This paper presents computer simulations of ${\mathrm{Cu}}_x{\mathrm{Zr}}_{100-x}(x=36,50,64)$ in the liquid and glass phases. The simulations are based on the effective-medium theory potentials. We find good invariance of both structure and dynamics in reduced units along the isomorphs of the systems. The state points studied involve a density variation of almost a factor of 2 and temperatures going from 1500 K to above 4000 K for the liquids and from 500 K to above 1500 K for the glasses. For comparison, results are presented also for similar temperature variations along isochores, showing little invariance. In general for a binary system the phase diagram has three axes: composition, temperature, and pressure (or density). When isomorphs are present, there are effectively only two axes, and for a fixed composition there is just one. We conclude that the liquid and glass parts of the thermodynamic phase diagram of this metallic glass former at a fixed composition are effectively one-dimensional in the sense that many physical properties are invariant along the same curves, implying that in order to investigate the phase diagram, it is only necessary to go across these curves.

If a localized quantum state in a tight-binding model with structural aperiodicity is subject to noisy evolution, then it is generally expected to result in diffusion and delocalization. In this work, the effects of strongly aperiodic kick sequences applied to a quasiperiodic and a translationally invariant model is studied. It is shown that the kicked Aubry-André-Harper (AAH) model is naturally robust to noise, provided that the kicks are delivered, on average, once every time period. However, if strong noisy perturbation is applied by randomly missing kicks, a sharp dynamical transition from an initial ballistic growth to a subsequent (asymptotic) diffusive growth phase is observed. Such sharp transitions are also present in translation invariant models, where they are related to the existence of flat bands. In both the models, the time scale at which the sharp transition takes place is related to the characteristics of noise, for which analytical support is presented. Remarkably, the spread of the wave function scales with the noise parameters. Further, using kick sequences modulated by a ‘coin toss,’ it is argued that the correlations in the noise are crucial to the observed sharp transitions.

We develop an ab initio method to simulate the infrared vibrational response of metallic systems in the framework of time-dependent density functional perturbation theory. By introducing a generalized frequency-dependent Born effective charge tensor, we show that phonon peaks in the reflectivity of metals can always be described by a Fano function, whose shape is determined by the complex nature of the frequency-dependent effective charges and electronic dielectric tensor. The IR vibrational properties of graphite, chosen as a representative test case to benchmark our method, are found to be accurately reproduced. Our approach offers a first-principles scheme for the prediction and understanding of IR reflectance spectra of metals, which may represent one of the few available tools of investigation of these materials when subjected to extremely high-pressure conditions.

The interplay of crystal symmetry, magnetism, band topology, and electronic correlation can be the origin of quantum phase transitions in condensed matter. Particularly, square-lattice materials have been serving as a versatile platform to study the rich phenomena resulting from that interplay. In this work we report a detailed magnetic study on the square-lattice-based magnetic topological semimetal ${\mathrm{GdSb}}_x{\mathrm{Te}}_{2-x-\delta }$. We report the H-T magnetic phase diagrams along three crystallographic orientations and show that, for those materials where a charge density wave distortion is known to exist, many different magnetic phases are identified. In addition, the data hints towards the existence of an antiferromagnetic skyrmion phase, which has been theoretically predicted but not experimentally confirmed in a bulk material yet.

We investigate the finite-temperature properties of the $S=\frac{1}{2}$ Shastry-Sutherland Heisenberg model using a quantum typicality method. In the intermediate plaquette state region, we naturally expect to realize the finite-temperature phase transition associated with breaking the mirror symmetry of this model. We reveal some signatures of the spontaneous phase transition within a two-point correlation level at moderate temperatures since the constructed typical state can sense the existence of the degenerated excited states depending on the initial random state. We also confirm that the local mirror order parameter shows the intriguing recovering phenomenon of the mirror symmetry in very low temperatures, which could be understood from the nature of the ground and excited states of the finite-size systems. We expect that this recovering feature will disappear; instead, a saturated behavior appears in the local mirror order parameter in the thermodynamic limit. We discuss the relationship to the recent experimental results on ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$ under high pressures.

We introduce a microscopic model for collinear multiferroics capable of reproducing, as a consequence of magnetic frustration and easy-axis anisotropy, the so-called “uudd” (or antiphase) magnetic ordering observed in several type-II multiferroic materials. The crucial role of lattice distortions in the multiferroic character of these materials enters into the model via an indirect magnetoelectric coupling, mediated by elastic degrees of freedom through a pantograph mechanism. Long-range dipolar interactions set electric dipoles in the antiferroelectric order. We investigate this model by means of extensive density matrix renormalization group computations and complementary analytical methods. We show that a lattice dimerization induces a spontaneous ${\mathbb{Z}}_2$ ferrielectric bulk polarization, with a sharp switch off produced by a magnetic field above a critical value. The topological character of the magnetic excitations makes this mechanism robust.

Miniature heaters are immersed in flows of quantum fluid and the efficiency of heat transfer is monitored versus velocity, superfluid fraction, and time. The fluid is ${}^4\mathrm{He}$ helium with a superfluid fraction varied from 71% down to 0% and an imposed velocity up to $3\mathrm{m}/\mathrm{s}$, while the characteristic sizes of heaters range from $1.3\mu \mathrm{m}$ up to a few hundreds of microns. At low heat fluxes, no velocity dependence is observed, in agreement with expectations. In contrast, some velocity dependence emerges at larger heat flux, as reported previously, and three nontrivial properties of heat transfer are identified. First, at the largest superfluid fraction (71%), a new heat transfer regime appears at non-null velocities and it is typically $10\%$ less conductive than at zero velocity. Second, the velocity dependence of the mean heat transfer is compatible with the square-root dependence observed in classical fluids. Surprisingly, the prefactor to this dependence is maximum for an intermediate superfluid fraction or temperature (around 2 K). Third, the heat transfer time series exhibit highly conductive short-lived events. These cooling glitches have a velocity-dependent characteristic time, which manifest itself as a broad and energetic peak in the spectrum of heat transfer time series, in the kHz range. After showing that the velocity dependence can be attributed to the breaking of superfluidity within a thin shell surrounding heaters, an analytical model of forced heat transfer in a quantum flow is developed to account for the properties reported above. We argue that large scale flow patterns must form around the heater, having a size proportional to the heat flux (here two decades larger than the heater diameter) and resulting in a turbulent wake. The observed spectral peaking of heat transfer is quantitatively consistent with the formation of a Von Kármán vortex street in the wake of a bluff body nearly two decades larger than the heater but its precise temperature and velocity dependence remains unexplained. An alternative interpretation for the spectral peaking is discussed, in connection with existing predictions of a bottleneck in the superfluid velocity spectra and energy equipartition.

The electronic and magnetic states of $\beta \text{−}{\mathrm{MnO}}_2$ in terms of hydrogen impurities have been investigated by the muon spin rotation technique combined with density-functional theory (DFT) calculations for muon as pseudohydrogen. We found that 85% of implanted muons are localized in the oxygen channels of the rutile structure and behave like interstitial protons (${\mathrm{Mu}}^{+})$ except those (7.6%) forming a charge-neutral state (${\mathrm{Mu}}^0)$ at 2.3 K, which indicates that interstitial hydrogen acts like a shallow donor within less than 0.1 meV of ionization energy. The residual 15% of muons are attributed to those related to lattice imperfection as Mn vacancies. Detailed analyses combined with the DFT approach suggested that the muon is localized at the center of the oxygen channel due to its large zero-point vibration energy.

We derive a scheme to calculate the Tomonaga-Luttinger liquid's (TLL) parameters and the holon velocity in a quasi-one-dimensional (quasi-1D) material that consists of two-leg ladders coupled through Coulomb interactions. First, we obtain an analytic formula for electron-electron interaction potential along the conducting axis for a generalized charge distribution in a plane perpendicular to it. Then, we introduce many-body screening that is present in a quasi-1D material by proposing an approximation for the charge susceptibility. Based on this we are able to find the TLL's parameters and velocities. We then show how to use these to validate the experimental angle-resolved photoemission spectroscopy data measured recently in $p$ polarization in ${\text{NbSe}}_3$. Although we focus our study on this specific material, it is applicable for any quasi-1D system that consists of two-leg ladders as its basic units.

In this paper we calculate the time-dependent forces between a swift electron traveling at constant velocity and a metallic nanoparticle made of either aluminum or gold. We consider that the nanoparticle responds as an electric point dipole and we use classical electrodynamics to calculate the force on both the nanoparticle and the electron. The values for the velocity of the electron and the radius of the nanoparticle were chosen in accordance with electron microscopy observations, and the impact parameter was selected to fulfill the constraints imposed by the dipole approximation. We find that there are times when the force on the nanoparticle is attractive and others when it is repulsive, and show that this is due to the delayed electromagnetic response of the nanoparticle. To establish the limits of validity of our approach, we calculate the total linear momentum transfer to the nanoparticle, and compare it with results obtained, in frequency space, using the full multipole expansion of the fields induced on the nanoparticle, considering the effects of electromagnetic radiation.

We show that the interband absorption of radiation in a two-dimensional (2D) Dirac material leads to a direct electric current flowing at sample edges. The photocurrent originates from the momentum alignment of electrons and holes and is controlled by the radiation polarization. We develop a microscopic theory of such an edge photogalvanic effect and calculate the photocurrent for gapped and gapless 2D Dirac materials, also in the presence of a static magnetic field which introduces additional imbalance between the electron and hole currents. Further, we show that the photocurrent can be considerably multiplied in a ratchetlike structure with an array of narrow strips.

We investigate the chemical reactivity of molecular populations confined inside a nanofluidic Fabry-Pérot cavity. Due to strong light-matter interactions developing between a resonant electromagnetic cavity mode and the electric dipole moment of the confined molecules, a collective polariton excitation is formed. The former gets dressed by environmental vibrational and rotational degrees of freedom of the solvent. We call the resulting polariton dressed by its cloud of environmental excitation a “reacton”, since it further undergoes chemical reactions. We characterize how the reacton formation modifies the kinetics of a photoisomerization chemical reaction involving an elementary charge-transfer process. We show that the reaction driving force and reorganization energy are both modulated optically by the reactant concentration, the vacuum Rabi splitting, and the detuning between the Fabry-Pérot cavity frequency and targeted electronic transition. Finally, we compute the ultrafast picosecond dynamics of the whole photochemical reaction. We predict that despite optical cavity losses and solvent-mediated nonradiative relaxation, measurable signatures of the reacton formation can be found in state-of-the-art pump-probe experiments.

Magnetic skyrmions in two-dimensional (2D) chiral magnets are often stabilized by a combination of a Dzyaloshinskii-Moriya interaction and an external magnetic field. Here, we show that skyrmions can also be stabilized in twisted moiré superlattices with a Dzyaloshinskii-Moriya interaction in the absence of an external magnetic field. Our setup consists of a 2D ferromagnetic layer twisted on top of an antiferromagnetic substrate. The coupling between the ferromagnetic layer and the substrate generates an effective alternating exchange field. We find a large region of the skyrmion crystal phase when the length scales of the moiré periodicity and skyrmions are compatible. Unlike chiral magnets under a magnetic field, skyrmions in moiré superlattices show an enhanced stability for the easy-axis (Ising) anisotropy which can be essential to realize skyrmions since most van der Waals magnets possess easy-axis anisotropy.

A metal-dielectric interface is usually modeled by a zero-width layer, where dielectric permittivity suffers a discontinuity jump. In reality, there is a narrow transition region, which includes an epsilon-near-zero (ENZ) layer. We show that due to a continuous dielectric function the dispersion of the surface plasmon is modified. The energy of the surface plasmon radiates through the ENZ layer, giving rise to additional radiative losses. Direct evidence of the effects related to the transition layer is given by the observation of plasmonic resonance in the presence of a strong electric field normal to the metal film. The electric field affects the electron density within the transition layer that leads to a measurable shift of the plasmonic resonance.

We characterize the optical coherence and energy-level properties of the 795-nm ${}^3\mathrm{H}_6$ to ${}^3\mathrm{H}_4$ transition of ${\mathrm{Tm}}^{3+}$ in a ${\mathrm{Ti}}^{4+}$:$\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ waveguide at temperatures as low as 0.65 K. Coherence properties are measured with varied temperature, magnetic field, optical excitation power and wavelength, and measurement timescale. We also investigate nuclear spin-induced hyperfine structure and population dynamics with varying magnetic field and laser excitation power. Except for accountable differences due to different ${\mathrm{Ti}}^{4+}$- and ${\mathrm{Tm}}^{3+}$-doping concentrations, we find that the properties of ${\mathrm{Tm}}^{3+}$:${\mathrm{Ti}}^{4+}$:$\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ produced by indiffusion doping are consistent with those of a bulk-doped ${\mathrm{Tm}}^{3+}$:$\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ crystal measured under similar conditions. Our results, which complement previous work in a narrower parameter space, support using rare-earth ions for integrated optical and quantum signal processing.

Floppy networks (FNs) provide valuable insight into the origin of anomalous mechanical and thermal properties in soft matter systems, from polymers, rubber, and biomolecules to glasses and granular materials. Here, we use the same FN concept to construct a quantitative microscopic theory of empty perovskites, a family of crystals with ${\mathrm{ReO}}_3$ structure, which exhibit a number of unusual properties. One remarkable example is ${\mathrm{ScF}}_3$, which shows a near-zero-temperature structural instability and large negative thermal expansion (NTE). We trace these effects to an FN-like crystalline architecture formed by strong nearest-neighbor bonds, which is stabilized by net electrostatic repulsion that plays a role similar to osmotic pressure in polymeric gels. NTE in these crystalline solids, which we conceptualize as Coulomb floppy networks, emerges from the tension effect of Coulomb repulsion combined with the FN's entropic elasticity and has the same physical origin as in gels and rubber. Our theory provides an accurate, quantitative description of phonons, thermal expansion, compressibility, and structural phase diagram, all in excellent agreement with experiments. The entropic stabilization of critical soft modes, which play only a secondary role in NTE, explains the observed phase diagram. Significant entropic elasticity resolves the puzzle of a marked, $\approx 50\%$ discrepancy between the experimentally observed bulk modulus and ab initio calculations. The Coulomb FN approach is potentially applicable to other important materials with markedly covalent bonds, from perovskite oxides to iron chalcogenides, whose anomalous vibrational and structural properties are still poorly understood.

We provide a principal Hugoniot of cryogenic liquid deuterium at a pressure of $27<P<240$ GPa, with reflected-shock data of up to $\sim 830$ GPa. The maximum density reaches $\sim 1.49$ $\mathrm{g}/{\mathrm{cm}}^3$, about 8.7 times the initial density. Our independent principal Hugoniot experimental data broadly support the wide-regime equation of state (WEOS) model, which well matches Fernandez-Pañella et al. [Phys. Rev. Lett. 122, 255702 (2019)] and Knudson et al. [Phys. Rev. Lett. 118, 035501 (2017)] experimental data over the observed pressure range up to 550 GPa, and most of three sets of reflected shock data are in accordance with our theory up to 1 TPa. Our high-precision experimental results establish an important benchmark equation of state of deuterium and conform to the WEOS model quite well. Our work is useful for the development of the high-pressure response of hydrogen's isotopes, directly related to inertial confinement fusion, planetary science, and metallization.

Breathing modes are closely related to many physical properties of nanoclusters. Decades of research, however, failed to formulate a general and unambiguous definition. Here we present a straightforward and widely applicable definition of breathing modes based on power spectra of geometric quantities, namely, surface area, volume, etc. Applying group theory, normal-mode analysis, and molecular dynamics simulations, we have explored breathing modes of several ${\mathrm{Al}}_n$ clusters with high and low symmetries. The results suggest that our definition is able to cover not only common breathing modes but also some hidden modes. Our consistent definition also allows us to make a comprehensive and in-depth comparison with Lamb's continuous medium model, which reveals some high-frequency breathing modes are explicable only at the atomic level.

In an era of new developments in nanomaterials analysis enabled by the unprecedented spatial and energy resolutions of electron energy-loss spectroscopy in the scanning transmission electron microscope (STEM-EELS), it remains that the vast majority of works concern collective or single-particle excitations that are well described by the electrostatic approximation, which neglects retardation and magnetic field effects. Here we demonstrate a simple case in which that approximation is fundamentally inadequate. When the beam energy is above the Cherenkov threshold and the geometric dimensions of the nanomaterial sample are on the order of the wavelength of light in the material, spatial variations in low-loss ($\lesssim 5$ eV) spectral maps from guided light modes may be observed. We demonstrate such observations for amorphous silicon disks and offer an interpretation of the results based on the waveguide modes of a cylinder. We also demonstrate explicitly that spatial variations from waveguide modes are manifest in analytic models for the especially simple geometry of a STEM beam penetrating a dielectric ribbon. We discuss how these modes relate to those that have been observed more generally in dielectric nanomaterials.

The breaking of the well-known Aubry-André model with a single-particle mobility edge (SPME) is a intriguing subject that has not yet been fully understood. In particular, how to accurately and efficiently recognize a SPME in an optical lattice is currently under active debate. In this work, we develop a data compression-based neural network (DCNN) approach to identify SPMEs in one-dimensional quasiperiodic optical lattice using eigenstates as the sole diagnostic. We find that such method can successfully identify SPMEs of a large system only using a small network trained by the small system data, without onerously and repetitively training a new and large-scale network by massive data of a large system. Furthermore, we show that this method is also applicable to recognize more complex phase transitions, such as many-body localization. Our DCNN approach first paves the way for the development of a generic tool for identifying unexplored phase transitions in large systems.