We have computed the correlated electronic structure of FeSe and its dependence on the $A_{1g}$ mode versus compression. Using the self-consistent density functional theory-dynamical mean field theory (DFT-DMFT) with continuous time quantum Monte Carlo, we find that there is greatly enhanced coupling between some correlated electron states and the $A_{1g}$ lattice distortion. Superconductivity in FeSe shows a very strong sensitivity to pressure, with an increase in $T_c$ of almost a factor of 5 within a few GPa, followed by a drop, despite monotonic pressure dependence of almost all electronic properties. We find that the maximum $A_{1g}$ deformation potential behaves similar to the experimental $T_c$. In contrast, the maximum deformation potential in DFT for this mode increases monotonically with increasing pressure.

In-plane magnetized films of CoTaZr demonstrate standing spin-wave resonances in the complex permeability spectra well below the ferromagnetic resonance frequency. From detailed analysis of the static field dependence of the modes combined with observation of the domains using Kerr microscopy, we show that these modes originate from spin waves within the Bloch walls where the magnetization rotates perpendicular to the plane of the film. Unlike spin waves typically described in perpendicularly magnetized films, the frequencies involved are much lower due to unique field dynamics inside the wall. This theory can be fully understood by incorporating these observations into an analytical model using the Landau-Lifshitz-Gilbert equation, with good agreement between model and experimental measurement.

We investigate the scaling of entanglement entropy in both the multiscale entanglement renormalization ansatz (MERA) and in its generalization, the branching MERA. We provide analytical upper bounds for this scaling, which take the general form of a boundary law with various types of multiplicative corrections, including power-law corrections all the way to a bulk law. For several cases of interest, we also provide numerical results that indicate that these upper bounds are saturated to leading order. In particular, we establish that, by a suitable choice of holographic tree, the branching MERA can reproduce the logarithmic multiplicative correction of the boundary law observed in Fermi liquids and spin-Bose metals in $D\ge 2$ dimensions.

${\mathrm{UPd}}_3$ is known to exhibit four antiferroquadrupolar ordered phases at low temperatures. We report measur-ements of the magnetization and magnetostriction of single crystal ${\mathrm{UPd}}_3$, along the principal symmetry directions, in fields up to 33 T. These results have been combined with recent inelastic neutron and x-ray resonant scattering measurements to construct a mean-field model of ${\mathrm{UPd}}_3$ including up to fourth nearest-neighbor interactions. In particular, we find that anisotropic quadrupolar interactions must be included in order to explain the low-temperature structures derived from the scattering data.

Time-resolved optical spin orientation is employed to study spin dynamics of $I^{*}$ and $I_1^{*}$ excitons bound to isoelectronic centers in bulk ZnO. It is found that spin orientation at the exciton ground state can be generated using resonant excitation via a higher lying exciton state located at about 4 meV from the ground state. Based on the performed rate equation analysis of the measured spin dynamics, characteristic times of subsequent hole, electron, and direct exciton spin flips in the exciton ground state are determined as being ${\tau }_h^s$ = 0.4 ns, ${\tau }_e^s\ge$ 15 ns, and ${\tau }_{eh}^s\ge$ 15 ns, respectively. This relatively slow spin relaxation of the isoelectronic bound excitons is attributed to combined effects of (i) weak $e$-$h$ exchange interaction, (ii) restriction of the exciton movement due to its binding at the isoelectronic center, and (iii) suppressed spin-orbit coupling for the tightly bound hole.

Understanding and manipulating coherent phonon transport in solids is of interest both for enhancing the fundamental understanding of thermal transport as well as for many practical applications, including thermoelectrics. In this study, we investigate phonon transmission across Si/Ge superlattices using the Green's function method with first-principles force constants derived from ab initio density functional theory. By keeping the period thickness fixed while changing the number of periods, we show that interface roughness partially destroys coherent phonon transport, especially at high temperatures. The competition between the low-frequency coherent modes and high-frequency incoherent modes leads to an optimum period length for minimum thermal conductivity. To destroy coherence of the low-frequency modes, scattering length scale on the order of period length is required. This finding is useful to guide the design of superlattices to reach even lower thermal conductivity.

Recent experimental results have demonstrated the ability of monolayer ${\mathrm{MoS}}_2$ to efficiently generate second harmonic fields with susceptibilities between 0.1 and 100 nm/V. However, few theoretical calculations exist with which to interpret these findings. In particular, it is of interest to theoretically estimate the modulus of the second harmonic response since experimental reports on this differ by almost three orders of magnitude. Here, we present calculations of the second harmonic response based on a tight-binding band structure and implementation of excitons in a Bethe-Salpeter framework. We compare directly with recent experimental findings demonstrating a good agreement with the excitonic theory regarding, e.g., peak position. Furthermore, we predict an off-resonance susceptibility on the order of 0.1 nm/V, while on-resonance values rise to 4 nm/V.

The structure of multilayer silicene formed on Ag(111) films was studied by low-energy electron diffraction. It turned out that the experimental data cannot be explained by proposed models that only consider buckling of silicon atoms. We have rather found that multilayer silicene on Ag(111) is actually a thin film of bulklike silicon terminated with the Si(111)$\sqrt{3}\times \sqrt{3}$-Ag surface. The results are compared to previous works and clearly show the importance to properly understand the structure of the system when discussing its electronic properties.

We study three-dimensional (3D) Dirac fermions with weak finite-range scalar potential disorder. In the clean system, the density of states vanishes quadratically at the Dirac point. Disorder is known to be perturbatively irrelevant, and previous theoretical work has assumed that the Dirac semimetal phase, characterized by a vanishing density of states, survives at weak disorder, with a finite disorder phase transition to a diffusive metal with a nonvanishing density of states. In this paper, we show that nonperturbative effects from rare regions, which are missed by conventional disorder-averaged calculations, instead give rise to a nonzero density of states for any nonzero disorder. Thus, there is no Dirac semimetal phase at nonzero disorder. The results are established both by a heuristic scaling argument and via a systematic saddle-point analysis. We also discuss transport near the Dirac point. At the Dirac point, we argue that transport is diffusive, and proceeds via hopping between rare resonances. As one moves in chemical potential away from the Dirac point, there are interesting intermediate-energy regimes where the rare regions produce scattering resonances that determine the dc conductivity. We derive a scaling theory of transport near disordered 3D Dirac points. We also discuss the interplay of disorder with attractive interactions at the Dirac point and the resulting granular superconducting and Bose glass phases. Our results are relevant for all 3D systems with Dirac points, including Weyl semimetals.

We explore the effect of the third-nearest neighbors on the magnetic properties of the Heisenberg model on an anisotropic triangular lattice. We obtain the phase diagram of the model using Schwinger boson mean-field theory. Competition between Néel, spiral, and collinear magnetically ordered phases is found as we vary the ratios of the nearest $J_1$, next-nearest $J_2$, and third-nearest $J_3$ neighbor exchange couplings. A spin-liquid phase is stabilized between the spiral and collinear ordered states when $J_2/J_1\gtrsim 1.8$, for rather small $J_3/J_1\lesssim 0.1$. The lowest-energy two-spinon dispersions relevant to neutron scattering experiments are analyzed and compared to semiclassical magnon dispersions finding significant differences in the spiral and collinear phases between the two approaches. The results are discussed in the context of the anisotropic triangular materials: Cs${}_2$CuCl${}_4$ and Cs${}_2$CuBr${}_4$ and layered organic materials, $\kappa$-(BEDT-TTF)${}_{2}X$, and $Y$[Pd(dmit)${}_2$]${}_2$.

We investigate the electron transport through the T-shaped quantum-dot (QD) structure theoretically, by coupling a Majorana zero mode to the terminal QD. It is found that in the double-QD configuration, the presence of the Majorana zero mode can efficiently dissolve the antiresonance point in the conductance spectrum while inducing a conductance peak to appear at the same energy position. In the case of asymmetric QD-lead coupling, such a valley-to-peak transition induced by the Majorana zero mode still exists. Next, we observe in the multi-QD case that at the zero-bias limit, the conductance values are always the same as the double-QD result, independent of the parity of the QD number. We believe that all these results can be helpful for understanding the properties of Majorana bound states.

We study localized modes at a surface of a multilayer structure made of graphene layers separated by dielectric layers. We demonstrate the existence of deeply subwavelength surface modes that can be associated with the electromagnetic Tamm states, with the frequencies in the THz frequency range the negative group velocities. We suggest that the dispersion properties of these Tamm surface modes can be tuned by varying the thickness of a dielectric cap layer.

Renormalization of the electronic band gap due to electron-phonon coupling in the tetrahedral semiconductors diamond, silicon, and cubic silicon carbide is studied from first principles. There is a marked difference between the coupling of the vibrational state to the valence band maximum and to the conduction band minimum. The strength of phonon coupling to the valence band maximum is similar between the three systems and is dominated by vibrations that change the bond length. The coupling strength to the conduction band minimum differs significantly in diamond, silicon carbide, and silicon. In diamond, the coupling is dominated by six small pockets of vibrational states in the phonon Brillouin zone that are ultimately responsible for the stronger electron-phonon coupling in this material. Our results represent a first step towards the development of an a priori understanding of electron-phonon coupling in semiconductors and insulators that should aid the design of materials with tailored electron-phonon coupling properties.

We have performed a study of thermally driven magnetic relaxation in building blocks of artificial kagome spin ice. For room-temperature measurements, we observe that low-energy states are accessed with high efficiency, particularly in structures with strong dipolar coupling and with low thicknesses. With carefully tuned heating experiments, we demonstrate how thermally active artificial spin ice systems relax magnetically from higher-energy states and eventually fall into low-energy states. The methods applied in our work offer the possibility to observe the thermodynamics of artificial spin ice systems in real space and time, and provide a way to directly investigate the nature of complex stochastic processes.

We study the anomalous Nernst effect (ANE) and anomalous Hall effect (AHE) in proximity-induced ferromagnetic palladium and platinum which is widely used in spintronics, within the Berry phase formalism based on the relativistic band-structure calculations. We find that both the anomalous Hall (${\sigma }_{xy}^A$) and Nernst (${\alpha }_{xy}^A$) conductivities can be related to the spin Hall conductivity (${\sigma }_{xy}^S$) and band exchange splitting (${\Delta }_{ex}$) by relations ${\sigma }_{xy}^A={\Delta }_{ex}\frac{e}{\hslash }{\sigma }_{xy}^S{\left(E_F\right)}^{\prime }$ and ${\alpha }_{xy}^A=-\frac{{\pi }^2}{3}\frac{k_B^{2}T{\Delta }_{ex}}{\hslash }{\sigma }_{xy}^s{\left(\mu \right)}^{\prime \prime }$, respectively. In particular, these relations would predict that the ${\sigma }_{xy}^A$ in the magnetized Pt (Pd) would be positive (negative) since the ${\sigma }_{xy}^S{\left(E_F\right)}^{\prime }$ is positive (negative). Furthermore, both ${\sigma }_{xy}^A$ and ${\alpha }_{xy}^A$ are approximately proportional to the induced spin magnetic moment ($m_s$) because the ${\Delta }_{ex}$ is a linear function of $m_s$. Using the reported $m_s$ in the magnetized Pt and Pd, we predict that the intrinsic anomalous Nernst conductivity (ANC) in the magnetic platinum and palladium would be gigantic, being up to ten times larger than, e.g., iron, while the intrinsic anomalous Hall conductivity (AHC) would also be significant.

We have studied the transition between pure potential flow and turbulent flow around a quartz tuning fork resonator in superfluid ${}^4\mathrm{He}$ at millikelvin temperatures. Turbulent flow is identified by an additional drag force on the fork prongs due to the creation of quantized vortices. When driven at a constant driving force amplitude, the transition to turbulence causes an abrupt decrease in the velocity amplitude of the prongs. For a range of driving forces, continuous switching is observed between the two flow states. We have made a statistical study of the switching characteristics and of the lifetimes of the unstable states. We find a characteristic velocity $v^{☆}$ which separates quasistable turbulent flow at higher velocities and quasistable potential flow at lower velocities. We show that the potential-to-turbulent flow transition is driven by random processes involving remanent vortices pinned to the prongs.

We report magnetization, specific heat, and NMR measurements of 3-Br-4-F-V [=3-(3-bromo-4-fluorophenyl)-1,5-diphenylverdazyl], a strong-rung $S=1/2$ Heisenberg spin ladder with ferromagnetic leg interactions. We explain the magnetic and thermodynamic properties based on the strong-rung regime. Furthermore, we find a field-induced successive phase transition in the specific heat and the nuclear spin-lattice relaxation rate $1/T_1$. ${}^{19}$F-NMR spectra for higher- and lower-temperature phases indicate partial magnetic order and incommensurate long-range order, respectively, evidencing the presence of frustration due to weak interladder couplings.

The spinel vanadates have become a model family for exploring orbital order on the frustrated pyrochlore lattice, and recent debate has focused on the symmetry of local crystal fields at the cation sites. Here, we present neutron scattering measurements of the magnetic excitation spectrum in ${\mathrm{FeV}}_2{\mathrm{O}}_4$, a recent example of a ferrimagnetic spinel vanadate which is available in single-crystal form. We report the existence of two emergent magnon modes at low temperatures, which draw strong parallels with the closely related material, ${\mathrm{MnV}}_2{\mathrm{O}}_4$. We were able to reproduce the essential elements of both the magnetic ordering pattern and the dispersion of the inelastic modes with semiclassical spin-wave calculations, using a minimal model that implies a sizable single-ion anisotropy on the vanadium sublattice. Taking into account the direction of ordered spins, we associate this anisotropy with the large trigonal distortion of VO${}_6$ octahedra, previously observed via neutron powder diffraction measurements. We further report on the spin gap, which is an order of magnitude larger than that observed in ${\mathrm{MnV}}_2{\mathrm{O}}_4$. By looking at the overall temperature dependence, we were able to show that the gap magnitude is largely associated with the ferro-orbital order known to exist on the iron sublattice, but the contribution to the gap from the vanadium sublattice is in fact comparable to what is reported in the Mn compound. This reinforces the conclusion that the spin canting transition is associated with the ordering of vanadium orbitals in this system, and closer analysis indicates closely related physics underlying orbital transitions in ${\mathrm{FeV}}_2{\mathrm{O}}_4$ and ${\mathrm{MnV}}_2{\mathrm{O}}_4$.

Low-temperature thermal conductivity ($\kappa$), as well as magnetization ($M$) and electric polarization ($P$), of multiferroic orthoferrite ${\text{DyFeO}}_3$ single crystals are studied with $H\parallel c$. When the crystal is cooled in zero field, $M$, $P$, and $\kappa$ all consistently exhibit irreversible magnetic-field dependencies. In particular, with 500 mK $<T\le$ 2 K, all these properties show two transitions at the first run of increasing field but only the higher-field transition is present in the subsequent field sweepings. Moreover, the ultra-low-$T$ ($T<$ 500 mK) $\kappa \left(H\right)$ shows a different irreversibility and there is only one transition when the field is swept both up and down. All the results indicate a complex low-$T$ $H\text{−}T$ phase diagram involving successive magnetic phase transitions of the Fe${}^{3+}$ spins. In particular, the ground state, obtained with cooling to sub-Kelvin temperatures in zero field, is found to be an unexplored phase.

By constructing $d_{x^2-y^2}-d_{z^2}$ two-orbital models from first principles, we have obtained a systematic correlation between the Fermi surface warping and theoretically evaluated $T_c$ for various bilayer as well as single-layer cuprates. This reveals that a smaller mixture of the $d_{z^2}$ orbital component on the Fermi surface leads simultaneously to larger Fermi-surface warping and higher $T_c$. The theoretical correlation strikingly resembles a systematic plot for the experimentally observed $T_c$ against the Fermi surface warping due to Pavarini et al. [Phys. Rev. Lett. 87, 047003 (2001)], and the present result unambiguously indicates that the $d_{z^2}$ mixture is one key factor that determines $T_c$ in the cuprates.