Distinct contribution of acoustic and optical phonons in light-induced lattice transformation was resolved at different time scales by monitoring the insulator-to-metal phase transition in epitaxial and nonepitaxial ${\mathrm{VO}}_2$ films. Applying the ultrafast angle-resolved light scattering technique we demonstrate a significant influence of internal misfit strain in epitaxial films on subpicosecond phase transition dynamics. This technique also allows for observing a contribution of structural defects in the evolution of the transient state. The ultrafast structural phase transition dynamics is discussed in terms of the Ginzburg-Landau formalism. Using a set of experimental data we reconstruct the thermodynamic potential of photoexcited ${\mathrm{VO}}_2$ and provide a phenomenological model of the ultrafast light-induced structural phase transition.

We investigate the role of chirality on the performance of a Maxwell demon implemented in a quantum Hall bar with a localized impurity. Within a stochastic thermodynamics description, we investigate the ability of such a demon to drive a current against a bias. We show that the ability of the demon to perform is directly related to its ability to extract information from the system. The key features of the proposed Maxwell demon are the topological properties of the quantum Hall system. The asymmetry of the electronic interactions felt at the localized state when the magnetic field is reversed joined to the fact that we consider energy-dependent (and asymmetric) tunneling barriers that connect such state with the Hall edge modes allow the demon to properly work.

Tuning band gaps in two-dimensional (2D) materials is of great interest for the fundamental and practical aspects of contemporary material sciences. Recently, black phosphorus (BP) consisting of stacked layers of phosphorene was experimentally observed to show a widely tunable band gap by means of the deposition of potassium (K) atoms on the surface, thereby allowing great flexibility in the design and optimization of electronic and optoelectronic devices. Here, based on density-functional theory calculations, we demonstrates that the donated electrons from K dopants are mostly localized in the topmost BP layer and this surface charging efficiently screens the K ion potential. It is found that, as the K doping increases, the extreme surface charging and its screening of K atoms shift the conduction bands down in energy, i.e., towards a higher binding energy, because they have more charge near the surface, while it has little influence on the valence bands having more charge in the deeper layers. This result provides a different explanation for the observed tunable band gap compared to the previously proposed giant Stark effect where a vertical electric field from the positively ionized K overlayer to the negatively charged BP layers shifts the conduction band minimum ${\mathrm{\Gamma }}_{1\mathrm{c}}$ (valence band minimum ${\mathrm{\Gamma }}_{8\mathrm{v}}$) downwards (upwards). The present prediction of ${\mathrm{\Gamma }}_{1\mathrm{c}}$ and ${\mathrm{\Gamma }}_{8\mathrm{v}}$ as a function of the K doping reproduces well the widely tunable band gap, anisotropic Dirac semimetal state, and band-inverted semimetal state, as observed in an angle-resolved photoemission spectroscopy experiment. Our findings shed new light on a route for tunable band gap engineering of 2D materials through the surface doping of alkali metals.

In this work, we study the short-time dynamics of a molecular junction described by Anderson-Holstein model using full-counting statistics after projective measurement. The coupling between the central quantum dot (QD) and two leads was turned on at remote past and the system is evolved to steady state at time $t=0$, when we perform the projective measurement in one of the lead. Generating function for the charge transfer is expressed as a Fredholm determinant in terms of Keldysh nonequilibrium Green's function in the time domain. It is found that the current is not constant at short times indicating that the measurement does perturb the system. We numerically compare the current behaviors after the projective measurement with those in the transient regime where the subsystems are connected at $t=0$. The universal scaling for high-order cumulants is observed for the case with zero QD occupation due to the unidirectional transport at short times. The influences of electron-phonon interaction on short-time dynamics of electric current, shot noise, and differential conductance are analyzed.

The interplay of van der Waals (vdW), electrostatic (ES), and short-range (SR) interactions on both the intra- and intermolecular contrast observed in high-resolution atomic force microscopy (HR-AFM) is explored in a hydrogen-bonded monolayer of triazine molecules. Our efficient model to simulate AFM images uses the three-dimensional (3D) charge distribution of both tip and sample to calculate the ES interaction, takes into account the tilting of the CO molecule, and reproduces with high accuracy density functional theory calculations. In spite of triazine's hexagonal structure, the intramolecular contrast has triangular symmetry, reflecting the charge density of the molecule. Stripelike intermolecular features, which join the molecules in the H-bond directions, originate from the overlap of the charge density of the atoms in neighboring molecules and are sharpened by the CO tilt. We demonstrate the existence of different potential energy surface minima for the CO tilt and discuss its influence on imaging. Our results clearly show that the ES interaction maps represent a local 3D average of the ES potential of the sample weighted by the tip's charge density, while the SR interaction resembles a local 3D average of the charge density of the sample. However, the strong cancellation of both contributions results in a net interaction dominated by the ES and vdW far from the molecules, and by the SR at short distance. This cancellation, which essentially removes the dependence on the detailed charge distribution of the tip, explains why AFM images can be reproduced using only sample properties such as the $z$ component of the electric field and the charge density of the molecule, and the success of simple models that only incorporate pairwise, point-charge interactions.

A nodal loop appears when two bands, typically one electronlike and one holelike, are crossing each other linearly along a one-dimensional manifold in reciprocal space. Here, we propose a type of nodal loop which emerges from the crossing between two bands which are both electronlike (or holelike) along a certain direction. Close to any point on such a loop (dubbed as a type-II nodal loop), the linear spectrum is strongly tilted and tipped over along one transverse direction, leading to marked differences in magnetic, optical, and transport responses compared with conventional (type-I) nodal loops. We show that the compound ${\mathrm{K}}_4{\mathrm{P}}_3$ is an example that hosts a pair of type-II nodal loops close to the Fermi level. Each loop traverses the whole Brillouin zone, and hence can only be annihilated in a pair when symmetry is preserved. The symmetry and topological protections of the loops as well as the associated surface states are discussed.

We study three-dimensional nodal line semimetals (NLSMs) with magnetic ordering and strong spin-orbit interaction. Two distinct classes of magnetic NLSMs are proposed. The first class is band-inversion NLSM where the accidental line node is induced by band inversion and locally protected by glide mirror plane and the combined time-reversal and inversion symmetries. This can be viewed as a trivial stacking of the two-dimensional antiferromagnetic Dirac semimetals. The second class is essential NLSM where the nodal features are filling enforced by specific magnetic symmetry group. We further provide two concrete tight-binding models for magnetic NLSMs which belong to these two different classes, respectively. We conclude with a brief discussion on the possible material venues and the experimental implications for such phases.

Landau level mixing plays an important role in the even denominator fractional quantum Hall states. In ZnO the Landau level gap is essentially an order of magnitude smaller than that in a GaAs quantum well. We introduce the screened Coulomb interaction in a single Landau level to deal with that situation. Here we study the incompressibility and the overlap of the ground state and the Pfaffian (or anti-Pfaffian) state at filling factors $\frac{5}{2}$ and $\frac{7}{2}$ with a general screened Coulomb interaction. For small Landau level gaps, the overlap is strongly system size dependent and the screening can stabilize the incompressibility of the ground state with particle-hole symmetry which suggests a newly proposed particle-hole symmetry Pfaffian ground state. When the ratio of Coulomb interaction to the Landau level gap $\kappa$ varies, we find a possible topological phase transition in the range $2<\kappa <3$, which was actually observed in an experiment. We then study how the width of quantum well combined with screening influences the system.

We study the magnetic field-driven metal-to-insulator transition in half-filled Hubbard model on the Bethe lattice, using the dynamical mean-field theory by solving the quantum impurity problem with density-matrix renormalization group algorithm. The method enables us to obtain a high-resolution spectral densities in the presence of a magnetic field. It is found that the Kondo resonance at the Fermi level splits at relatively high magnetic field: the spin-up and -down components move away from the Fermi level and finally form a spin-polarized band insulator. By calculating the magnetization and spin susceptibility, we clarify that an applied magnetic field drives a transition from a paramagnetic metallic phase to a band insulating phase. In the weak interaction regime, the nature of the transition is continuous and captured by the Stoner's description, while in the strong interaction regime the transition is very likely to be metamagnetic, evidenced by the hysteresis curve. Furthermore, we determine the phase boundary by tracking the kink in the magnetic susceptibility, and the steplike change of the entanglement entropy and the entanglement gap closing. Interestingly, the phase boundaries determined from these two different ways are largely consistent with each other.

We theoretically study a one-dimensional (1D) mutually incommensurate bichromatic lattice system, which has been implemented in ultracold atoms to study quantum localization. It has been universally believed that the tight-binding version of this bichromatic incommensurate system is represented by the well-known Aubry-Andre model capturing all the essential localization physics in the experimental cold atom optical lattice system. Here we establish that this belief is incorrect and that the Aubry-Andre model description, which applies only in the extreme tight-binding limit of a very deep primary lattice potential, generically breaks down near the localization transition due to the unavoidable appearance of single-particle mobility edges (SPME). In fact, we show that the 1D bichromatic incommensurate potential system manifests generic mobility edges, which disappear in the tight-binding limit, leading to the well-studied Aubry-Andre physics. We carry out an extensive study of the localization properties of the 1D incommensurate optical lattice without making any tight-binding approximation. We find that, for the full lattice system, an intermediate phase between completely localized and completely delocalized regions appears due to the existence of the SPME, making the system qualitatively distinct from the Aubry-Andre prediction. Using the Wegner flow approach, we show that the SPME in the real lattice system can be attributed to significant corrections of higher-order harmonics in the lattice potential, which are absent in the strict tight-binding limit. We calculate the dynamical consequences of the intermediate phase in detail to guide future experimental investigations for the observation of 1D SPME and the associated intermediate (i.e., neither purely localized nor purely delocalized) phase. We consider effects of interaction numerically, and conjecture the stability of SPME to weak interaction effects, thus leading to the exciting possibility of an experimentally viable nonergodic extended phase in interacting 1D optical lattices. Our work provides precise quantitative protocols for future optical lattice based experiments searching for mobility edges in one-dimensional bichromatic incommensurate lattices, both in noninteracting and interacting systems.

Cerium is a fascinating element exhibiting, with its different phases, long-range magnetic order and superconductivity in bulk form. The coupling of the $4f$ electron to $sd$ conduction electrons and to the lattice is responsible for unique structural and electronic properties like the isostructural first-order solid-solid transition from the cubic $\gamma$ phase to the cubic $\alpha$ phase, which is accompanied by a huge volume collapse of 14%. We report experiments aiming at disentangling the $4f$ contribution to the electric conductance of the different phases. On single-atom Ce contacts we observe a strongly enhanced conductance $G$. By controlling the content of $\alpha$-Ce employing different rates of cooling, we find a strong correlation between the fraction of $\alpha$-Ce and the magnitude of $G$ at the last conductance plateau before the contact breaks. We attribute the enhanced conductance of $\alpha$-Ce to the additional contribution of the $4f$ level.

Coherent manipulation of nuclear spins is a highly desirable tool for both quantum metrology and quantum computation. However, most of the current techniques to control nuclear spins lack fast speed, impairing their robustness against decoherence. Here, based on stimulated Raman adiabatic passage, and its modification including shortcuts to adiabaticity, we present a fast protocol for the coherent manipulation of nuclear spins. Our proposed $\mathrm{\Lambda }$ scheme is implemented in the microwave domain and its excited-state relaxation can be optically controlled through an external laser excitation. These features allow for the initialization of a nuclear spin starting from a thermal state. Moreover we show how to implement Raman control for performing Ramsey spectroscopy to measure the dynamical and geometric phases acquired by nuclear spins.

The low-temperature behavior of tetragonal copper sulfide, ${\mathrm{Cu}}_2\mathrm{S}$, was investigated by powder and single-crystal x-ray diffraction, calorimetry, electrical resistance measurements, and ambient temperature optical absorption spectroscopy. The experiments were complemented by density-functional-theory-based calculations. High-quality, polycrystalline samples and single crystals of tetragonal copper sulfide were synthesized at 5 GPa and 700 K in a large volume multianvil press. Tetragonal ${\mathrm{Cu}}_2\mathrm{S}$ undergoes a temperature-induced phase transition to an orthorhombic structure at around 202 K with a hysteresis of $\pm 21$ K, an enthalpy of reaction of 1.3(2) $\mathrm{kJ}{\mathrm{mol}}^{-1}$, and an entropy of reaction of 6.5(2) $\mathrm{J}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-1}$. The temperature dependence of the heat capacity at the transition temperature indicates that the transition from the tetragonal to the low-temperature polymorph is not a single process. The structure of the low-temperature polymorph at 100 K was solved in space group $Pna{2}_1$. The structure is based on a slightly distorted cubic close packing of sulfur with copper in threefold coordination similar to the structure of tetragonal copper sulfide. The electrical resistance changes several orders of magnitude at the transition following the temperature hysteresis. The activation energy of the conductivity for the tetragonal phase and the low-temperature polymorph are 0.15(2) and 0.22(1) eV, respectively. The direct band gap of the tetragonal polymorph is found to be 1.04(2) eV with the absorption spectrum following Urbach's law. The activation energies and the band gaps of both phases are discussed with respect to the results of the calculated electronic band structures.

We investigate the time dependence of correlation functions in the central spin model, which describes the electron or hole spin confined in a quantum dot, interacting with a bath of nuclear spins forming the Overhauser field. For large baths, a classical description of the model yields quantitatively correct results. We develop and apply various algorithms in order to capture the long-time limit of the central spin for bath sizes from 1000 to infinitely many bath spins. Representing the Overhauser field in terms of orthogonal polynomials, we show that a carefully reduced set of differential equations is sufficient to compute the spin correlations of the full problem up to very long times, for instance up to ${10}^5\hslash /J_{\mathrm{Q}}$ where $J_{\mathrm{Q}}$ is the natural energy unit of the system. This technical progress renders an analysis of the model with experimentally relevant parameters possible. We benchmark the results of the algorithms with exact data for a small number of bath spins and we predict how the long-time correlations behave for different effective numbers of bath spins.

The effect of hydrostatic pressure on complex magnetic phase diagram of $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4$ has been investigated by various magnetic measurements in the range of 0–12 kbar. The temperature of the ferrimagnetic (fM) transition ($T_{\mathrm{N}}\approx 245\mathrm{K}$) practically does not change with increasing pressure, while the temperature of the appearance of mixed fM and antiferromagnetic (AFM) state at $\sim 200\mathrm{K}$ increases with the rate $d{T}_{\mathrm{C}}/dP\approx 0.8\mathrm{K}/\mathrm{kbar}$. Moreover, an applied pressure suppresses the magnetization, enhances the field of metamagnetic transition, and extends the range of existence of mixed fM and AFM state. Suppression of the ordered magnetic moment under pressure induces a transition from a counterclockwise (CCW) thermal hysteresis below $T\approx 220\mathrm{K}$ to a clockwise (CW) thermal hysteresis in a magnetic field $H=1\mathrm{kOe}$. Nevertheless, field induced metamagnetic transition from mixed fM and AFM state to fM state results in a narrowing of the temperature range of thermal hysteresis and in restoring of CCW type of thermal hysteresis. As a consequence of metamagnetic transition, the thermal hysteresis completed at $H=10\mathrm{kOe}$ remains one of the CCW type under applied pressure.

We report structure, susceptibility, and specific heat studies of stoichiometric and off-stoichiometric poly- and single crystals of the $A$-site spinel compound $\mathrm{FeS}{\mathrm{c}}_2{\mathrm{S}}_4$. In stoichiometric samples, no long-range magnetic order is found down to 1.8 K. The magnetic susceptibility of these samples is field independent in the temperature range 10–400 K and does not show irreversible effects at low temperatures. In contrast, the magnetic susceptibility of samples with iron excess shows substantial field dependence at high temperatures and manifests a pronounced magnetic irreversibility at low temperatures with a difference between zero-field cooled (ZFC) and field cooled (FC) susceptibilities and a maximum at 10 K, reminiscent of a magnetic transition. Single-crystal x-ray diffraction of the stoichiometric samples revealed a single phase spinel structure without site inversion. In single crystalline samples with Fe excess, in addition to the main spinel phase, a second ordered single-crystal phase was detected with the diffraction pattern of a vacancy-ordered superstructure of iron sulfide, close to the 5C polytype $\mathrm{F}{\mathrm{e}}_9{\mathrm{S}}_{10}$. Specific heat studies reveal a broad anomaly, which evolves below 20 K in both stoichiometric and off-stoichiometric crystals. We show that the low-temperature specific heat can be well described by considering the low-lying spin-orbital electronic levels of $\mathrm{F}{\mathrm{e}}^{2+}$ ions. Our results demonstrate significant influence of excess Fe ions on intrinsic magnetic behavior of $\mathrm{FeS}{\mathrm{c}}_2{\mathrm{S}}_4$ and provide support for the spin-orbital liquid scenario proposed in earlier studies for the stoichiometric compound.

The series of multiferroics $R{\mathrm{Mn}}_2{\mathrm{O}}_5$ is extensively studied for its quasicollinear spin arrangement, which results in an electrical polarization according to the exchange-striction model. Variations of the interatomic distances modified by the external pressure can strongly influence the multiferroic properties. Understanding this influence is of great importance, especially for the future realization of multiferroic devices. As ${\mathrm{PrMn}}_2{\mathrm{O}}_5$ is paraelectric at ambient pressure, it is the most suitable candidate to search for pressure induced multiferroicity. In this paper, we report the emergence of a new pressure induced magnetic phase in ${\mathrm{PrMn}}_2{\mathrm{O}}_5$ determined by powder neutron diffraction under pressure. This new magnetic phase presenting at relatively low pressure becomes completely exclusive at 8 GPa. The determination of its magnetic structure has thus been possible for the first time. More importantly, the magnetic structure stabilized under pressure should induce a strong spontaneous electric polarization due to the nearly perfect collinearity of the ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ spins.

Analytical solutions to the problem of mode splitting caused by a thin plain magnetized slab placed in a rectangular electromagnetic cavity are found in two special cases: an infinite slab between two ideal infinite plain mirrors (1D model) and a 2D model of a cavity confined in the direction of permanent magnetization. Under the resonance conditions, the time-dependent magnetization vectors inside the slab are strongly enhanced, with opposite directions in the split modes. For realistic parameters, the frequency splitting does not depend on the dielectric constant of the slab and the Landau-Lifshitz-Gilbert damping coefficient, whereas the loaded cavity quality factor is not sensitive to the slab position.

Recent ac susceptibility and polar Kerr effect measurements in the skutterudite superconductor ${\mathrm{PrOs}}_4{\mathrm{Sb}}_{12}$ (POS) (E. M. Levenson-Falk, E. R. Schemm, M. B. Maple, and A. Kapitulnik, arXiv:1609.07535) uncovered the nature of the superconducting double transition from a high-temperature, high-field, time-reversal symmetric phase (or the A phase) to a low-temperature, low-field, time-reversal symmetry-broken phase (or the B phase). Starting from a microscopic model, we derive a Ginzburg-Landau expansion relevant to POS that describes this entrance into the time-reversal symmetry-broken phase along the temperature axis. We also provide a study of the low-energy inelastic (Raman) response in both the A and B phases of POS, and seek additional signatures which could help reveal the exact form of the gap functions previously proposed in these phases. By appropriately manipulating the incoming and scattered light geometries, along with additional subtraction procedures and suitable assumptions, we show that one can access the various irreducible representations contained in the point group describing POS. We demonstrate how to use this technique on example order parameters proposed in POS. Depending on whether there exist nodes along the $c$ axis, we find additional low-energy spectral weight within the superconducting gap in the $E_g$ geometry, a feature that could pinpoint the location of nodes on the Fermi surface.