We develop the finite-size scaling (FSS) theory at quantum transitions. We consider various boundary conditions, such as open and periodic boundary conditions, and characterize the corrections to the leading FSS behavior. Using renormalization-group (RG) theory, we generalize the classical scaling ansatz to describe FSS in the quantum case, classifying the different sources of scaling corrections. We identify nonanalytic corrections due to irrelevant (bulk and boundary) RG perturbations and analytic contributions due to regular backgrounds and analytic expansions of the nonlinear scaling fields. To check the general predictions, we consider the quantum $XY$ chain in a transverse field. For this model exact or numerically accurate results can be obtained by exploiting its fermionic quadratic representation. We study the FSS of several observables, such as the free energy, the energy differences between low-energy levels, correlation functions of the order parameter, etc., confirming the general predictions in all cases. Moreover, we consider bipartite entanglement entropies, which are characterized by the presence of additional scaling corrections, as predicted by conformal field theory.

The heat capacity of helical magnets ${\mathrm{Cu}}_2\mathrm{O}\mathrm{Se}{\mathrm{O}}_3$ and MnSi has been investigated at high pressures by the ac-calorimetric technique. Despite the differing nature of their magnetic moments, ${\mathrm{Cu}}_2\mathrm{O}\mathrm{Se}{\mathrm{O}}_3$ and MnSi demonstrate a surprising similarity in behavior of their magnetic and thermodynamic properties at the phase transition. Two characteristic features of the heat capacity at the phase transitions of both substances (peak and shoulder) behave also in a similar way at high pressures if analyzed as a function of temperature. This probably implies that the longitudinal spin fluctuations typical of weak itinerant magnets like MnSi contribute little to the phase transition. The shoulders of the heat capacity curves shrink with decreasing temperature suggesting that they arise from classical fluctuations. In the case of MnSi the sharp peak and shoulder at the heat capacity disappear simultaneously probably signifying the existence of a tricritical point and confirming the fluctuation nature of the first-order phase transition in MnSi as well as in ${\mathrm{Cu}}_2\mathrm{O}\mathrm{Se}{\mathrm{O}}_3$.

We study the quantum phase transition between the $Z_2$ spin liquid and valence bond solid (VBS) orders on a triangular lattice. With a fully isotropic triangular lattice, the transition from a columnar or resonating-plaquette VBS order can either be first order or there could be two transitions with an intermediate phase. If the transition splits into two, then the $Z_2$ spin liquid will first experience a first-order $q=3$ Potts transition to a new nematic $Z_2$ spin liquid that breaks the $2\pi /3$ lattice rotation symmetry (but retain translation symmetry, unlike the VBS states). The second transition will then take this new nematic $Z_2$ spin liquid to a columnar or resonating-plaquette VBS state through a second-orderthree-dimensional (3D) $X{Y}^{*}$ transition. On a distorted triangular lattice, the degeneracy between some of the different columnar VBS orders is lifted, and the phase transition can reduce to a single 3D $X{Y}^{*}$ transition.

We analyze the Purcell relaxation rate of a superconducting qubit coupled to a resonator, which is coupled to a transmission line and pumped by an external microwave drive. Considering the typical regime of the qubit measurement, we focus on the case when the qubit frequency is significantly detuned from the resonator frequency. Surprisingly, the Purcell rate decreases when the strength of the microwave drive is increased. This suppression becomes significant in the nonlinear regime. In the presence of the microwave drive, the loss of photons to the transmission line also causes excitation of the qubit; however, the excitation rate is typically much smaller than the relaxation rate. Our analysis also applies to a more general case of a two-level quantum system coupled to a cavity.

We studied the electronic structure of the Kondo lattice system ${\mathrm{Ce}}_2{\mathrm{Co}}_{0.8}{\mathrm{Si}}_{3.2}$ by angle-resolved photoemission spectroscopy. The spectra obtained below the coherence temperature consist of a Kondo resonance, its spin-orbit partner, and a number of dispersing bands. The quasiparticle weight related to the Kondo peak depends strongly on Fermi vectors associated with bulk bands. This indicates a highly anisotropic hybridization between conduction-band and $4f$ electrons—$V_{cf}$ in ${\mathrm{Ce}}_2{\mathrm{Co}}_{0.8}{\mathrm{Si}}_{3.2}$.

Resonant metamaterials usually exhibit substantial dispersion, which is considered a shortcoming for many applications. Here we take advantage of the ability to tailor the dispersive response of a metamaterial, introducing a new method of group-velocity dispersion compensation in telecommunication systems. The method consists of stacking a number of highly dispersive sheet metamaterials and is capable of compensating the dispersion of optical fibers with either negative or positive group-velocity dispersion coefficients. We demonstrate that the phase-engineered metamaterial can provide strong group-velocity dispersion management without being adversely affected by large transmission loss, while at the same time offering high customizability and a small footprint.

The influence of dot size and shape on the excitonic structure of GaAs/AlGaAs quantum dots (QDs) is studied experimentally and theoretically. Almost strain-free GaAs QDs are fabricated by epitaxially filling of nanoholes in an AlGaAs surface. The nanoholes are formed in a self-assembling fashion by local droplet etching. As an important point, the size of the QDs can be adjusted by the hole filling level. As a consequence, the exciton recombination energy can be controllably varied over a range of 130 meV by the dot size. We present micro- and macro-photoluminescence measurements of the $s$-shell recombinations. With the hole shape determined using atomic force microscopy and the amount of GaAs filled into the holes, we have quite precise input parameters for a simulation of the excitonic states in the QDs. The measured exciton and biexciton recombination energies are well reproduced by calculations based on eight band $\mathit{k}·\mathit{p}$ theory and configuration interaction scheme with a deviation of less than 7 meV.

Nonlinear dynamics of nuclear spin ensembles driven by a two-electron system in a double quantum dot in the Pauli spin blockade (SB) regime is studied experimentally in conjunction with numerical simulation. Dynamic nuclear spin polarization (DNP) is systematically studied by evaluating the current level and its fluctuations. We interpret large current noise in the SB regime as stable feedback noise, where identical Overhauser fields of the two dots are preferred. In contrast, stepwise increases of current in the shallow Coulomb blockade region can be understood as unstable dynamics with significant imbalance of the Overhauser fields, which cancels the external magnetic field in one of the two dots. There, an extremely small transverse Overhauser field can easily lift the SB transport, giving the highest current level, when longitudinal components cancel the applied field.

We present a comprehensive analysis of the role of $\epsilon$-near-zero (ENZ) materials with realistic losses in enhanced light transmission through subwavelength channels. In this work, we utilize a bulk ENZ material consisting of heavily doped semiconductor, operating at the semiconductor's plasma frequency (in mid-infrared regime). Resonant transmission peaks for transverse magnetic polarized light have been experimentally observed when such material is used as a coupling layer and/or inside the subwavelength slit. In the process, we developed a generalized mode-matching-based approach to calculate and analyze light propagation through slits and waveguides embedded inside planar layer stacks and applied this approach to the particular case of ENZ-filled subwavelength slit. The developed formalism inherently provides the individual amplitudes of the guided modes inside the slit and their coupling efficiencies to the surrounding media. Using the calculated coupling efficiencies, we were successful in explaining the resonant transmission behavior of these structures and also were able to track the origin of enhancements. Our analysis demonstrates that in the presence of losses, the transmission efficiency is dominated by the bulk plasma resonances of the ENZ coupling layer and minimal advantage is gained by filling the slit with ENZ material. The enhancements were also observed to be dependent on the thickness of the substrate layer. Further analysis of the calculated amplitudes using the numerical algorithm developed an approximate analytical model for systems with deep subwavelength slits.

Using local-probe magnetic-characterization techniques of muon spin relaxation and electron spin resonance we have investigated the Mn-induced magnetism of the wide-band-gap perovskite SrTiO${}_3$. Our results clearly demonstrate that this diluted magnetic oxide remains paramagnetic down to low temperatures for both doping cases, i.e., when Mn substitutes for Sr or Ti. In addition, both experimental techniques have revealed that the distribution of individual Mn${}^{2+}$ and Mn${}^{4+}$ ions is nonrandom, as these ions partially aggregate into nanosized clusters.

We have measured the interfacial magnetization depth profile in ferromagnet/antiferromagnet exchange-coupled NiFe/CoO bilayers. Both a polycrystalline and an epitaxial-(111) bilayer were examined. We find that the nonswitchable magnetization profile in the biased state is highly correlated with the magnetization profile in the unbiased state. The nonswitchable moment distributions are shown to be consistent with the predictions of a previously reported model for the magnetic and microstructural features of the interfacial region.

We study elementary excitations of a system of one-dimensional bosons with weak contact repulsion. We show that the Gross-Pitaevskii regime, in which the excitations are the well-known Bogoliubov quasiparticles and dark solitons, does not extend to the low-energy limit. Instead, the spectra of both excitations have finite curvatures at zero momentum, in agreement with the phenomenological picture of fermionic quasiparticles. We describe analytically the crossover between the Gross-Pitaevskii and the low-energy regimes, and discuss implications of our results for the behavior of the dynamic structure factor.

The ${\mathrm{P}}_2{\mathrm{O}}_5$ compound is an archetypical glass-forming oxide with a record high hygroscopicity, which makes its study extremely difficult. We present the in situ x-ray diffraction study of the pressure-temperature phase diagram of ${\mathrm{P}}_2{\mathrm{O}}_5$ and, particularly, of the liquid ${\mathrm{P}}_2{\mathrm{O}}_5$ structure under high pressure up to 10 GPa. Additionally, quenching from the melt has been used to extend the melting curve up to 15 GPa. We found that structural transformation in the liquid ${\mathrm{P}}_2{\mathrm{O}}_5$ under pressure is unique and includes three stages: first, the disappearance of the intermediate range order of the melt together with a slow increase in the average first-coordination number $\langle N_1\rangle$ (P-O and O-P neighbors) up to 4 GPa; second, the “normal” compression almost without structural modification at higher pressures up to 8–9 GPa; and, finally, the abrupt change of the short-range order structure of the liquid with the jumplike $\langle N_1\rangle$ increase at 9–10 GPa. The last stage correlates with the melting curve maximum (≈1250 °C) at ≈10 GPa and can be interpreted as a transformation to the liquid phase with entirely fivefold-coordinated phosphorus and twofold-coordinated oxygen atoms.

Using microemulsion methods, CoO-Pt core-shell nanoparticles, with diameters of nominally 4 nm, were synthesized and characterized by high-resolution transmission electron microscopy and a suite of x-ray spectroscopies, including diffraction, absorption, absorption near-edge structure, and extended absorption fine structure, which confirmed the existence of CoO cores and pure Pt surface layers. Using a commercial magnetometer, the ac and dc magnetic properties were investigated over a range of temperature (2 K $\le T\le$ 300 K), magnetic field ($\le 50$ kOe), and frequency ($\le 1$ kHz). The data indicate the presence of two different magnetic regimes whose onsets are identified by two maxima in the magnetic signals, with a narrow maximum centered at 6 K and a large one centered at 37 K. The magnetic responses in these two regimes exhibit different frequency dependencies, where the maximum at high temperature follows a Vogel-Fulcher law, indicating a superparamagnetic blocking of interacting nanoparticle moments and the maximum at low temperature possesses a power-law response characteristic of a collective freezing of the nanoparticle moments in a superspin glass state. This co-existence of blocking and freezing behaviors is consistent with the nanoparticles possessing an antiferromagnetically ordered core, with an uncompensated magnetic moment, and a magnetically disordered interlayer between the CoO core and the Pt shell.

Half-quantum vortices and ordinary vortices in a rotating thin-film superfluid ${}^3$He under a strong magnetic field are considered. It is shown that $2n+1$ half-quantum vortices interpolate between $n$ singular vortices and $n+1$ singular vortices as the angular velocity is changed when the external magnetic field is strong enough. The phase diagram of the vortex configurations in the angular velocity–magnetic field space is obtained for $p=0$.

This work investigates the Mn${}^{2+}$ electronic structure and exciton dynamics in one-dimensional (1D) N(CH${}_3$)${}_4$MnCl${}_3$ (TMMC) through time-resolved excitation/emission spectroscopy and absorption measurements in the 0–10 GPa pressure range for different Cu${}^{2+}$ doping concentrations. The local and crystal structures have been analyzed by Raman spectroscopy and x-ray absorption measurements at the Mn $K$ edge showing that the 1D chain structure is maintained in the whole explored pressure range. We show that both the first Mn${}^{2+}$ absorption band, ${}^{4}T$${}_1\left(G\right)$, and its associated emission band experience very large pressure redshifts, which are associated with the crystal anisotropy providing large axial ligand fields at the Mn${}^{2+}$ site that increase with pressure. The red emission at 633 nm shows a large pressure variation of 22 nm/GPa (50 meV/GPa) making TMMC a suitable probe for using as a photoluminescence (PL) pressure gauge in the low-pressure regime. The energy-transfer exciton dynamics and trapping at non-PL centers have been explained through changes of the intrachain Mn-Mn exchange interaction and Cu${}^{2+}$-trap concentration carried out by applying pressure and doping, respectively. The model demonstrates that an increase of exchange interaction favors both the pumping capability and energy transfer yielding exciton migration. Under these conditions, we show that pressure enhances the PL efficiency of TMMC provided that the Cu${}^{2+}$ concentration responsible for the PL quenching is below 0.001 mol %. However, between 0.001% and 0.1%, the PL intensity reduces with pressure, and above 0.1%, the PL is practically quenched even at ambient conditions.

The effects of Tm substitution on the dynamical magnetic response of Yb${}_{1-x}$Tm${}_x$B${}_{12}$ $(x=0$, 0.08, 0.15, and 0.75) and Lu${}_{0.92}$Tm${}_{0.08}$B${}_{12}$ compounds have been studied using time-of-flight inelastic neutron scattering. Major changes were observed in the spectral structure and temperature evolution of the Yb contribution to the inelastic response for a rather low content of magnetic Tm ions. A sizable influence of the $R$B${}_{12}$ host (YbB${}_{12}$, as compared to LuB${}_{12}$ or pure TmB${}_{12}$) on the crystal-field splitting of the Tm${}^{3+}$ ion is also reported. The results point to a specific effect of impurities carrying a magnetic moment (Tm, as compared to Lu or Zr) in a Kondo insulator, which is thought to reflect the “undercompensation” of Yb magnetic moments, originally Kondo screened in pure YbB${}_{12}$. A parallel is made with the strong effect of Tm substitution on the temperature dependence of the Seebeck coefficient in Yb${}_{1-x}$Tm${}_x$B${}_{12}$, which was reported previously.

We discuss topological electronic states described by the Dirac Hamiltonian plus an additional one in three dimensions. When the additional Hamiltonian is an element of an Abelian group, electronic states become topologically nontrivial even in the absence of the fundamental symmetries such as the time-reversal symmetry and the particle-hole one. Such symmetry-breaking topological states are characterized by the Chern number defined in the two-dimensional partial Brillouin zone. The topological insulators in Zeeman fields are an example of the symmetry-breaking topological electric state. We show the crossover from the topological insulating phase to the topological semimetal one in strong Zeeman fields.

In the limit of low adatom concentration, we obtain exact analytic expressions for the local and total density of states (LDOS, TDOS) for a tight-binding model of adatoms on graphene. The model is not limited to nearest-neighbor hopping but can include hopping between carbon atoms at any separation. We also find an analytical expression for the spectral function $A(\mathbf{k},E)$ of an electron of Bloch vector $\mathbf{k}$ and energy $E$ on the graphene lattice, to first order in the adatom concentration. We treat the electron-electron interaction by including a Hubbard term on the adatom, which we solve within a mean-field approximation. For finite Hubbard $U$, we find the spin-polarized LDOS, TDOS, and spectral function self-consistently. For any choice of parameters of the tight-binding model within mean-field theory, we find a critical value of $U$ above which a moment develops on the adatom. Preliminary calculations also indicate that this moment can be switched on and off by varying the Fermi energy. For most choices of parameters, we find a substantial charge transfer from the adatom to the graphene host.