In this theoretical study we qualitatively and quantitatively investigate the electric dipole spin resonance (EDSR) in a single Si/SiGe quantum dot in the presence of a magnetic field gradient, e.g., produced by a ferromagnet. We model a situation in which the control of electron spin states is achieved by applying an oscillatory electric field, inducing real-space oscillations of the electron inside the quantum dot. One of the goals of our study is to present a microscopic theory of valley-dependent $g$ factors in Si/SiGe quantum dots and investigate how valley relaxation combined with a valley-dependent $g$ factor leads to a novel electron spin dephasing mechanism. Furthermore, we discuss the interplay of spin and valley relaxations in Si/SiGe quantum dots. Our findings suggest that the electron spin dephases due to valley relaxation, and are in agreement with recent experimental studies [Nat. Nanotechnol. 9, 666 (2014)].

The orbital magnetic moment of a finite piece of matter is expressed in terms of the one-body density matrix as a simple trace. We address a macroscopic system, insulating in the bulk, and we show that its orbital moment is the sum of a bulk term and a surface term, both extensive. The latter only occurs when the transverse conductivity is nonzero and it is due to conducting surface states. Simulations on a model Hamiltonian validate our theory.

We use a real-space implementation of pseudopotentials within the density functional theory to investigate the structural and magnetic properties of cobalt clusters with up to 365 atoms. We find from structural optimization that hcp and icosahedral clusters are lower in energy than bcc and fcc clusters. The calculated magnetic moments generally decrease as a function of increasing cluster size. For clusters of several hundred atoms the bulk limit becomes apparent. However, the decrease is not monotonic. It depends on the details of the interior structure of the cluster and the corresponding surface geometry. By analyzing the detailed evolution of the local magnetic moment, we find the spin moment is bulklike in the cluster interior and increases in the vicinity of the surface and can be correlated with coordination. The calculated behavior accounts for the observed variations in the measured moments.

We introduce a mechanism in which coupling between fluctuating spin magnetic dipole moments and polar optical phonons leads to a nonzero ferroic ordering of quasistatic magnetoelectric multipoles. Using first-principles calculations within the $\mathrm{LSDA}+U$ method of density functional theory, we calculate the magnitude of the effect for the prototypical cuprate superconductor ${\mathrm{HgBa}}_2{\mathrm{CuO}}_4$. We show that our proposed mechanism is consistent, to our knowledge, with all experimental data for the onset of the pseudogap phase and therefore propose the quasistatic magnetoelectric multipole as a possible pseudogap order parameter. Finally, we show that our mechanism embraces some key aspects of previous theoretical models, in particular the description of the pseudogap phase in terms of orbital currents.

In this paper we resolve a contradiction between the fact that the method based on the Majorana representation of spin-$\frac{1}{2}$ is exact and its failure to reproduce the perturbative Bloch-Redfield relaxation rates. Namely, for the spin-boson model, direct application of this method in the leading order allows for a straightforward computation of the transverse spin correlations; however, for the longitudinal spin correlations it apparently fails in the long-time limit. Here we indicate the reason for this failure. Moreover, we suggest how to apply this method to allow, nevertheless, for simple and accurate computations of spin correlations. Specifically, we demonstrate that accurate results are obtained by avoiding the use of the longitudinal Majorana fermion and that correlations of the remaining transverse Majorana fermions can be easily evaluated using an effective Gaussian action.

We study the ferromagnetic superconductor UCoGe at ambient pressure under $ab$-plane magnetic fields $\mathbf{H}$, which are perpendicular to the ferromagnetic easy axis. It is shown that, by taking into account the Dyaloshinskii-Moriya interaction arising from the zigzag chain crystal structure of UCoGe, we can qualitatively explain the experimentally observed in-plane anisotropy for critical magnetic fields of the paramagnetic transition. Because of this strong dependence on the magnetic field direction, upper critical fields of superconductivity, which is mediated by ferromagnetic spin fluctuations, also become strongly anisotropic. The experimental observation of $\mathsf{S}$-shaped $H_{c2}\parallel b$ axis is qualitatively explained as a result of enhancement of the spin fluctuations due to decreased Curie temperature by the $b$-axis magnetic field. We also show that the $\mathsf{S}$-shaped $H_{c2}$ is accompanied by a rotation of the $d$ vector, which would be a key to understand the experiments not only at ambient pressure but also under pressure.

We introduce ${\mathrm{Nd}}_3{\mathrm{Sb}}_3{\mathrm{Mg}}_2{\mathrm{O}}_{14}$ with ideal kagome lattices of neodymium ions in ABC stacking. Thermodynamic measurements show a Curie-Weiss temperature of ${\mathrm{\Theta }}_{\text{CW}}=-0.12$ K, a ${\mathrm{Nd}}^{3+}$ spin-$\frac{1}{2}$ Kramers doublet ground state, and a second-order phase transition at $T_N=0.56\left(2\right)$ K. Neutron scattering reveals noncoplanar scalar chiral $\mathbf{k}=\mathbf{0}$ magnetic order with a correlation length exceeding $400\text{Å}=55a$ and an ordered moment of $1.79\left(5\right){\mu }_B$. This order includes a canted ferromagnetic component perpendicular to the kagome planes favored by Dzyaloshinskii-Moriya interactions.

The metal-insulator switching characteristics of ${\mathrm{VO}}_2$ play a crucial role in the performances of ${\mathrm{VO}}_2$-based devices. In this paper we study high-quality (010)-oriented epitaxial films grown on (001) sapphire substrates by means of electron-beam evaporation and investigate the role of interface defects and thermal strain on the parallel evolution of the metal-insulator transition (MIT) and structural phase transition (SPT) between the monoclinic (insulator) and rutile (metal) phases. It is demonstrated that the highly-mismatched ${\mathrm{VO}}_2/{\mathrm{Al}}_2{\mathrm{O}}_3$ interface promotes a domain-matching epitaxial growth process where the film grows in a strain-relaxed state and the lattice distortions are confined at the interface in regions with limited spatial extent. Upon cooling down from the growth temperature, tensile strain is stored in the films as a consequence of the thermal expansion mismatch between ${\mathrm{VO}}_2$ and ${\mathrm{Al}}_2{\mathrm{O}}_3$. The thinnest films exhibit the highest level of tensile strain in the interfacial plane resulting in a shift of both the MIT and the SPT temperatures towards higher values, pointing to a stabilization of the monoclinic/insulating phase. Concomitantly, the electrical switching characteristics are altered (lower resistivity ratio and broader transition) as a result of the presence of structural defects located at the interface. The SPT exhibits a similar evolution with, additionally, a broader hysteresis due to the formation of an intermediate, strain-stabilized phase in the M1-R transition. Films with thickness ranging between 100–300 nm undergo a partial strain relaxation and exhibit the best performances, with a sharp ($10{}^{\circ }\mathrm{C}$ temperature range) and narrow (hysteresis $<4{}^{\circ }\mathrm{C}$) MIT extending over more than four orders of magnitude in resistivity $\left(6\times {10}^4\right)$.

We present temperature dependent Raman spectra of single crystals of two different samples of the spin-orbital liquid candidate ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$. The “hexagonal” sample is known to show no magnetic order down to low temperatures, while the “orthorhombic” sample undergoes a crossover into an orthorhombic crystal structure below $T_{\text{JT}}$ of approximately 200 K and shows spin freezing at 110 $\mu \mathrm{K}$. Our Raman scattering results demonstrate a strong influence of disorder in both samples. The frequencies of stretching vibrations of oxygens associated with ${\mathrm{CuSbO}}_9$ octahedra indicate a difference in the crystal structure between the hexagonal and orthorhombic samples even at room temperature. On cooling below $T_{\text{JT}}$ we observe new bands in the spectra of the orthorhombic sample due to a lowering of symmetry of the unit cell and a collective Jahn-Teller distortion. The spectra of the hexagonal sample show that average hexagonal symmetry is maintained at least down to 20 K. An analysis of the band shape of stretching oxygen vibrations suggests a weak disordered local Jahn-Teller distortion in the hexagonal sample of ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$ which increases slightly on cooling. This Jahn-Teller distortion is either static or dynamic with a frequency below 1.5 THz.

The interplay between the singlet ground state of the antiferromagnetic Heisenberg model and the experimentally measured Néel state of antiferromagnets is studied. To verify the hypothesis [M. I. Katsnelson et al., Phys. Rev. B 63, 212404 (2001)] that the latter can be considered to be a result of local measurements destroying the entanglement of the quantum ground state, we have performed systematic simulations of the effects of von Neumann measurements for the case of a one-dimensional antiferromagnetic spin-1/2 system for various types and degrees of magnetic anisotropies. It is found that in the ground state, a magnetization measurement can create decoherence waves [M. I. Katsnelson et al., Phys. Rev. A 62, 022118 (2000)] in the magnetic sublattices, and that a symmetry breaking anisotropy does not lead to alignment of the spins in a particular direction. However, for an easy-axis anisotropy of the same order magnitude as the exchange constant, a measurement on the singlet ground state can create Néel ordering in finite systems of experimentally accessible size.

In this paper, we calculate the entanglement negativity in free-fermion systems by use of the overlap matrices. For a tripartite system, if the ground state can be factored into triples of modes, we show that the partially transposed reduced density matrix can be factorized and the entanglement negativity has a simple form. However, the factorability of the ground state in a tripartite system does not hold in general. In this situation, the partially transposed reduced density matrix can be expressed in terms of the Kronecker product of matrices. We explicitly compute the entanglement negativity for the Su-Schrieffer-Heeger model, the integer Quantum Hall state, and a homogeneous one-dimensional chain. We find that the entanglement negativity for the integer quantum Hall states shows an area law behavior. For the entanglement negativity of two adjacent intervals in a homogeneous one-dimensional gas, we find agreement with the conformal field theory. Our method provides a numerically feasible way to study the entanglement negativity in free-fermion systems.

For a gapped disordered many-body system with both internal and translation symmetry, one can define the corresponding weak and strong symmetry protected topological (SPT) phases. A strong SPT phase is protected by the internal symmetry $G$ only while a weak SPT phase, fabricated by alignment of a strong SPT state in a lower dimension, requires additional discrete translation symmetry protection. In this paper, we construct a phase transition between weak and strong SPT phase in a strongly interacting boson system. The starting point of our construction is the superconducting Dirac fermions with pair density wave (PDW) order in $2d$. We first demonstrate that the nodal line of the PDW contains a $1d$ boson SPT phase. We further show that melting the PDW stripe and condensing the nodal line provoke the transition from weak to strong SPT phase in $2d$. The phase transition theory contains an O(4) nonlinear-$\sigma \mathrm{model}(\mathrm{NL}\sigma \mathrm{M})$ with topological $\mathrm{\Theta }$ term emerging from the proliferation of domain walls bound to an SPT chain. A similar scheme also applies to weak-strong SPT transition in other dimensions and predicts possible phase transition from $2d$ to $3d$ topological order.

We use first-principles free-energy calculations to predict a pressure-induced spin crossover in the liquid planetary material (Mg,Fe)O, whereby the magnetic moments of Fe ions vanish gradually over a range of hundreds of GPa. Because electronic entropy strongly favors the nonmagnetic low-spin state of Fe, the crossover has a negative effective Clapeyron slope, in stark contrast to the crystalline counterpart of this transition-metal oxide. Diffusivity of liquid (Mg,Fe)O is similar to that of MgO, displaying a weak dependence on element and spin state. Fe-O and Mg-O coordination increases from approximately 4 to 7 as pressure goes from 0 to 200 GPa. We find partitioning of Fe to induce a density inversion between the crystal and melt, implying separation of a basal magma ocean from a surficial one in the early Earth. The spin crossover induces an anomaly into the density contrast, and the oppositely signed Clapeyron slopes for the crossover in the liquid and crystalline phases imply that the solid-liquid transition induces a spin transition in (Mg,Fe)O.

Based on first-principles calculations, we predict a substantial increase in the optical dielectric function of ${\mathrm{LiAsSe}}_2$ under pressure. We find that the optical dielectric constant is enhanced threefold under compression along all three axes by 3%. This enhancement is mainly due to the dimerization strength reduction of the one-dimensional (1D) As–Se chains in ${\mathrm{LiAsSe}}_2$, which significantly alters the wave function phase mismatch between two neighboring chains and changes the transition intensity. By developing a tight-binding model of the interacting 1D chains, the essential features of the low-energy electronic structure of ${\mathrm{LiAsSe}}_2$ are captured. Our findings are important for understanding the fundamental physics of ${\mathrm{LiAsSe}}_2$ and provide a feasible way to enhance the material optical response that can be applied to light harvesting for energy applications.

Cd(Se,Te) quantum dots (QDs) in a ZnSe barrier typically exhibit a very high spectral density, which precludes investigation of single dot photoluminescence. We design, grow, and study individual Cd(Se,Te)/ZnSe QDs of low spectral density of emission lines achieved by the implementation of Mn-assisted epitaxial growth. We find an unusually large variation of exciton-biexciton energy difference ($3\text{meV}\le \mathrm{\Delta }{E}_{X-XX}\le 26$ meV) and of exciton radiative recombination rate in the statistics of QDs. We observe a strong correlation between the exciton-biexciton energy difference, exciton recombination rate, splitting between dark and bright excitons, and additionally the exciton fine-structure splitting ${\delta }_1$ and the Landé factor. The above results indicate that the values of the ${\delta }_1$ and of the Landé factor in the studied QDs are dictated primarily by the electron and hole respective spatial shift and wave functions overlap, which vary from dot to dot due to a different degree of localization of electrons and holes in, respectively, CdSe- and CdTe-rich QD regions.

We systematically investigate the plasmonic “dolmen” geometry and its constituent elements using electron energy-loss spectroscopy and cathodoluminescence spectroscopy. In particular, we study the effects of the particle size and spacing on the resonant behavior and interparticle coupling. Because we apply both techniques on the same structures we can directly compare the results and investigate the radiative versus nonradiative character of the different modes. We find that the cathodoluminescence response is significantly lower than the electron energy-loss response for higher-energy modes because strong absorption reduces the scattering efficiency in this regime. Furthermore, we show that the overall resonant response roughly scales with size as expected for plasmonic structures but that the transverse resonant modes do become more dominant in larger structures due to a relative reduction in Ohmic dissipation. Using EELS and CL we can rigorously study coupling between the elements and show that the coupling diminishes for larger spacings.

We report a nonsaturating linear magnetoresistance in charge-compensated bilayer graphene in a temperature range from 1.5 to 150 K. The observed linear magnetoresistance disappears away from charge neutrality, ruling out the traditional explanation of the effect in terms of the classical random resistor network model. We show that experimental results qualitatively agree with a phenomenological two-fluid model taking into account electron-hole recombination and finite-size sample geometry.

Coherent twin boundaries form periodic lamellar twinning in a wide variety of semiconductor nanowires, and they are often viewed as near-perfect interfaces with reduced phonon and electron scattering behaviors. Such unique characteristics are of practical interest for high-performance thermoelectrics and optoelectronics; however, insufficient understanding of twin-size effects on thermal boundary resistance poses significant limitations for potential applications. Here, using atomistic simulations and ab initio calculations, we report direct computational observations showing a crossover from diffuse interface scattering to superlatticelike behavior for thermal transport across nanoscale twin boundaries present in prototypical bulk and nanowire Si examples. Intrinsic interface scattering is identified for twin periods $\ge 22.6$ nm, but it also vanishes below this size to be replaced by ultrahigh Kapitza thermal conductances. Detailed analysis of vibrational modes shows that modeling twin boundaries as atomically thin $6H\text{−}\mathrm{Si}$ layers, rather than phonon scattering interfaces, provides an accurate description of effective cross-plane and in-plane thermal conductivities in twinning superlattices, as a function of the twin period thickness.