The interplay of magnons and phonons can induce strong temperature variations in the magnetic exchange interactions, leading to changes in the magnetothermal response. This is a central mechanism in many magnetic phenomena, and in the new field of Spin Caloritronics, which focuses on the combination of heat and spin currents. Boson model systems have previously been developed to describe the magnon-phonon coupling but, until recently, studies rely on empirical parameters. In this paper, we propose a first-principles approach to describe the dependence of the magnetic exchange integrals on phonon renormalization, leading to changes in the magnon dispersion as a function of temperature. The temperature enters into the spin dynamics (by introducing fluctuations) as well as in the magnetic exchange itself. Depending on the strength of the coupling, these two temperatures may or may not be equilibrated, yielding different regimes. We test our approach in typical and well-known ferromagnetic materials: Ni, Fe, and Permalloy. We compare our results to recent experiments on the spin-wave stiffness, and discuss departures from Bloch's law and parabolic dispersion.

The doubly ordered perovskites $\mathrm{Na}Ln\mathrm{MnW}{\mathrm{O}}_6$ ($Ln=\mathrm{La}$, Nd, and Tb), $\mathrm{Na}Ln\mathrm{FeW}{\mathrm{O}}_6$ ($Ln=\mathrm{La}$ and Nd), and $\mathrm{Na}Ln\mathrm{CoW}{\mathrm{O}}_6$ ($Ln=\mathrm{Sm}-\mathrm{Er}$, Yb, and Y) have been reported to be potential multiferroics because of their polar crystal structure ($P{2}_1$) and antiferromagnetic ordering of the transition-metal ions at low temperatures. In these materials, the octahedral rotation resulting from the layered ordering of cations at the $A$-site and the rock-salt ordering of cations at the $B$-site is suggested to be responsible for ferroelectric polarization. However, the ferroelectricity of these compounds has never been confirmed experimentally. Here we report an experimental demonstration of nonswitchable electric polarization below the magnetic ordering temperatures in two representative compounds, namely $\mathrm{NaYMnW}{\mathrm{O}}_6$ and $\mathrm{NaHoCoW}{\mathrm{O}}_6,$ with smaller $Ln$-ions synthesized by high pressure and high temperatures. These compounds crystallize in the polar structure ($P{2}_1$) and undergo antiferromagnetic ordering at $T_{\mathrm{N}}\sim 9\mathrm{K}$, where a dielectric anomaly occurs in zero magnetic field. While the conventional $P-E$ loop measurements suggest the absence of ferroelectric polarization in the paramagnetic state, the pyroelectric current measurements across the magnetic ordering temperature show a nonswitchable change in polarization ($\mathrm{\Delta }P$) at $T_{\mathrm{N}}$, indicating a large energy barrier between the two polar states. The change in polarization at $T<T_{\mathrm{N}}$ under magnetic field reveals magnetoelectric coupling.

We present detailed calculations of the electric field gradient (EFG) using a point charge approximation in ${\mathrm{Ba}}_2{\mathrm{NaOsO}}_6$, a Mott insulator with strong spin-orbit interaction. Recent ${}^{23}\mathrm{Na}$ nuclear magnetic resonance (NMR) measurements found that the onset of local point symmetry breaking, likely caused by the formation of quadrupolar order [Chen, Pereira, and Balents, Phys. Rev. B 82, 174440 (2010)], precedes the formation of long range magnetic order in this compound [Lu et al., Nat. Commun. 8, 14407 (2017); Liu et al., Physica B 536, 863 (2018)]. An extension of the static ${}^{23}\mathrm{Na}$ NMR measurements as a function of the orientation of a 15 T applied magnetic field at 8 K in the magnetically ordered phase is reported. Broken local cubic symmetry induces a nonspherical electronic charge distribution around the Na site and thus finite EFG, affecting the NMR spectral shape. We combine the spectral analysis as a function of the orientation of the magnetic field with calculations of the EFG to determine the exact microscopic nature of the lattice distortions present in low temperature phases of this material. We establish that orthorhombic distortions, constrained along the cubic axes of the perovskite reference unit cell, of oxygen octahedra surrounding Na nuclei are present in the magnetic phase. Other common types of distortions often observed in oxide structures are considered as well.

Atomic-scale computational modeling of technologically relevant permanent magnetic materials faces two key challenges. First, a material's magnetic properties depend sensitively on temperature, so the calculations must account for thermally induced magnetic disorder. Second, the most widely used permanent magnets are based on rare-earth elements, whose highly localized $4f$ electrons are poorly described by standard electronic structure methods. Here, we take two established theories, the disordered local moment picture of thermally induced magnetic disorder and self-interaction-corrected density functional theory, and devise a computational framework to overcome these challenges. Using this approach, we calculate magnetic moments and Curie temperatures of the rare-earth cobalt (${\mathrm{RECo}}_5$) family for RE = Y-Lu. The calculations correctly reproduce the experimentally measured trends across the series and confirm that, apart from the hypothetical compound ${\mathrm{EuCo}}_5, {\mathrm{SmCo}}_5$ has the strongest magnetic properties at high temperature. An order-parameter analysis demonstrates that varying the RE has a surprisingly strong effect on the Co-Co magnetic interactions determining the Curie temperature, even when the lattice parameters are kept fixed. We propose the origin of this behavior is a small contribution to the density from $f$-character electrons located close to the Fermi level.

The recently demonstrated x-ray scattering approach using a free electron laser with a high field pulsed magnet has opened new opportunities to explore the charge-density-wave (CDW) order in cuprate high temperature superconductors. Using this approach, we substantially degrade the superconductivity with magnetic fields up to 33 T to investigate the onset of CDW order in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ at low temperatures near a putative quantum critical point (QCP) at $p_1\sim 0.08$ holes per Cu. We find no CDW can be detected in a sample with a doping concentration smaller than $p_1$. Our results indicate that the onset of the CDW ground state lies inside the zero-field superconducting dome, and broken translational symmetry is associated with the putative QCP at $p_1$.

We study the effect of coupling between the superconducting current and magnetization in the superconductor/ferromagnet/superconductor Josephson junction under an applied circularly polarized magnetic field. Manifestation of ferromagnetic resonance in the frequency dependence of the amplitude of the magnetization and the average critical current density is demonstrated. The IV characteristics show subharmonic steps that form devil's staircases, following a continued fraction algorithm. The origin of the found steps is related to the effect of the magnetization dynamics on the phase difference in the Josephson junction. The dynamics of our system is described by a generalized RCSJ model coupled to the Landau-Lifshitz-Gilbert equation. We justify analytically the appearance of the fractional steps in IV characteristics of the superconductor/ferromagnet/superconductor Josephson junction.

The spin-$1/2$ Heisenberg antiferromagnet on the honeycomb bilayer lattice is shown to display a rich variety of semiclassical and genuinely quantum phases, controlled by the interplay between intralayer frustration and interlayer exchange. Employing a complementary set of techniques, comprising spin rotationally invariant Schwinger boson mean-field theory, bond operators, and series expansions, we unveil the quantum phase diagram, analyzing low-energy excitations and order parameters. By virtue of Schwinger bosons we scan the complete range of exchange parameters, covering both long-range-ordered and quantum disordered ground states, and reveal the existence of an extended, frustration-induced lattice nematic phase in a range of intermediate exchange unexplored so far.

Several condensed-matter platforms have been proposed recently to realize the Sachdev-Ye-Kitaev (SYK) model in their low-energy limit. In these proposed realizations, the characteristic SYK behavior is expected to occur under certain assumptions about the underlying physical system that (i) render all bilinear terms small compared to four-fermion interactions and (ii) ensure that the coupling constants are approximately all-to-all and independent random variables. In this work we explore, both analytically and numerically, the family of models that arises when we relax these assumptions in ways motivated by real physical systems. By relaxing (i) and allowing large bilinear terms, we obtain a novel, exactly solvable cousin of the $\mathrm{SYK}$ model. It exhibits two distinct phases separated by a quantum phase transition characterized by a power-law, ${\sim \left|\omega \right|}^{-1/3}$, scaling of the low-energy spectral density, despite being a noninteracting model. By relaxing (ii), we obtain close relatives of the $\mathrm{SYK}$ model which exhibit interesting behaviors, including a chaotic non-Fermi liquid phase with continuously varying fermion scaling dimension, and a phase transition to a disordered Fermi liquid as a function of interaction range and disorder length scale.

Using a nonequilibrium implementation of the extended dynamical mean-field theory (EDMFT) we simulate the relaxation after photoexcitation in a strongly correlated electron system with antiferromagnetic spin interactions. We consider the $t-J$ model and focus on the interplay between the charge and spin dynamics in different excitation and doping regimes. The appearance of string states after a weak photoexcitation manifests itself in a nontrivial scaling of the relaxation time with the exchange coupling and leads to a correlated oscillatory evolution of the kinetic energy and spin-spin correlation function. A strong excitation of the system, on the other hand, suppresses the spin correlations and results in a relaxation that is controlled by hole scattering. We discuss the possibility of detecting string states in optical and cold-atom experiments.

The functional renormalization group (FRG) has been used widely to investigate phase diagrams, in particular the one of the two-dimensional Hubbard model. So far, the study of one-dimensional models has not attracted as much attention. We use the FRG to investigate the phases of a one-dimensional spinless tight-binding chain with nearest and next-nearest neighbor interactions at half filling. The phase diagram of this model has already been established with other methods, and phase transitions from a metallic phase to ordered phases take place at intermediate to strong interactions. The model is thus well suited to analyze the potential and the limitations of the FRG in this regime of interactions. We employ flow equations that are exact up to second order in the interaction, which implies that we take into account the frequency dependence of the two-particle vertex as well as the feedback of the dynamic self-energy. For intermediate nearest neighbor interactions, our scheme captures the phase transition from a metallic phase to a charge density wave with alternating occupation. The critical interaction, at which this transition occurs, is underestimated due to our approximations. Similarly, for intermediate next-nearest neighbor interactions, we observe a transition to a charge density wave with occupation pattern $...00110011...$. We show that taking into account a feedback of the two-particle vertex in the flow equation is essential for the detection of those phases.

We present an experimental and theoretical study of the electronic structure of the transparent conductive oxide semiconductor ${\mathrm{MgGa}}_2{\mathrm{O}}_4$. Its valence band and the core levels have been measured experimentally by angle-resolved photoemission spectroscopy and compared to theoretical ab initio density-functional calculations. The bands stem from oxygen orbitals and have a dispersion of about 0.6 eV and high effective masses, in agreement with the calculations. Angle-resolved measurements of the Ga 3d core levels indicate a sizable upward band bending, which in combination with an exact model potential yielded a quantitative estimate of the spatial extension of the depletion region, the effective Debye length in the material, and the extrinsic bulk carrier density.

We study the band structure of phases induced by depositing bilayer graphene on a transition-metal dichalcogenide monolayer. Tight-binding and low-energy effective Hamiltonian calculations show that it is possible to induce topologically nontrivial phases that should exhibit the quantum spin Hall effect in these systems. We classify bulk insulating phases through calculation of the $Z_2$ invariant, which unequivocally identifies the topology of the structure. The study of these and similar hybrid systems under an applied gate voltage opens the possibility for tunable topological structures in real experimental systems.

Reports of weak local minima in the magnetoresistance at $\nu =2+3/5, 2+3/7, 2+4/9, 2+5/9, 2+5/7$, and $2+5/8$ in the second Landau level of the electron gas in GaAs/AlGaAs left open the possibility of fractional quantum Hall states at these filling factors. In a high-quality sample we found that the magnetoresistance exhibits peculiar features near these filling factors of interest. These features, however, cannot be associated with fractional quantum Hall states; instead, they originate from magnetoresistive fingerprints of the electronic bubble phases. We found only two exceptions: at $\nu =2+2/7$ and $2+5/7$ there is evidence for incipient fractional quantum Hall states at intermediate temperatures. As the temperature is lowered, these fractional quantum Hall states collapse due to a phase competition with bubble phases.

The emergent dynamical processes associated with magnetic excitations in heavy-fermion ${\mathrm{SmB}}_6$ are investigated. By imposing multiorbital interactions on a first-principles model, we find the interplay between spin and orbital fluctuations in the $f$ manifold is highly sensitive to local correlations. The magnetic phase diagram constructed at zero temperature reveals quantum critical features with the existence of several competing phases. Within the random phase approximation, we perform a comprehensive study of the spin-spin correlation function, and our results agree with neutron scattering experiments. Spectral weight analysis shows the low-energy spin excitations are selectively accompanied by orbital fluctuations, indicating a nontrivial entanglement between the spin and orbital degree of freedom driven by relativistic couplings.

We employ matrix-product state techniques to numerically study the zero-temperature spin transport in a finite spin-$\frac{1}{2}$ XXZ chain coupled to fermionic leads with a spin bias voltage. Current-voltage characteristics are calculated for parameters corresponding to the gapless XY phase and the gapped Néel phase. In both cases, the low-bias spin current is strongly suppressed unless the parameters of the model are fine tuned. For the XY phase, this corresponds to a conducting fixed point where the conductance agrees with the Luttinger-liquid prediction. In the Néel phase, fine tuning the parameters similarly leads to an unsuppressed spin current with a linear current-voltage characteristic at low bias voltages. However, with increasing the bias voltage, there occurs a sharp crossover to a region where the current-voltage characteristic is no longer linear and a smaller differential conductance is observed. We furthermore show that the parameters maximizing the spin current minimize the Friedel oscillations at the interface, in agreement with the previous analyses of the charge current for inhomogeneous Hubbard and spinless fermion chains.

In monolayer transitional metal dichalcogenides, the two excitonic states which are eigenstates of Coulomb interaction are coherent superposition of electron and holes in the two valleys. The exciton band which couples to the transverse electric mode of light has parabolic dispersion for the center-of-mass momentum, whereas the one which couples to the transverse magnetic mode has both parabolic and linear components. In this paper we show that the signatures of this band structure of excitons can be observed in exciton diffusion experiments. In particular, it is demonstrated that by pumping the system with linearly polarized light the exciton transport is anisotropic compared to circularly polarized pump and that such anisotropy is absent for the cases of two linearly or two quadratically dispersing bands. We show that the results persist for the moderate level of disorder present in realistic systems. Finally, we demonstrate that similar effects can be obtained for positively detuned exciton polaritons, in less stringent experimental requirements compared to the bare exciton case.

We investigate the effect of the valley degree of freedom on Pauli-spin blockade readout of spin qubits in silicon. The valley splitting energy sets the singlet-triplet splitting and thereby constrains the detuning range. The valley phase difference controls the relative strength of the intra- and intervalley tunnel couplings, which, in the proposed Pauli-spin blockade readout scheme, couple singlets and polarized triplets, respectively. We find that high conversion fidelity is possible for a wide range of phase differences, while taking into account experimentally observed valley splittings and tunnel couplings. We also show that the control of the valley splitting together with the optimization of the readout detuning can compensate the effect of the valley phase difference. To increase the measurement fidelity and extend the relaxation time we propose a latching protocol that requires a triple quantum dot and exploits weak long-range tunnel coupling. These opportunities are promising for scaling spin qubit systems and improving qubit readout fidelity.