Magnonic excitations in the two-dimensional (2D) van der Waals (vdW) ferromagnet chromium triiodide (${\mathrm{CrI}}_3$) are studied. We find that bulk magnons exhibit a nontrivial topological band structure without the need for Dzyaloshinskii-Moriya interaction. This is shown in vdW heterostructures, consisting of single-layer ${\mathrm{CrI}}_3$ on different 2D materials such as ${\mathrm{MoTe}}_2$, ${\mathrm{HfS}}_2$, and ${\mathrm{WSe}}_2$. We find numerically that the proposed substrates substantially modify the out-of-plane magnetic anisotropy on each sublattice of the ${\mathrm{CrI}}_3$ subsystem. The induced staggered anisotropy, combined with a proper band inversion, leads to the opening of a topological gap of the magnon spectrum. Since the gap is opened nonsymmetrically at the ${\mathbf{K}}^{+}$ and ${\mathbf{K}}^{-}$ points of the Brillouin zone, an imbalance in the magnon population between these two valleys can be created under a driving force. This phenomenon has a close analogy to the so-called valley Hall effect and is thus termed the magnon valley Hall effect. In linear response to a temperature gradient, we quantify this effect by the evaluation of the temperature dependence of the magnon thermal Hall effect. These findings open a different avenue by adding the valley degrees of freedom besides the spin in the study of magnons.

Motivated by recent observation of nematicity in moiré systems, we study three different orbital orders that potentially can happen in moiré systems: (1) the nematic order, (2) the valley polarization, and (3) the “compass order.” Each order parameter spontaneously breaks part of the spatial symmetries of the system. We explore physics caused by the quantum fluctuations close to the order-disorder transition of these order parameters. Especially, we recognize that the symmetry of the moiré systems leads to a crucial difference of the effective theory describing the nematic order from the standard Hertz-Millis formalism. We demonstrate that this key difference may lead to a special non-Fermi liquid behavior near the order-disorder nematic transition, different from the standard non-Fermi liquid behavior usually expected when a Fermi surface is coupled to the critical fluctuations of orbital orders. We also discuss the interplay of the three order parameters and the possible rich phase diagram at finite temperature. Within the three orbital orders, the valley polarization and the compass order likely strongly compete with the superconductor.

The Samarium orthovanadates particles were prepared using the sol-gel method, and the effect of sintering temperature on the crystallinity was studied in detail through x-ray diffraction (XRD) and the Rietveld refinement of the obtained XRD data. Density functional theory (DFT) based calculations were used to describe the dynamic properties in Samarium orthovanadate. The data obtained from the DFT calculations were made instrumental in confirming the vibrational properties, and both the Raman and infrared modes obtained through the experiments. Temperature-dependent Raman spectroscopy was used to provide a deeper insight into all the eigenmodes and their behavior with varying temperature. A temperature-dependent (experiment) and pressure-dependent (DFT) Raman spectra were produced for the investigation of eigenmodes in the samples. A correlation between the two methods was seen, and the alignment between the two data sets was found to be ineluctable. The Grüneisen parameter of individual modes were calculated with the aid of the theoretically obtained variation in the Raman shift with respect to the bulk modulus. Ultraviolet-visible spectroscopy of the samples was explored to study possible correlation with temperature, and theoretical bandgaps were obtained through standard DFT and hybrid exchange-correlation functional calculations. Photoluminescence spectroscopy was performed in the hopes of unveiling possible applications of the material.

We investigate the localization-delocalization transition in one-dimensional non-Hermitian quasiperiodic lattices with exponential short-range hopping, which possess parity-time ($\mathcal{PT}$) symmetry. The localization transition induced by the non-Hermitian quasiperiodic potential is found to occur at the $\mathcal{PT}$-symmetry-breaking point. Our results also demonstrate the existence of energy-dependent mobility edges, which separate the extended states from localized states and are only associated with the real part of eigenenergies. The level statistics and Loschmidt echo dynamics are also studied.

Motivated by a recent experiment on manganese oxide thin films, we theoretically study the topological Hall effect in a weakly canted antiferromagnet with textured Néel ($\mathit{n}$) and uniform ($\mathit{l}$) components of magnetization. Treating the Néel texture by a spin gauge field and the uniform component perturbatively, we obtain an analytical expression for the topological Hall conductivity. The result is proportional to the emergent magnetic field, $\mathbf{\nabla }\times {\mathit{A}}_{\mathrm{AF}}$, where $A_{\mathrm{AF},i}=\mathit{l}·({\partial }_i\mathit{n}\times \mathit{n})$ is an emergent vector potential in antiferromagnets, which consists of the spin-chirality density, $\widehat{\mathit{l}}·({\partial }_x\widehat{\mathit{l}}\times {\partial }_y\widehat{\mathit{l}})$, formed by the normalized uniform moment $\widehat{\mathit{l}}=\mathit{l}/\left|\mathit{l}\right|$, and one formed by the Néel and uniform components in the presence of spatial variation of canting. The result is discussed in comparison with the previous study on ferromagnets in the weak-coupling regime.

We study a hybrid system of a magnetic oxide thin film on a ferroelectric substrate. We show that magnetic and transport properties of the magnetic oxide film strongly depend on the dielectric constant and polarization of the ferroelectric substrate. Tuning these parameters one can induce the metal-insulator transition in the magnetic oxide film accompanied by the ferromagnetic to superparamagnetic transition. Ferroelectric properties can be tuned by varying temperature or applying an electric field.

Many current descriptions of the pseudogap in underdoped cuprates envision a doping-dependent transition line $T^{*}\left(p\right)$ which descends monotonically toward zero just beyond optimal doping. There is much debate as to the location of the terminal point $p^{*}$ where $T^{*}\left(p\right)$ vanishes, whether or not there is a phase transition at $T^{*}$ and exactly how $T^{*}\left(p\right)$ behaves below $T_{\mathrm{c}}$ within the superconducting dome. One perspective sees $T^{*}\left(p\right)$ cutting the dome and continuing to descend monotonically to zero at $p^{*}\approx 0.19$ holes/Cu—referred to here as entrant behavior. Another perspective derived from photoemission studies is that $T^{*}\left(p\right)$ intersects the dome near $p\approx 0.23$ holes/Cu then turns back below $T_{\mathrm{c}}$, falling to zero again around $p^{*}\approx 0.19$—referred to here as reentrant behavior. By examining field-dependent thermodynamic data for ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ we show that neither entrant nor reentrant behavior is supported. Rather, $p^{*}$ sharply delimits the pseudogap regime: For $p<0.19$ the pseudogap is always present, independent of $T$. Similar results are found for ${\mathrm{Y}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$. For both materials, $T^{*}\left(p\right)$ is not a temperature but a crossover scale, $\approx E^{*}\left(p\right)/2k_B$, reflecting instead the underlying pseudogap energy $E^{*}\left(p\right)$ which vanishes as $p\to p^{*}\approx 0.19$.

The correlation between magnetism and electric polarization in a chiral insulating magnet $\mathrm{B}{\mathrm{a}}_3\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_5\mathrm{C}{\mathrm{l}}_2$ has been investigated. The temperature dependence of magnetic susceptibility shows an anomaly at 564 K, suggesting the onset of antiferromagnetic order of $\mathrm{F}{\mathrm{e}}^{3+}$ moments. Electric polarization appears in a magnetic field below $T_{\mathrm{N}}$ due to the spin-direction-dependent metal-ligand hybridization. The direction of the electric polarization can be controlled in three-dimensional space by changing the direction of an external magnetic field. This compound also shows an antiferromagnetic-to-weak-ferromagnetic transition accompanied by a structural phase transition at 140 K.

Diamond samples containing silicon and nitrogen are shown to be heavily photochromic, with the dominant visible changes due to simultaneous change in total ${\mathrm{SiV}}^{0/-}$ concentration. The photochromism treatment is not capable of creating or destroying SiV defects, and thus we infer the presence of the optically inactive ${\mathrm{SiV}}^{2-}$ . We measure spectroscopic signatures we attribute to substitutional silicon in diamond, and identify a silicon-vacancy complex decorated with a nearest-neighbor nitrogen $\mathrm{SiVN}$, supported by theoretical calculations.

Motivated by the recent report on the high-$T_c$ ferromagnetic insulating state of a single transition metal containing a double perovskite compound, ${\mathrm{Sr}}_3{\mathrm{OsO}}_6$ [Wakabayashi et al., Nat Commun. 10, 535 (2019)], we study this curious behavior by employing first-principles calculations in conjunction with exact diagonalization of the full $t_{2g}$ multiplet problem of two Os sites. Our analysis highlights the fact that stabilization of ${\mathrm{Sr}}_3{\mathrm{OsO}}_6$ in the cubic phase in epitaxially grown thin film is the key to both ferromagnetic correlation and the high-temperature scale associated with it. It also provides a natural explanation for why the sister compound, ${\mathrm{Ca}}_3{\mathrm{OsO}}_6$, exhibits low-$T_N$ antiferromagnetism in its monoclinic structure. Furthermore, the insulating property is found to be driven by the opening of a Mott gap in the half-filled spin-orbit coupled $j=3/2$ manifold of $d^2$ Os. We point out that ${\mathrm{Sr}}_2{\mathrm{CaOsO}}_6$, which naturally forms in the cubic phase, would be worthwhile to explore as a future candidate to exhibit a high-$T_c$ ferromagnetic insulating state in bulk form.

We study an effective spin model derived perturbatively from a random transverse-field Ising model on the pyrochlore lattice. The model consists of spin configurations on the pyrochlore lattice, restricted to the spin-ice subspace, with spins interacting with random Ising exchange couplings as well as ring exchanges along the hexagons of the lattice. This model is studied by exact diagonalization up to $N=64$ site systems. We calculate spin-glass correlation functions and local entanglement entropy $S_T$ between spins in a single tetrahedron and the rest of the system. We find that the model undergoes two phase transitions. At weak randomness the model is in a quantum spin-ice phase where $S_T=ln6$. Increasing randomness, at low transverse fields, first leads to a frozen phase, with long-range spin-glass order and $S_T=ln2$ corresponding to the Cat states associated with Ising order. Further increase in randomness leads to a random resonating-hexagon phase with a frozen backbone of spins and a broad distribution of entanglement entropies. The implications of these studies for non-Kramers rare-earth pyrochlores are discussed.

We consider the case of a chiral soliton lattice subjected to uniaxial elastic strain applied perpendicular to the chiral axis and derive through analytical modeling the phase diagram of magnetic states supported in the presence of an external magnetic field. The strain induced anisotropies give rise to three distinct nontrivial spin textures, depending on the nature of the strain, and we show how these states may be identified by their signatures in Lorentz transmission electron microscopy (TEM). Experimental TEM measurements of the Fresnel contrast in a strained sample of the prototypical monoaxial chiral helimagnet ${\mathrm{CrNb}}_3{\mathrm{S}}_6$ are reported and compare well with the modeled contrast. Our results demonstrate an additional degree of freedom that may be used to tailor the magnetic properties of helimagnets for fundamental research and applications in the areas of spintronics and the emerging field of strain manipulated spintronics.

Higgs spectroscopy is a new field in which Higgs modes in nonequilibrium superconductors are analyzed to gain information about the ground state. One experimental setup in which the Higgs mode in $s$-wave superconductors was observed is periodic driving with terahertz (THz) light, which shows resonances in the third-harmonic generation (THG) signal if twice the driving frequency matches the energy of the Higgs mode. We derive expressions of the driven gap oscillations for arbitrary gap symmetry and calculate the THG response. We demonstrate that the possible Higgs modes for superconductors with nontrivial gap symmetry can lead to additional resonances if twice the driving frequency matches the energy of these Higgs modes and we disentangle the influence of charge density fluctuations (CDF) to the THG signal within our clean-limit analysis. With this we show that THG experiments on unconventional superconductors allow for detection of their Higgs modes. This paves the way for future studies on realistic systems including additional features to understand the collective excitation spectra of unconventional superconductors.

This paper presents a method of computing approximate conservation laws and eigenstates of integrability-broken models using the concept of adiabatic continuation. Given some Hamiltonian, eigenstates and conserved operators may be computed by using those of a simple Hamiltonian close by in parameter space, dressed by some unitary rotation. However, most adiabatic continuation analyses only use this unitary implicitly. In this work, approximate adiabatic gauge potentials are used to construct a state dressing using variational methods, to compute eigenstates via a rotated truncated spectrum approximation. These methods allow construction of both low- and high-energy approximate nonthermal eigenstates, as well as quasilocal almost-conserved operators, in models where integrability may be nonperturbatively broken. These concepts will be demonstrated in the mixed-field Ising model.

Interparticle coulombic decay (ICD) is an efficient energy transfer process between two weakly interacting systems. ICD was recently proposed as the underlying fundamental mechanism for technological purposes based on quantum dot nanostructures, such as wavelength-sensitive detectors. Via ICD, an excited donor quantum dot releases its excess energy by ionizing a neighboring acceptor dot. Here, we demonstrate that the presence of a third (ICD inactive) quantum dot can serve as a bridge between the two dots, which is shown to result in an enhancement of the efficiency of the ICD-mediated energy transfer. Furthermore, our results show that this enhancement is found to be robust against change in the characteristics of the bridge quantum dot, particularly the depth and size. On the other hand, its relative position with respect to the donor and acceptor dots is found to foster ICD when it is located in between the two dots. Our findings provide new insights for the development of ICD-based nanostructure technologies, particularly for rational design of three coupled quantum dots.

Bilayer graphene in a magnetic field hosts a variety of ordered phases built from eight Landau levels close in energy to the neutrality point. These levels are characterized by orbital $n=0,1$, valley $\xi =+,-$, and spin $\sigma =\uparrow ,\downarrow$; their relative energies depend strongly on the Coulomb interaction, magnetic field, and interlayer bias. We treat interactions at the Hartree-Fock level, including the effects of metallic gates, layer separation, spatial extent of the $p_z$ orbitals, all Slonczewski-Weiss-McClure tight-binding parameters, and pressure. We obtain the ground state as function of the applied magnetic field, bias, and pressure. The gates, layer separation, and extent of the $p_z$ orbitals weaken the Coulomb interaction at different length scales; these effects distort the phase diagram but do not change its topology. However, previously predicted continuous transitions become discontinuous when all tight-binding parameters are included nonperturbatively. We find that pressure increases the importance of the noninteracting scale with respect to the Coulomb energy, which drives phase transitions to occur at lower fields. This brings two orbitally polarized states not yet predicted or observed into the experimentally accessible region of the phase diagram, in addition to previously identified valley-polarized, spin-polarized, and partially orbitally polarized states.

We provide an analytical tool to calculate the energies of Andreev bound states (ABSs) in long imperfect SNS junctions, which at present can only be described using numerical tools. We model an NS junction as a $\delta$-function “Andreev” impurity, i.e., a localized potential that scatters an electron into a hole with opposite spin. We show using the scattering matrix formalism that, quite surprisingly, an “Andreev” impurity is equivalent to an NS junction characterized by both Andreev reflection and a finite amount of normal scattering. The ABS energies are then calculated using the $T$-matrix formalism applied to a system with two Andreev impurities. Our results lie between those for a perfect long SNS junction limit described by the Andreev approximation (ABS energies depend linearly on the phase and are independent of the chemical potential) and the particle-in-the-box limit (bound state energies are independent of the phase and have a linear dependence on the chemical potential). Moreover, we recover a closed-form expression for the ABS energies by expanding around the particle-in-the-box limit.

We propose a mechanism to directly measure the chiral anomaly in disorder Weyl semimetals (WSMs) by the Kondo effect. We find that in a magnetic and electric-field-driven WSM, the locations of the Kondo peaks can be modulated by the chiral chemical potential, which is proportional to $\mathbf{E}·\mathbf{B}$. The Kondo peaks come from spin fluctuations within the impurities, which apart from the temperature, relate closely to the host's Fermi level. In WSMs, the chiral-anomaly-induced chirality population imbalance will shift the local Fermi levels of the paired Weyl valleys toward opposite directions in energy and then affect the Kondo effect. Consequently, the Kondo effect can be tunable by an external electric field via the chiral chemical potential. This is unique to the chiral anomaly. Based on this, we argue that the electrically tunable Kondo effect can serve as a direct measurement of the chiral anomaly in WSMs. The Kondo peaks are robust against the disorder effect and therefore, the signal of the chiral anomaly survives for a relatively weak magnetic field.