We investigate the in-plane oscillations of relaxed-shape graphene due to externally applied tensile edge stress along both the armchair and zigzag directions. We show that the total elastic energy density is enhanced with temperature for the case of applied tensile edge stress along the zigzag direction. Thermoelectromechanical effects are treated via pseudomorphic vector potentials to analyze the influence of these coupled effects on the band structures of bilayer-graphene quantum dots. We report that the level crossing between the ground and first-excited states in the localized edge states can be achieved with accessible values of temperature. In particular, the level-crossing point extends to higher temperatures with decreasing externally applied tensile edge stress along the armchair direction. This kind of level crossing is absent in the states formed at the center of the graphene sheet due to the presence of threefold symmetry.

We analyze out-of-equilibrium fluctuations in a driven spin system and relate them to the noise of spin susceptibility. In the spirit of the linear response theory we further relate the noise of susceptibility to a 4-spin correlation function in equilibrium. We show that, in contrast to the second noise (noise of noise), the noise of susceptibility is a direct measure of non-Gaussian fluctuations in the system. We develop a general framework for calculating the noise of susceptibility using the Majorana representation of spin-1/2 operators. We illustrate our approach by a simple example of noninteracting spins coupled to a dissipative (Ohmic) bath.

Exploring two-dimensional layered materials, such as molybdenum disulfide $\left({\mathrm{MoS}}_2\right)$, for (opto)electronic applications requires detailed knowledge of their electronic band structures. Using first-principles calculations we trace the evolution of the band structure as a function of the number of layers, starting from a monolayer, which has a direct gap, to the bulk material, which has an indirect gap. We find that, with respect to the vacuum level, the valence-band maximum (VBM) increases rapidly with the number of layers, while the conduction-band minimum (CBM) remains almost constant. For two or more layers the VBM always occurs at $\Gamma$ and the CBM occurs at K. These findings are analyzed in terms of the orbital composition of the valence- and conduction-band edges at the various high-symmetry points in the Brillouin zone.

Using first principles calculations we showed that the electronic and optical properties of single-layer black phosphorus (BP) depend strongly on the applied strain. Due to the strong anisotropic atomic structure of BP, its electronic conductivity and optical response are sensitive to the magnitude and the orientation of the applied strain. We found that the inclusion of many body effects is essential for the correct description of the electronic properties of monolayer BP; for example, while the electronic gap of strainless BP is found to be 0.90 eV by using semilocal functionals, it becomes 2.31 eV when many-body effects are taken into account within the $G_0{W}_0$ scheme. Applied tensile strain was shown to significantly enhance electron transport along zigzag direction of BP. Furthermore, biaxial strain is able to tune the optical band gap of monolayer BP from 0.38 eV (at $-8\%$ strain) to 2.07 eV (at 5.5%). The exciton binding energy is also sensitive to the magnitude of the applied strain. It is found to be 0.40 eV for compressive biaxial strain of $-8\%$, and it becomes 0.83 eV for tensile strain of 4%. Our calculations demonstrate that the optical response of BP can be significantly tuned using strain engineering which appears as a promising way to design novel photovoltaic devices that capture a broad range of solar spectrum.

We report a determination of the complex in-plane dielectric function of monolayers of four transition-metal dichalcogenides: $\mathrm{Mo}{\mathrm{S}}_2$, $\mathrm{Mo}\mathrm{S}{\mathrm{e}}_2$, $\mathrm{W}{\mathrm{S}}_2$, and $\mathrm{WS}{\mathrm{e}}_2$, for photon energies from 1.5 to 3 eV. The results were obtained from reflection spectra using a Kramers-Kronig constrained variational analysis. From the dielectric functions, we obtain the absolute absorbance of the monolayers. We also provide a comparison of the dielectric function for the monolayers with the corresponding bulk materials.

An effect of metallization of a magnonic crystal surface on band-gap formation in the spectra of a surface spin wave (SSW) is studied both theoretically and experimentally. The structures under consideration are one-dimensional magnonic crystals based on yttrium iron garnet with an array of etched grooves, with and without a metal screen on the top of the corrugated surface. Due to nonreciprocity of propagation of the SSW, the shift of the band gap to higher frequency and from the border of the Brillouin zone in the presence of a conducting overlayer was measured in a transmission line experiment. Results of numerical calculations and model analysis are in agreement with experimental data and give further insight into the origin of the band gap and properties of the nonreciprocal SSW in metalized magnonic crystals. This gives a positive answer to the outstanding question about the possibility of detection of magnonic band gaps in the spectra of spin waves with nonreciprocal dispersion in magnonic crystals, and creates the potential for new applications and improvements of already existing prototype magnonic devices.

Spin liquids represent exotic types of quantum matter that evade conventional symmetry-breaking order even at zero temperature. Exhaustive classifications of spin liquids have been carried out in several systems, particularly in the presence of full SU(2) spin-rotation symmetry. Real magnetic compounds, however, generically break SU(2) spin symmetry as a result of spin-orbit coupling—which in many materials provides an “order one” effect. We generalize previous works by using the projective symmetry group method to classify ${\mathbb{Z}}_2$ spin liquids on the square lattice when SU(2) spin symmetry is maximally lifted. We find that, counterintuitively, the lifting of spin symmetry actually results in vastly more spin-liquid phases compared to SU(2)-invariant systems. A generic feature of the SU(2)-broken case is that the spinons naturally undergo $p+ip$ pairing; consequently, many of these ${\mathbb{Z}}_2$ spin liquids feature a topologically nontrivial spinon band structure supporting gapless Majorana edge states. We study in detail several spin-liquid phases with varying numbers of gapless edge states and discuss their topological protection. The edge states are often protected by a combination of time reversal and lattice symmetries and hence resemble recently proposed topological crystalline superconductors.

Combined electric- and magnetic-field control of magnetization orientation and reversal is studied using anomalous Hall effect in an ultrathin ferromagnetic (Ga,Mn)As/(Ga,Mn)(As,P) bilayer. Its anisotropy results from the electrically tunable competition between the in-plane and out-of-plane anisotropies of both layers. The magnetic hysteresis loop shape is sensitive to the bias electric field. In the loop reversible part, an electric-field variation is found to reorient reversibly the magnetization. In this case, the magnetization direction follows the easy anisotropy direction controlled by electric field. In contrast, in the hysteretic part, an almost complete nonreversible magnetization reversal is achieved. This is interpreted as resulting from the electric-field-induced enhancement of domain nucleation and domain-wall propagation.

This study aims at inquiring into the role of hydrostatic pressure in driving the relaxor behavior within a local-symmetry-based approach to relaxor ferroelectrics. Results reveal the occurrence of a pressure-induced ferroelectric-to-relaxor crossover, clearly reflected in the experiment-matching temperature-pressure phase diagram of lanthanum-modified lead zirconate titanate. Relaxor behavior is found to occur under pressure and upon cooling due to the nucleation of local order within fractal regions, as an incipient state towards percolating ferroelectric order. Further analysis of the geometrical features of ordering process points to a manifest nontrivial disruption of the balance between competing interactions under the conjugated effects of pressure and local-symmetry constraints.

Solid ${}^4\mathrm{He}$ is a unique example of crystal where dislocations may move at macroscopic speeds with impurities attached to them. In ${}^4\mathrm{He}$ crystals, the only impurities are ${}^3\mathrm{He}$ atoms, whose concentration can be reduced to zero and measured down to the ppt $\left(10{}^{-12}\right)$ level. We present measurements of the mobility of dislocations dressed with ${}^3\mathrm{He}$ impurities as a function of the crystal purity. They show that the damping of dislocation motion is proportional to the concentration of ${}^3\mathrm{He}$ bound to these dislocations. It has allowed us to measure the ${}^3\mathrm{He}$ binding energy $E_B$ to dislocations without any ambiguity. Our results solve the controversy concerning $E_B$: We confirm our previously measured value $0.7 \pm$ 0.1 K, and we demonstrate that it cannot be 0.2 or 0.4 K as estimated by other authors. Finally, we present a simple model for the damping magnitude, where dissipation is due to the emission by moving ${}^3\mathrm{He}$ impurities of transverse waves along the dislocation lines.

Thermal conductivity measurements in the superconducting state of the ferromagnet UCoGe were performed at very low temperatures and under magnetic field on samples of different qualities and with the heat current along the three crystallographic axis. This allows one to disentangle intrinsic and extrinsic effects, confirm the situation of multigap superconductivity, and shed new light on the situation expected or claimed for the gap in these ferromagnetic superconductors, such as evidence of the absence of “partially gapped” Fermi surfaces.

By evaluating the upper and thermodynamic critical fields $H_{c2}$ and $H_c$ and their ratio $H_{c2}/H_c$ at arbitrary temperatures, we argue that situations are possible when a type-II material is transformed into type I by adding magnetic impurities.

We numerically test an experimentally realizable method for the extraction of the critical Casimir force based on its thermodynamic definition as the derivative of the excess free energy with respect to system size. Free energy differences are estimated for different system sizes by integrating the order parameter along an isotherm. The method could be developed for experiments on magnetic systems and could give access to the critical Casimir force for any universality class. By choosing an applied field that opposes magnetic ordering at the boundaries, the Casimir force is found to increase by an order of magnitude over zero-field results.

We study the quantum phase transition of an $S=\frac{1}{2}$ anisotropic $\alpha$ $(\equiv J_z/J_{xy})$ Heisenberg antiferromagnet on a triangular lattice. We calculate the sublattice magnetization and the long-range helical order parameter and their Binder ratios on finite systems with $N\le 36$ sites. The $N$ dependence of the Binder ratios reveals that the classical ${120}^{\circ }$ Néel state occurs for $\alpha \lesssim 0.55$, whereas a critical collinear state occurs for $1/\alpha \lesssim 0.6$. This result is at odds with a widely held belief that the ground state of a Heisenberg antiferromagnet is the ${120}^{\circ }$ Néel state, but it also provides a possible mechanism explaining experimentally observed spin liquids.

In nature, many systems exhibit modulated phases with periodic macroscopic patterns and textures mainly due to the competitive interactions of different phases. Vortex systems in superconductors, which are easy to access, offer the possibility of tuning the ratio between the competitive interactions, providing a unique tool to study the evolution and equilibrium of similar patterns. The $\kappa -T$ phase diagram of clean superconductors shows the transition from type-I to type-II superconductivity via a narrow $\kappa$ range near the dual point $\kappa =1/\sqrt{2}$ where the vortices attract each other at long distances and repel each other at short distances. This $\kappa$ range, which is termed the type-II/1 phase, becomes larger with decreasing temperature. The direct imaging at the scale of individual vortices of the vortex pattern transition would provide valuable information. Therefore, by using scanning Hall probe microscopy, we have performed direct visualization of the vortex pattern transition in a ${\mathrm{ZrB}}_{12}$ single crystal across the type-II and type-II/1 phases. By gradually lowering the temperature, and thereby tuning vortex interactions, a transition is observed from the ordered Abrikosov vortex lattice to a disordered vortex pattern with large areas of Meissner phase, vortex chains, and vortex clusters. The formation of vortex chains and clusters has been found to arise from the combined effect of quenched disorder and the attractive vortex-vortex interaction in the type-II/1 phase. The clusters and chains serve as the vortex reservoir to enable the formation of a triangular vortex lattice of the type-II phase at high temperatures.

Motivated by the proposed topological state in ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$, we study the possibility of phonon-mediated odd-parity superconductivity in spin-orbit coupled systems with time-reversal and inversion symmetry. For such systems, we show that, in general, pure electron-phonon coupling can never lead to a triplet state with a higher critical temperature than the leading singlet state. The Coulomb pseudopotential, which is the repulsive part of the electron-electron interaction and is typically small in weakly correlated systems, is therefore critical to stabilizing the triplet state. We introduce a chirality quantum number, which identifies the electron-phonon vertex interactions that are most favorable to the triplet channel as those that conserve chirality. Applying these results to ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$, we find that a phonon-mediated odd-parity state may be realized in the presence of weak electronic correlations if the chirality-preserving electron-phonon vertices are much stronger than the chirality-flipping vertices.

We study the nuclear spin relaxation in a ballistic nanowire with hyperfine and Rashba spin-orbit interactions (SOI) and in the presence of magnetic field and electron interactions. The relaxation rate shows pronounced peaks as a function of magnetic field and chemical potential due to van Hove singularities in the Rashba bands. As a result, the regimes of weak and strong SOIs can be distinguished by the number of peaks in the rate. The relaxation rate increases with increasing magnetic field if both Rashba subbands are occupied, whereas it decreases if only the lowest one is occupied.

The ability to discriminate between simultaneously occurring noise sources in the local environment of semiconductor InGaAs quantum dots, such as electric and magnetic field fluctuations, is key to understanding their respective dynamics and their effect on quantum dot coherence properties. We present a discriminatory approach to all-optical sensing based on two-color resonance fluorescence of a quantum dot charged with a single electron. Our measurements show that local magnetic field fluctuations due to nuclear spins in the absence of an external magnetic field are described by two correlation times, both in the microsecond regime. The nuclear spin bath dynamics show a strong dependence on the strength of resonant probing, with correlation times increasing by a factor of 4 as the optical transition is saturated. We interpret the behavior as motional averaging of both the Knight field of the resident electron spin and the hyperfine-mediated nuclear spin-spin interaction due to optically induced electron spin flips.

We develop and discuss appropriate methods based on x-ray diffraction and generalized infrared spectroscopic ellipsometry to identify wurtizte and zinc-blende polymorphs, and quantify their volume fractions in mixed-phase epitaxial films taking InN as an example. The spectral signatures occurring in the azimuth polarization (Müller matrix) maps of mixed-phase epitaxial InN films are discussed and explained in view of polymorphism (zinc-blende versus wurtzite), volume fraction of different polymorphs and their crystallographic orientation, and azimuth angle. A comprehensive study of the structural, phonon and free electron properties of zinc-blende InN films containing inclusions of wurtzite InN is also presented. Thorough analysis on the formation of the zinc-blende and wurtzite phases is given and the structural evolution with film thickness is discussed in detail. The phonon properties of the two phases are determined and discussed together with the determination of the bulk free-charge carrier concentration, and electron accumulation at the mixed-phase InN film surfaces.