We propose an electromagnetically tunable thermal diode based on a two-phase multiferroic composite. Analytical and full numerical calculations for a prototypical heterojunction composed of iron on barium titanate in the tetragonal phase demonstrate a strong heat rectification effect that can be controlled externally by a moderate electric field. This finding is important for thermally based information processing and sensing and can also be integrated in (spin) electronic circuits for heat management and recycling.

One of the main goals of spintronics is to improve transport of information carriers and to achieve new functionalities with ultra-low dissipation. A most promising strategy for this holy grail is to use pure magnon currents created and transported in insulating magnets, in the complete absence of any conducting metallic elements. Here we propose a realistic solution to this fundamental challenge by analyzing magnon and heat transport in insulating ferromagnetic junctions. We calculate all transport coefficients for magnon transport and establish Onsager relations between them. We theoretically discover that magnon transport in junctions has a universal behavior, i.e., is independent of material parameters, and establish a magnon analog of the celebrated Wiedemann-Franz law, which governs charge transport at low temperatures. We calculate the Seebeck and Peltier coefficients, which are crucial quantities for spin caloritronics, and demonstrate that they assume universal values in the low-temperature limit. Finally, we show that our predictions are within experimental reach with current device and measurement technologies.

The effect of electronic dispersion over a wide variety of ${\mathrm{SiO}}_2$ polymorphs (faujasite, ferrierite, $\alpha$-cristobalite, $\alpha$-quartz, coesite, and stishovite) is investigated using state-of-the-art density functional theory. Different functionals and dispersion correction schemes are compared, ranging from the local density approximation to fully nonlocal exchange-correlation functionals. It is shown that both empirical dispersion corrections and fully nonlocal functionals improve the energetics and give correct volumetric data. However, the correct volume results come from error cancellation between an overestimation of the Si-O distance and an underestimation of the Si-O-Si angle. Quantum Monte Carlo is used to compute the quartz-cristobalite energy difference within an accuracy of $0.2$ kCal/mol per ${\text{SiO}}_2$ unit. This demonstrates the feasability of achieving subchemical accuracy on extended systems, and confirms the validity of the Slater-Jastrow ansatz for describing ${\mathrm{SiO}}_2$ polymorphs.

We present a complete analytical and numerical solution of the typical medium theory (TMT) for the Anderson metal-insulator transition. This approach self-consistently calculates the typical amplitude of the electronic wave functions, thus representing the conceptually simplest order-parameter theory for the Anderson transition. We identify all possible universality classes for the critical behavior, which can be found within such a mean-field approach. This provides insights into how interaction-induced renormalizations of the disorder potential may produce qualitative modifications of the critical behavior. We also formulate a simplified description of the leading critical behavior, thus obtaining an effective Landau theory for Anderson localization.

The static and dynamic magnetic behavior of 5-nm-thick $\mathrm{C}{\mathrm{o}}_{49}\mathrm{P}{\mathrm{t}}_{51}$ films with strong perpendicular magnetic anisotropy (PMA) grown at different deposition temperatures $\left(T_{\mathrm{d},\mathrm{CoPt}}\right)$ was investigated using complementary techniques such as vibrating sample magnetometry (VSM), magneto-optical Kerr effect (MOKE) imaging, and Brillouin light scattering (BLS). Our previous study on these films demonstrated the evolution of phase transformation from an $A3$-disordered (hexagonal) to an $L{1}_1$-ordered (rhombohedral) structure against increasing $T_{\mathrm{d},\mathrm{CoPt}}$ from room temperature (RT) to 350°C. Along these lines, the changes in the domain configuration, magnetization reversal, and spin wave behavior of the 5-nm-thick $\mathrm{CoPt}$ films due to varying $T_{\mathrm{d},\mathrm{CoPt}}$ are emphasized in this study. The VSM out-of-plane hysteresis loops confirmed the existence of strong PMA for all the $\mathrm{CoPt}$ films, irrespective of $T_{\mathrm{d},\mathrm{CoPt}}$. MOKE studies revealed that the films deposited at RT and at $150{}^{\circ }\mathrm{C}$ containing hard and soft magnetic areas, while the films grown at higher $T_{\mathrm{d},\mathrm{CoPt}}$, 250 and $350{}^{\circ }\mathrm{C}$, are more uniform and homogeneous. The MOKE findings are validated by the BLS spectra in terms of high and low frequency lines corresponding to the hard and soft magnetic areas, respectively. A suitable model is hypothesized to interpret the frequency variation of BLS modes corresponding to the easy saturated regions of the $\mathrm{CoPt}$ films. By this means, a good correlation between both static and dynamic behavior of the 5-nm-thick $\mathrm{CoPt}$ films has been established in this study.

Confinement of superfluid ${}^3\mathrm{He}$ on length scales comparable to the radial size of the $p$-wave Cooper pairs can greatly alter the phase diagram by stabilizing broken symmetry phases not observed in bulk ${}^3\mathrm{He}$. We consider superfluid ${}^3\mathrm{He}$ confined within long cylindrical channels of radius $100\text{nm}$, and report new theoretical predictions for the equilibrium superfluid phases under strong confinement. The results are based on the strong-coupling formulation of Ginzburg-Landau (GL) theory with precise numerical minimization of the free energy functional to identify the equilibrium phases and their regions of stability. We introduce an extension of the standard GL strong-coupling theory that accurately accounts for the phase diagram at high pressures, including the tricritical point and $T_{AB}\left(p\right)$ line defining the region of stability for the bulk A phase. We also introduce tuneable boundary conditions that allow us to explore boundary scattering ranging from maximal to minimal pairbreaking, and report results for the phase diagram as a function of pressure, temperature, and boundary conditions. Four stable phases are found: a polar phase stable in the vicinity of $T_c$, a strongly anisotropic, cylindrical analog of the bulk B phase stable at sufficiently low temperatures, and two chiral A-like phases with distinctly different orbital symmetry, one of which spontaneously breaks rotation symmetry about the axis of the cylindrical channel. The relative stability of these phases depends sensitively on pressure and the degree of pairbreaking by boundary scattering. The broken symmetries exhibited by these phases give rise to distinct signatures in transverse nuclear magnetic resonance (NMR) spectroscopy. We present theoretical results for the transverse NMR frequency shifts as functions of temperature, the rf pulse tipping angle, and the static NMR field orientation.

We present an analysis of the electronic structure of perovskite-related iridates, 5d electron compounds where a subtle interplay between spin-orbit coupling, tetragonal distortions, and electron correlations determines the electronic structure properties. We suggest via electronic structure calculations that a noncollinear calculation is required to obtain solutions close to the usually quoted $j_{\mathrm{eff}}=1/2$ state to describe the $t_{2g}$ hole in the ${\mathrm{Ir}}^{4+}$:${\mathrm{d}}^5$ cation; while a collinear calculation yields a different solution, the hole is in a simpler $xz/yz$ complex combination with a smaller $L_z/S_z$ ratio. We describe what the implications of this are in terms of the electronic structure; surprisingly, both solutions barely differ in terms of their band structure and are similar to the one obtained by a tight-binding model involving $t_{2g}$ orbitals with mean field interactions. We also analyze how the electronic structure and magnetism evolve with strain, spin-orbit coupling strength and on-site Coulomb repulsion; we suggest the way the band structure gets modified and draw some comparisons with available experimental observations.

We introduce a structural modeling technique, called force-enhanced atomic refinement (FEAR). The technique incorporates interatomic forces in reverse Monte Carlo (RMC) simulations for structural refinement by fitting experimental diffraction data using the conventional RMC algorithm, and minimizes the total energy and forces from an interatomic potential. We illustrate the usefulness of the approach by studying $a\text{−}{\text{SiO}}_2$ and $a\text{−}\text{Si}$. The structural and electronic properties of the FEAR models agree well with experimental neutron and x-ray diffraction data and the results obtained from previous molecular dynamics simulations of $a\text{−}{\text{SiO}}_2$ and $a\text{−}\text{Si}$. We have shown that the method is more efficient than the conventional molecular dynamics simulations via “melt quench.” The computational time in FEAR has been observed to scale quadratically with the number of atoms.

Optical control strategies in semiconductor nanostructures have almost exclusively relied on heavy-hole exciton and trion states. Interestingly, optical access to both light- and heavy-hole trions may provide the missing resource for consolidating all single qubit operations in a single magnetic field configuration. In this work, we describe the conditions in which nearly degenerate light- and heavy-hole trions can be exploited for implementing electron spin initialization, control, and single-shot readout in systems of $C_s$ or higher symmetry. Then, we experimentally demonstrate that a previously unidentified nitrogen pair configuration in GaAs exhibiting $C_s$ symmetry binds both light- and heavy-hole excitons and negative trions. A detailed analysis of the energy and polarization of the emission allows determining the crystal-field and exchange coefficients, the electron, hole, and exciton Zeeman coefficients, and the light- and heavy-hole character of all states. This analysis also reveals that light- and heavy-hole trion states provide the $\mathrm{\Lambda }$ level structure necessary for initialization and control along with an energetically protected cycling transition compatible with single-shot readout.

The energies of Andreev bound states (ABS) forming in an $N$-terminal junction are affected by $N-1$ independent macroscopic phase differences between superconducting leads and can be regarded as energy bands in $\left(N-1\right)$-periodic solids owing to the $2\pi$ periodicity in all phases. We investigate the singularities and peculiarities of the resulting ABS spectrum combining phenomenological and analytical methods and illustrating with the numerical results. We pay special attention to spin-orbit (SO) effects. We consider Weyl singularities with a conical spectrum that are situated at zero energy in the absence of SO interaction. We show that the SO interaction splits the spectrum in spin like a Zeeman field would do. The singularity is preserved while departing from zero energy. With SO interaction, points of zero energy form an $\left(N-2\right)$-dimensional manifold in an $\left(N-1\right)$-dimensional space of phases, while this dimension is $N-3$ in the absence of SO interaction. Singularities of other types are situated near the superconducting gap edge. In the absence (presence) of SO interaction, the ABS spectrum at the gap edge is mathematically analogous to that at zero energy in the presence (absence) of SO interaction. We demonstrate that the gap edge touching (GET) points of the spectrum in principle form an $N-2 \left(N-3\right)$ dimensional manifold when the SO interaction is absent (present). Certain symmetry lines in the Brillouin zone of the phases are exceptions from this rule, and GET there should be considered separately. We derive and study the effective Hamiltonians for all the singularities under consideration.

The structure, thermodynamics, and band gaps in graphene/graphene, boron nitride/boron nitride, and graphene/boron nitride bilayers are determined using several different corrections to first-principles approaches to account for the dispersion interactions. While the density functional dispersion correction, van der Waals density functional, meta–generalized gradient approximation, and adiabatic fluctuation-dissipation theorem methods (ACFDT-RPA) all lead to qualitatively similar predictions, the best accuracy is obtained through the application of the computationally expensive ACFDT-RPA method. We present an accurate ACFDT-RPA-based method to determine bilayer structure, generalized stacking-fault energy (GSFE), and band gaps as a function of the relative translation states of the two layers. The GSFE data clearly identify all of the stable and metastable bilayer translations as well as the barriers between them. This is key for predicting the sliding, formation, and adhesion energies for homo- and hetero-bilayers, as well as for the determination of defects in such multilayer van der Waals systems. These, in turn, provide an accurate approach for determining and manipulating the spatial variation of electronic structure.

The author reports on high-fidelity simulations of charge carriers in the high-$T_c$ cuprate materials using quantum Monte Carlo techniques applied to the first-principles Hamiltonian. With this high accuracy technique, the doped ground state is found to be a spin polaron, in which charge is localized through a strong interaction with the spin. This spin polaron has calculated properties largely similar to the phenomenology of the cuprates, and may be the object which forms the Fermi surface and charge inhomogeneity in these materials. The spin polaron has some unique features that should be visible in x-ray, EELS, and neutron experiments.

Whether at the zero spin density $m=0$ and finite temperatures $T>0$ the spin stiffness of the spin-$1/2XXX$ chain is finite or vanishes remains an unsolved and controversial issue, as different approaches yield contradictory results. Here we explicitly compute the stiffness at $m=0$ and find strong evidence that it vanishes. In particular, we derive an upper bound on the stiffness within a canonical ensemble at any fixed value of spin density $m$ that is proportional to $m^{2}L$ in the thermodynamic limit of chain length $L\to \infty$, for any finite, nonzero temperature, which implies the absence of ballistic transport for $T>0$ for $m=0$. Although our method relies in part on the thermodynamic Bethe ansatz (TBA), it does not evaluate the stiffness through the second derivative of the TBA energy eigenvalues relative to a uniform vector potential. Moreover, we provide strong evidence that in the thermodynamic limit the upper bounds on the spin current and stiffness used in our derivation remain valid under string deviations. Our results also provide strong evidence that in the thermodynamic limit the TBA method used by X. Zotos [Phys. Rev. Lett. 82, 1764 (1999)] leads to the exact stiffness values at finite temperature $T>0$ for models whose stiffness is finite at $T=0$, similar to the spin stiffness of the spin-$1/2$ Heisenberg chain but unlike the charge stiffness of the half-filled 1D Hubbard model.

Maximally localized Wannier functions are widely used in electronic structure theory for analyses of bonding, electric polarization, orbital magnetization, and for interpolation. The state of the art method for their construction is based on the method of Marzari and Vanderbilt. One of the practical difficulties of this method is guessing functions (initial projections) that approximate the final Wannier functions. Here we present an approach based on optimized projection functions that can construct maximally localized Wannier functions without a guess. We describe and demonstrate this approach on several realistic examples.

We study theoretically the optical response of a nanohybrid comprising a symmetric quantum dimer emitter coupled to a metal nanoparticle (MNP). The interactions between the exitonic transitions in the dimer and the plasmons in the MNP lead to interesting effects in the composite's input-output characteristics for the light intensity and the absorption spectrum, which we study in the linear and nonlinear regimes. We find that the exciton-plasmon hybridization leads to optical bistability and hysteresis for the one-exciton transition and enhancement of excitation for the two-exciton transition. The latter leads to a significant decrease in the field strength needed to saturate the system. In the linear regime, the absorption spectrum has a dispersive (Fano-like) line shape. The spectral position and shape of this spectrum depend on the detuning of the dimer's one-exciton resonance relative to the plasmon resonance and the ratio of the exciton-plasmon coupling constant to the exciton dephasing rate. When the applied field intensity to the nonlinear regime is increased, the Fano-like singularities in the absorption spectra are smeared and they disappear due to saturation of the dimer, which leads to the MNP dominating the spectrum. The above effects, for which we provide physical explanations, allow one to tailor the Fano-like shape of the absorption spectrum, by changing either the detuning or the input power.

We report on molecular dynamics studies of phonon (lattice vibrations) transport in bilayer silicon thin films stuck together via van der Waals interactions. Results indicate that for bilayer thin films with an atomically smooth interface, the in-plane thermal conductivity of the bilayer films is the same as that of a single layer; however, the in-plane thermal conductivity of bilayer films is higher than that of single films if roughness is introduced at the interface. These observations are explained by the effects of interfacial roughness on phonon specularity parameters for transmitted and reflected phonons.

Optical properties and lattice dynamics of hexagonal $2H-\mathrm{BaMn}{\mathrm{O}}_3$ single crystals are studied experimentally in a wide temperature range by means of rotating analyzer ellipsometry and Raman scattering. The magnitude of the direct electronic band gap is found to be $E_g=3.2\mathrm{eV}$. At room temperature the far-infrared (IR) ellipsometry spectra reveal six IR-active phonons; two of them are polarized along the $c$ axis and four are polarized within the a-b plane. Seven phonon modes are identified in the Raman scattering experiments. Group theoretical mode analysis and complementary density functional theory lattice dynamics calculations are consistent with the $2H-\mathrm{BaMn}{\mathrm{O}}_3$ structure belonging to the polar $P{6}_{3}mc$ space group at room temperature. All observed vibrational modes are assigned to specific eigenmodes of the lattice. The neutron diffraction measurements reveal a structural phase transition upon cooling below $T_C=130\pm 5\mathrm{K}$, which is accompanied by a lattice symmetry change from $P{6}_{3}mc$ to $P{6}_{3}cm$. Simultaneously, at temperatures below $T_C$ several additional IR- and Raman-active modes are detected in experimental spectra. This confirms the occurrence of a structural transition, which is possibly associated with the appearance of electrical polarization along the $c$ axis and a previously known tripling of the primitive cell volume at low temperatures.