FeSe single crystals have been studied by soft point-contact Andreev reflection spectroscopy. Superconducting gap features in the differential resistance $dV/dI\left(V\right)$ of point contacts such as a characteristic Andreev reflection double-minimum structure have been measured versus temperature and magnetic field. Analyzing $dV/dI$ within the extended two-gap Blonder-Tinkham-Klapwijk model allows one to extract both the temperature and magnetic field dependence of the superconducting gaps. The temperature dependence of both gaps is close to the standard BCS behavior. Remarkably, the magnitude of the double-minimum structure gradually vanishes in magnetic field, while the minima position only slightly shifts with field, indicating a weak decrease of the superconducting gaps. Analyzing the $dV/dI\left(V\right)$ spectra for 25 point contacts results in the averaged gap values $\langle {\mathrm{\Delta }}_L\rangle =1.8\pm 0.4\mathrm{meV}$ and $\langle {\mathrm{\Delta }}_S\rangle =1.0\pm 0.2$ meV and reduced values $2\langle {\mathrm{\Delta }}_L\rangle /k_B{T}_c=4.2\pm 0.9$ and $2\langle {\mathrm{\Delta }}_S\rangle /k_B{T}_c=2.3\pm 0.5$ for the large ($L$) and small ($S$) gap, respectively. Additionally, the small gap contribution was found to be within tens of percent, decreasing with both temperature and magnetic field. No signatures in the $dV/dI$ spectra were observed, testifying to a gapless superconductivity or the presence of even smaller gaps.

Superconductivity develops in bulk doped ${\mathrm{SrTiO}}_3$ and at the ${\mathrm{LaAlO}}_3$/${\mathrm{SrTiO}}_3$ interface with a dome-shaped density dependence of the critical temperature $T_c$, despite different dimensionalities and geometries. We propose that the $T_c$ dome of ${\mathrm{LaAlO}}_3$/${\mathrm{SrTiO}}_3$ is a shape resonance due to quantum confinement of superconducting bulk ${\mathrm{SrTiO}}_3$. We substantiate this interpretation by comparing the exact solutions of a three-dimensional and quasi-two-dimensional two-band BCS gap equation. This comparison highlights the role of heavy bands for $T_c$ in both geometries. For bulk ${\mathrm{SrTiO}}_3$, we extract the density dependence of the pairing interaction from the fit to experimental data. We apply quantum confinement in a square potential well of finite depth and calculate $T_c$ in the confined configuration. We compare the calculated $T_c$ to transport experiments and provide an explanation as to why the optimal $T_c$'s are so close to each other in two-dimensional interfaces and the three-dimensional bulk material.

We report a comprehensive ${}^{139}\mathrm{La}$ and ${}^{63}\mathrm{Cu}$ nuclear magnetic resonance study on ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ $\left(0.07\le x\le 0.2\right)$ single crystals. The ${}^{139}\mathrm{La}$ spin-lattice relaxation rate ${}^{139}\mathrm{T}_1^{-1}$ is drastically influenced by Sr doping $x$ at low temperatures. A detailed field dependence of ${}^{139}\mathrm{T}_1^{-1}$ at $x=1/8$ suggests that charge ordering induces the critical slowing down of spin fluctuations toward glassy spin order and competes with superconductivity. On the other hand, the ${}^{63}\mathrm{Cu}$ relaxation rate ${}^{63}\mathrm{T}_1^{-1}$ is well described by a Curie-Weiss law at high temperatures, yielding the Curie-Weiss temperature $\mathrm{\Theta }$ as a function of doping. $\mathrm{\Theta }$ changes sharply through a critical hole concentration $x_c\sim 0.09$. $x_c$ appears to correspond to the delocalization limit of doped holes, above which the bulk nature of superconductivity is established.

Motivated by recent experimental progress on measuring various correlation functions in systems of ultracold atoms in optical lattices, we study properties of the Bose-Hubbard model in external synthetic magnetic field to describe transport phenomena in a multiband strongly interacting bosonic systems. We calculate the conductivity both in the Mott insulator and superfluid phases and investigate its two main contributions: intra- and interband. It appears that the interband processes dominate the transport properties by at least an order of magnitude. Also, at finite temperatures, additional transport channels appear due to coupling of the thermally excited particles or holes to the external field.

We analyze the electronic properties of the recently discovered stoichiometric superconductor ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ by combining an ab initio approach and a projection of the band structure to a low-energy tight-binding Hamiltonian, based on the maximally localized Wannier orbitals of the $3d$ Fe states. We identify the key symmetries as well as differences and similarities in the electronic structure between ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ and the parent systems ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ and ${\mathrm{KFe}}_2{\mathrm{As}}_2$. In particular, we find ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ to have a significantly more quasi-two-dimensional electronic structure than the latter systems. Finally, we study the superconducting instabilities in ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ by employing the leading angular harmonics approximation and find two potential $A_{1g}$-symmetry representations of the superconducting gap to be the dominant instabilities in this system.

A density functional theory study of atomic defects and dopants in ternary Z-phase transition-metal nitrides $\mathrm{Cr}M\mathrm{N}$ with $M=\mathrm{V}$, Nb, or Ta is presented. Various defect formation energies of native point defects and of substitutional atoms of other metal elements which are abundant in the steel as well are evaluated. The dependence thereof on the thermodynamic environment, i.e., the chemical conditions of a growing Z-phase precipitate, is studied, and different growth scenarios are compared. The results obtained may help to relate results of experimental atomic-scale analysis by atom probe tomography or transmission electron microscopy to the theoretical modeling of the formation process of the Z phase from binary transition-metal nitrides.

Kinetic Monte Carlo simulations have been performed to investigate the evolution of ordered domains in model alloys under irradiation. The alloys investigated were equiatomic binary alloys on a simple square lattice with first and second nearest-neighbor interactions, chosen so that a $2\times 2$ ordered structure is the equilibrium phase below a critical order-disorder transition temperature $T_{\mathrm{c}}$. The ratio of second to first nearest-neighbor interactions $R$ was varied from 0 to 0.45 to explore the effect of the thermodynamic frustrations induced by the proximity of the $2\times 1$ phase boundary, which occurs at $R=0.5$ for $T=0$. The atomic mixing produced by nuclear collisions was modeled by forcing the ballistic exchange of pairs of atoms at a controlled rate ${\mathrm{\Gamma }}_{\mathrm{b}}$. This disordering process competed with thermodynamic reordering, resulting in nonequilibrium steady states. Two trivial steady states were found, a disordered state at high ${\mathrm{\Gamma }}_{\mathrm{b}}$ and low $T$, and a long-range ordered state at low ${\mathrm{\Gamma }}_{\mathrm{b}}$ and low $T$. In the $R=0.45$ alloy, however, a third steady state was identified at intermediate ${\mathrm{\Gamma }}_{\mathrm{b}}$ and $T$ values, where multiple long-range ordered domains coexisted dynamically. It is shown that this state of patterning of order resulted from the coupling of the thermodynamic frustrations present in that alloy with the disorder introduced by irradiation. The practical relevance of this novel mechanism for patterning of order under irradiation is discussed in the context of recent observations of domain coexistence in irradiated ${\mathrm{Cu}}_3\mathrm{Au}$.

For a molecule moving near a single metal surface at equilibrium, following von Oppen and coworkers [N. Bode, S. V. Kusminskiy, R. Egger, and F. von Oppen, Beilstein J. Nanotechnol. 3, 144 (2012)] and using a nonequilibrium Green's-function (NEGF) approach, we derive a very general form of electronic friction that includes non-Condon effects. We then demonstrate that the resulting NEGF friction tensor agrees exactly with the Head-Gordon–Tully model, provided that finite temperature effects are incorporated correctly. The present results are in agreement with our recent claim that there is only one universal electronic friction tensor arising from the Born-Oppenheimer approximation [W. Dou, G. Miao, and J. E. Subotnik, Phys. Rev. Lett. 119, 046001 (2017)].

We use the color diffusion (CD) algorithm in nonequilibrium (accelerated) ab initio molecular dynamics simulations to determine Ti monovacancy jump frequencies in NaCl-structure titanium nitride (TiN), at temperatures ranging from 2200 to 3000 K. Our results show that the CD method extended beyond the linear-fitting rate-versus-force regime [Sangiovanni et al., Phys. Rev. B 93, 094305 (2016)] can efficiently determine metal vacancy migration rates in TiN, despite the low mobilities of lattice defects in this type of ceramic compound. We propose a computational method based on gamma-distribution statistics, which provides unambiguous definition of nonequilibrium and equilibrium (extrapolated) vacancy jump rates with corresponding statistical uncertainties. The acceleration-factor achieved in our implementation of nonequilibrium molecular dynamics increases dramatically for decreasing temperatures from 500 for $T$ close to the melting point $T_m$, up to 33 000 for $T\approx 0.7T_m$.

The thermalization of fast electrons by phonons is studied in CsI, NaI, ${\mathrm{SrI}}_2$, and ${\mathrm{YAlO}}_3$. This numerical study uses an improvement to a recently developed method based on a density functional perturbation description of the phonon modes that provides a way to go beyond widely used phonon models based on binary crystals. The method is compared to standard ab initio approaches to the electron-phonon interaction. Improvements to this method are described, and scattering rates are presented and discussed. The relative activity of the numerous phonon modes in materials with complicated structures is discussed, and a simple criterion for finding the modes that scatter strongly is presented.

We study the magnetic properties of epitaxial double-wedge CoO/Fe bilayers grown on MgO(001). We present a comprehensive set of data derived from the hysteresis loops measured as a function of temperature in a wide range of CoO and Fe thicknesses using Kerr microscopy imaging. We focus on relatively high temperatures and CoO layers with small thicknesses to address the onset of the exchange bias. We identify a characteristic CoO thickness of 30–40 Å, above which the films can be considered bulklike. However, a considerable exchange bias is still observed even for CoO films as thin as 12 Å if the temperature is sufficiently low. From the results of the x-ray photoelectron spectroscopy measurements, we deduce the presence of an interfacial iron oxide layer of mixed stoichiometry and a uniform composition of the CoO layer along the CoO wedge.

We explore remanent magnetization $\left(\mu \right)$ as a function of time and temperature, in a variety of rhombohedral antiferromagnets (AFMs) which are also weak ferromagnets (WFMs) and piezomagnets (PzMs). These measurements, across samples with length scales ranging from nano to bulk, firmly establish the presence of a remanence that is quasistatic in nature and exhibits a counterintuitive magnetic field dependence. These observations unravel an ultraslow magnetization relaxation phenomenon related to this quasistatic remanence. This feature is also observed in a defect-free single crystal of $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$, which is a canonical WFM and PzM. Notably, $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ is not a typical geometrically frustrated AFM, and in single crystal form it is also devoid of any size or interface effects, which are the usual suspects for a slow magnetization relaxation phenomenon. The underlying pinning mechanism appears exclusive to those AFMs which either are symmetry allowed WFMs, driven by Dzyaloshinskii-Moriya interaction, or can generate this trait by tuning of size and interface. The qualitative features of the quasistatic remanence indicate that such WFMs are potential piezomagnets, in which magnetization can be tuned by stress alone.

Co doped ZnO films have been deposited by a laser-molecular beam epitaxy system. X-ray diffraction and UV spectra analysis show that Co effectively substitutes the Zn site. Transmission electron microscopy (TEM) and secondary ion mass spectroscopy analysis indicate that there are no clusters. Co dopants are uniformly distributed in ZnO film. Ferromagnetic ordering is observed in all samples deposited under an oxygen partial pressure, ${\mathrm{PO}}_2={10}^{-3}, {10}^{-5}$, and ${10}^{-7}$ torr, respectively. However, the magnetization of ${\mathrm{PO}}_2={10}^{-3}$ and ${10}^{-5}$ is very small at room temperature. At low temperature, the ferromagnetic ordering is enhanced. Muon spin relaxation ($\mu \mathrm{SR}$) measurements confirm the ferromagnetism in all samples, and the results are consistent with magnetization measurements. From $\mu \mathrm{SR}$ and TEM analysis, the film deposited under ${\mathrm{PO}}_2={10}^{-7}$ torr shows intrinsic ferromagnetism. However, the volume fraction of the ferromagnetism phase is approximately 70%, suggesting that the ferromagnetism is not carrier mediated. Resistivity versus temperature measurements indicate Efros variable range hopping dominates the conductivity. From the above results, we can confirm that a bound magnetic polaron is the origin of the ferromagnetism.