Heating of a metal film with a weak oscillating heat source leads to oscillations of the electron temperature $T_e$. Because of the temperature dependence of phonon contribution to electrical conductivity, the oscillations of $T_e$ result in oscillations of voltage across the current-carrying film. Using the equation for $T_e$ and the expression for the phonon contribution to the electrical conductivity, we have found the connection of the frequency dependence of the amplitude of the voltage oscillations across the film with the time of electron-phonon collisions (${\tau }_e$) and the mean phonon escape time from the film to the substrate (${\tau }_{es}$). Renormalization of electron-phonon interaction by impurities and defects is considered phenomenologically. It is discussed how the times ${\tau }_e$ and ${\tau }_{es}$ can be found experimentally using the peculiarities of the frequency dependence of the voltage oscillations across the film.

Neutron diffraction measurements are presented measuring the responses of both magnetic and structural order parameters of parent and lightly Co-doped Ba(${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$)${}_2{\mathrm{As}}_2$ under the application of uniaxial pressure. We find that the uniaxial pressure induces a thermal shift in the onset of antiferromagnetic order that grows as a percentage of $T_N$ as Co doping is increased and the superconducting phase is approached. Additionally, as uniaxial pressure is increased within parent and lightly doped Ba(${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$)${}_2{\mathrm{As}}_2$ on the first-order side of the tricritical point, we observe a decoupling between the onsets of the orthorhombic structural distortion and antiferromagnetism. Our findings place needed constraints on models exploring the nematic susceptibility of the bilayer pnictides in the tetragonal, paramagnetic regime.

Growth of ferroelectric nanotori is reported and first-principles-based effective Hamiltonian simulations were performed on these new objects. They could reproduce the nonpolar (phase I) and homogeneously toroidized (phase II) states of an isolated nanotorus. Computation of an axial hypertoroidal moment leads to numerical observation of two new phases: (i) a homogeneously hypertoroidized one (phase III) that can be switched by a homogeneous electric field and (ii) another one with striking new polarization patterns (phase IV) due to azimuthal variations of the hypertoroidal moment. In both phases, hypertoroidization coexists with a homogeneous axial toroidal moment.

We study the behavior of skyrmions in thin films under the action of stochastic torques arising from thermal fluctuations. We find that the Brownian motion of skyrmions is described by a stochastic Thiele's equation and its corresponding Fokker-Planck equation. The resulting Fokker-Planck equation is recognized as the one for a high-friction Brownian particle which has been studied extensively in different physical contexts. It is shown that thermal fluctuations favor the skyrmion motion allowing a finite mobility even in presence of pinning traps. We calculate explicitly the mobility tensor of skyrmions in linear response to an electric current finding that it increases with temperature.

We investigate the Hubbard model on the anisotropic triangular lattice with two hopping parameters $t$ and $t^{\prime }$ in different spatial directions, interpolating between decoupled chains ($t=0$) and the isotropic triangular lattice ($t=t^{\prime }$). Variational wave functions that include both Jastrow and backflow terms are used to compare spin-liquid and magnetic phases with different pitch vectors describing both collinear and coplanar (spiral) order. For relatively large values of the on-site interaction $U/t^{\prime }\gtrsim 10$ and substantial frustration, i.e., $0.3\lesssim t/t^{\prime }\lesssim 0.8$, the spin-liquid state is clearly favored over magnetic states. Spiral magnetic order is only stable in the vicinity of the isotropic point, while collinear order is obtained in a wide range of interchain hoppings from small to intermediate frustration.

The experimental study of the modulation of the envelope of spin-echo signals due to internal and external fields is an important spectroscopic tool to detect very small internal magnetic fields. We derive the free induction decay and the frequency spectrum and amplitude of spin-echo signals for arbitrary orientation of fields with respect to crystalline axis for nuclei in a crystal of orthorhombic symmetry. Results reproduce the results that no modulation should be observed in tetragonal crystals for fields either along the $c$ axis or any direction in the basal plane and give details of the signal as a function of the orthorhombicity parameter. These results correct the theoretical expressions given in some papers along with experiments and provide guidelines for future experiments. We also note that the exact theoretical results obtained disagree with the experimental results reported which give large oscillations in a tetragonal crystal where symmetry dictates that none should be found. With known external fields applied to an orthorhombic crystal, the oscillations expected can also be exactly calculated. The experiments disagree severely with these calculations also. The disagreements of the experimental results with results required by symmetry and those that can be exactly calculated call into question the experimental methods used.

The interplay of spin orbit coupling and electron electron interaction condensing new phases of matter is an important new phenomena in solid state physics. In this paper we explore the nature of excitonic phases induced in Weyl semimetals by long-range Coulomb repulsion. It has been previously shown that within mean field short-range repulsion leads to a ferromagnetic insulator, while short-range attraction results in a charge density wave state. Here we show that the charge density wave is the energetically favored state in the presence of long-range repulsion.

We consider a recently proposed network model of the integer quantum Hall (IQH) effect in a weak magnetic field. Using a supersymmetry approach, we reformulate the network model in terms of a superspin ladder. A subsequent analysis of the superspin ladder and the corresponding supersymmetric nonlinear sigma model allows us to establish the phase diagram of the network model, and the form of the critical line of the weak-field IQH transition. Our results confirm the universality of the IQH transition, which is described by the same sigma model in strong and weak magnetic fields. We apply the suspersymmetry method to several related network models that were introduced in the literature to describe the quantum Hall effect in graphene, the spin-degenerate Landau levels, and localization of electrons in a random magnetic field.

Donors in silicon are now demonstrated as one of the leading candidates for implementing qubits and quantum information processing. Single qubit operations, measurements, and long coherence times are firmly established, but progress on controlling two qubit interactions has been slower. One reason for this is that the interdonor exchange coupling has been predicted to oscillate with separation, making it hard to estimate in device designs. We present a multivalley effective mass theory of a donor pair in silicon, including both a central cell potential and the effective mass anisotropy intrinsic in the Si conduction band. We are able to accurately describe the single donor properties of valley-orbit coupling and the spatial extent of donor wave functions, highlighting the importance of fitting measured values of hyperfine coupling and the orbital energy of the $1s$ levels. Ours is a simple framework that can be applied flexibly to a range of experimental scenarios, but it is nonetheless able to provide fast and reliable predictions. We use it to estimate the exchange coupling between two donor electrons and we find a smoothing of its expected oscillations, and predict a monotonic dependence on separation if two donors are spaced precisely along the [100] direction.

The in-plane shear ($C$) modes between neighbor layers of 2-4 layer graphenes (2-4LGs) and the corresponding 2-4 layer graphene scrolls (2-4LGSs) rolled up by 2-4LGs at edges are investigated by Raman scattering. In contrast to the result that only one $C$ mode is observed in 3-4LGs, all the $C$ modes of 3-4LGs are observed in 3-4LGSs, whose frequencies agree with the theoretical prediction by the force-constant and linear chain models. The results indicate that the $C$ mode intensity of 2-4LGSs is resonantly enhanced by the electronic transition gaps of band structures of 2-4LGS structures at edges, which makes it possible to observe all the $C$ modes. Indeed, for a simple assumption, the calculated band structures of twisted ($n+n$)LGs ($n=2$,3,4) show parallel conduction and valence bands and the corresponding Van Hove singularities in the joint density of states along $\Gamma$-M, $\Gamma$-K and/or K-M directions. The intensity resonance of the $C$ modes provides direct evidence to explain how the band structure of few layer graphenes can be sensitive to local stacking configurations. This result can be extended to $n$ layer graphene ($n>4$) for understanding the basic phonon and electronic properties of multilayer graphenes. This observation of all the $C$ modes in graphene scrolls can be foreseen in other two-dimensional materials with similar scroll structures.

First, we study a square fermionic lattice that supports the existence of massless Dirac fermions, where the Dirac points are protected by a hidden symmetry. We also consider two modified models with a staggered potential and the diagonal hopping terms, respectively. In the modified model with a staggered potential, the Dirac points exist in some range of magnitude of the staggered potential, and move with the variation of the staggered potential. When the magnitude of the staggered potential reaches a critical value, the two Dirac points merge. In the modified model with the diagonal hopping terms, the Dirac points always exist and just move with the variation of amplitude of the diagonal hopping. We develop a mapping method to find hidden symmetries evolving with the parameters. In the two modified models, the Dirac points are protected by this kind of hidden symmetry, and their moving and merging process can be explained by the evolution of the hidden symmetry along with the variation of the corresponding parameter.

We derive thermomagnonic torque and its “$\beta$-type” dissipative correction from the stochastic Landau-Lifshitz-Gilbert equation. The $\beta$-type dissipative correction describes viscous coupling between magnetic dynamics and magnonic current and it stems from spin mistracking of the magnetic order. We show that thermomagnonic torque is important for describing temperature gradient induced motion of skyrmions in helical magnets while dissipative correction plays an essential role in generating transverse Magnus force. We propose to detect such skyrmionic motion by employing the transverse spin Seebeck effect geometry.

Multiband models with hot spots are of current interest partly because of their relevance for the iron-based superconductors. In these materials, the momentum-dependent scattering off spin fluctuations and the ellipticity of the electron Fermi pockets are responsible for anisotropy of the lifetimes of excitations around the Fermi surface. The deep minima of the lifetimes—the so-called hot spots—have been assumed to contribute little to the transport as is indeed predicted by a simple relaxation-time approach. Calculating forward-scattering corrections to this approximation, we find that the effective transport times are much more isotropic than the lifetimes and that, therefore, the hot spots contribute to the transport even in the case of strong spin-fluctuation scattering. We discuss this effect on the basis of an analytical solution of the Boltzmann equation and calculate numerically the temperature and doping dependence of the resistivity and the Hall, Seebeck, and Nernst coefficients.

We report theoretical calculations of electronic and transport properties mediated by topological helical states on the walls of three-dimensional topological insulator (TI) nanowires. A universal regime of quantized conductance and fluctuations is found that is induced by disorder. The average conductance of the disordered nanowire scales as a function of the number of transmission channels $N$ in a universal form $\langle G\rangle =\alpha N+\beta$, independent of the system details. For instance, for ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ nanowires cleaved along the $x$ or $y$ direction with the quintuple layers along the $z$ direction, $\langle G_{xx}\rangle =(5/12)N+1/2$. The universal and quantized behavior is due to the topological physics happening on the walls of the nanowire under the influence of disorder.

We consider optical signatures of valence band mixing in positive trion and exciton complexes in vertically stacked InGaAs quantum dots. We use the configuration interaction method and an axially symmetric four-band Luttinger-Kohn Hamiltonian (KL) that allows for heavy-hole and light-hole band mixing due to spin-orbit interaction. A scalar effective hole mass model is also included for comparison. We found essential differences (i.e., different recombination patterns) between the KL and separated-bands model spectra. In the weak-coupling regime for KL model, we obtained a good agreement with experimentally observed X patterns in contrast to the scalar effective mass model.

We investigate the mechanical resonance characteristics of semiconductor rolled-up tubes containing a high-mobility two-dimensional electron gas (HM2DEG) by optical and electrical means. The observed mode frequencies are in an excellent agreement with the theoretically calculated frequencies for the ground bending and excited bending and axial modes. The effect of the curvature is to increase the frequencies of the ground bending modes and the axial wave modes, while decreasing the frequencies of the first excited bending modes. We find significant splitting of the bending and twisting modes by the residual stress effects due to axial shear relaxation in $z$-dependent modes. The HM2DEG interacts with the mechanical motion due to Eddy currents and embedded impedances. A prominent asymmetry appears in the vibration amplitude with respect to the direction of the magnetic field. This originates from the broken symmetry of the HM2DEG on the curved surfaces.

We develop a first-principles theoretical approach to doping in field-effect devices. The method allows for calculation of the electronic structure as well as complete structural relaxation in field-effect configuration using density-functional theory. We apply our approach to ionic-liquid-based field-effect doping of monolayer, bilayer, and trilayer ZrNCl and analyze in detail the structural changes induced by the electric field. We show that, contrary to what is assumed in previous experimental works, only one ZrNCl layer is electrochemically doped and that this induces large structural changes within the layer. Surprisingly, despite these structural and electronic changes, the density of states at the Fermi energy is independent of the doping. Our findings imply a substantial revision of the phase diagram of electrochemically doped ZrNCl and elucidate crucial differences with superconductivity in Li-intercalated bulk ZrNCl.

Polarized Raman spectra of oriented $\mathrm{Ca}{\mathrm{Mn}}_7{\mathrm{O}}_{12}$ single crystals have been studied in a broad temperature range (5–500 K) covering the crystallographic, ferroelectric, and magnetic transitions. From the spectra obtained at room temperature (RT) 10 out of the total 12 Raman active $6A_g+6E_g$ modes of the trigonal $R\overline{3}$ phase have been identified and assigned to definite atomic motions in close comparison to results of lattice-dynamics modeling. With the decrease in temperature below 250 K some Raman bands split, and additional spectral features, not expected for the $R\overline{3}$ structure, arise well above the magnetic transitions near 90 and 50 K. Their appearance, however, correlates with the transformation due to incommensurate structural modulation revealed by recent x-ray and neutron-diffraction studies. As temperature increases above RT the intensity of the Raman lines strongly decreases, and entering the high-temperature $Im\overline{3}$ phase the spectrum transforms its shape to that expected for a structure with disordered Jahn-Teller distortions.