In this study, we systematically investigate the temperature dependence of effective damping in FeNi (permalloy or Py)/Gd bilayer based on broadband ferromagnetic resonance. Despite a significant increase in the effective damping of the Py/Gd bilayer with decreasing temperature, we show that the spin pumping is not responsible for the result. By combining our findings with systematic measurement data of Py/Cu/Gd contrast samples, we conclude that an effective damping enhancement from a decrease in temperature is due to the temperature-dependent magnetic moment of Gd and antiferromagnetic coupling between Py and Gd. Further, we perform a semiquantitative analysis of the experimental data using the Landau-Lifshitz-Gilbert equations that consider the antiferromagnetic coupling between Py and Gd, and estimated the damping parameters in the range of 0.3–0.4 for Gd films with thicknesses of 5 and 8 nm. Our results suggest that to analyze the spin transport in heterostructure with several magnetic layers, it is necessary to commence from the fundamentals of magnetization dynamics considering interlayer coupling and avoid the overuse of spin pumping.

The effect of chemical substitution on the ground state of the geometrically frustrated antiferromagnet ${\mathrm{CsCrF}}_4$ has been investigated through a neutron powder diffraction experiment. Magnetic Fe-substituted ${\mathrm{CsCr}}_{0.94}{\mathrm{Fe}}_{0.06}{\mathrm{F}}_4$ and nonmagnetic Al-substituted ${\mathrm{CsCr}}_{0.98}{\mathrm{Al}}_{0.02}{\mathrm{F}}_4$ samples are measured, and magnetic Bragg peaks are clearly observed in both samples. Magnetic structure analysis revealed a ${120}^{\circ }$ structure having a magnetic propagation vector ${\mathit{k}}_{\mathrm{mag}}=(0,0,1/2)$ in ${\mathrm{CsCr}}_{0.94}{\mathrm{Fe}}_{0.06}{\mathrm{F}}_4$. For ${\mathrm{CsCr}}_{0.98}{\mathrm{Al}}_{0.02}{\mathrm{F}}_4$, a quasi-${120}^{\circ }$ structure having ${\mathit{k}}_{\mathrm{mag}}=(1/2,0,1/2)$ is formed. It is notable that the identified magnetic structure in ${\mathrm{CsCr}}_{0.94}{\mathrm{Fe}}_{0.06}{\mathrm{F}}_4$ belongs to a different phase of ground states from those in ${\mathrm{CsCr}}_{0.98}{\mathrm{Al}}_{0.02}{\mathrm{F}}_4$ and the parent ${\mathrm{CsCrF}}_4$. These results suggest that the Fe substitution strongly influences the ground state of ${\mathrm{CsCrF}}_4$.

We study the in-plane critical magnetic field of two-dimensional Ising superconducting systems and propose the microscopic calculation for these disordered two-dimensional superconducting systems with or without inversion symmetry. Protected by certain specific spin-orbit interaction which polarizes the electron spin to the out-of-plane direction, the in-plane critical fields largely surpass the Pauli limit and show remarkable upturn in the zero-temperature limit. The impurity scattering and Rashba spin-orbit interaction, treated on equal footing in the microscopic framework, both weaken the critical field but in qualitatively different manners. The microscopic theory is consistent with recent experimental results in stanene and Pb superconducting ultrathin films.

We develop a theory of conductivity of type-II superconductors in the flux flow regime taking into account random spatial fluctuations of the system parameters, such as the gap magnitude $\mathrm{\Delta }\left(\mathbf{r}\right)$ and the diffusion coefficient $D\left(\mathbf{r}\right)$. We find a contribution to the conductivity that is proportional to the inelastic relaxation time ${\tau }_{\mathrm{in}}$, which is much longer than the elastic relaxation time. This contribution is due to Debye-type relaxation, and it can be much larger than the conventional flux flow conductivity due to Bardeen and Stephen. The new contribution is expected to dominate in clean superconductors at low temperatures and in magnetic fields much smaller than $H_{\mathrm{c}2}$.

The polar Kerr effect in superconducting ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ implies finite ac anomalous Hall conductivity. Since intrinsic anomalous Hall effect (AHE) is not expected for a chiral superconducting pairing developed on the single Ru $d_{xy}$ orbital, multiorbital chiral pairing actively involving the Ru $d_{xz}$ and $d_{yz}$ orbitals has been proposed as a potential mechanism. Here we propose that AHE could still arise even if the chiral superconductivity is predominantly driven by the $d_{xy}$ orbital. This is demonstrated through two separate models which take into account subdominant orbitals in the Cooper pairing, one involving the oxygen $p_x$ and $p_y$ orbitals in the $\mathrm{Ru}{\mathrm{O}}_2$ plane, and another the $d_{xz}$ and $d_{yz}$ orbitals. In both models, finite orbital mixing between the dominant $d_{xy}$ and the other orbitals may induce interorbital pairing between them, and the resultant states support intrinsic AHE, with Kerr rotation angles that could potentially reconcile with the experimental observation. Our proposal therefore sheds new light on the microscopic pairing in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. We also show that intrinsic Hall effect is generally absent for nonchiral states such as $\mathcal{S}+i\mathcal{D}, \mathcal{D}+i\mathcal{P}$, and $\mathcal{D}+i\mathcal{G}$, which provides a clear constraint on the symmetry of the superconducting order in this material.

The deeply buried, yet most important part of any spintronic device is the interface. This is even more interesting and much more complex when soft, light materials like organic molecules are in contact with an inorganic metallic electrode. Hence, exceptional care is required to better understand the phenomena driven by this type of organic/inorganic interfaces. To this end, ferromagnetic nuclear resonance (FNR) spectroscopy studies were performed to investigate the morphology and the magnetic properties of the hybrid organic-inorganic interfaces when zinc tetraphenyl porphyrin (ZnTPP) molecules are at the vicinity of ferromagnetic metallic cobalt (Co) layers. The FNR experimental results show that when ZnTPP is deposited on top of Co the resulting interface is smoother and sharper compared to the more extended interface obtained when Co is deposited on top of ZnTPP. The shape of the spectra suggests that no chemical bonds take place between the interfacial Co atoms and the ZnTPP molecules and that interactions at the interfaces are governed by weak van der Waals forces. Finally, FNR also showed that the magnetic anisotropy at the Co-ZnTPP hybrid interfaces is reduced compared to the magnetic anisotropy of the Co atoms inside the Co films.

The effects of nonmagnetic Al impurities on a magnetic bath ${\mathrm{YMnO}}_3$ are studied by magnetization, heat capacity, neutron diffraction, and NMR measurements. Suppression of the Néel order with an increase in the doping rate is evidenced by the heat capacity and $M\text{−}T$ curve, also showing vacancy-induced magnetization. Theory predicts that the vacancies in triangular antiferromagnets release the local frustration partially by a readjustment of the spin directions, which results in a net magnetic moment of $0.04S$ around the impurity site, where $S$ is the single spin moment. Interestingly, our NMR results show that the amplitude of the impurity moment is similar to $S$, which is one order of magnitude larger than a previous prediction. The direction of the impurity moment and neighboring spins generating a hyperfine field at a nonmagnetic impurity site are random. Above $T_N$, the release of frustration due to a spin-vacancy is evidenced by the enhancement of the antiferromagnetic correlation of spins near impurity sites. Based on careful consideration about the structural distortion, we suggest that this huge spin-disorder originates from the magnetic correlation between impurity moments.

Using first-principles alloy theory, we perform a systematic study of the Co-Cu phase diagram. Calculations are carried out for ferromagnetic and paramagnetic ${\mathrm{Co}}_{1-x}{\mathrm{Cu}}_x$ solid solutions with face-centered-cubic (fcc) crystal structure. We find that the equilibrium volumes and magnetic states are crucial for a quantitative description of the thermodynamics of the Co-Cu system at temperatures up to 1400 K. In particular, the paramagnetic state of Cu-rich alloys with persisting local magnetic moments is shown to be responsible for the solubility of a small amount of Co in fcc Cu whereas the excess entropy in the ferromagnetic Co-rich region critically depends on the adopted lattice parameters. None of the common local or semilocal density functional theory approximations have the necessary accuracy for the lattice parameters when compared to the experimental data. The predicted ab initio Co-Cu phase diagram is in good agreement with the measurements and CALPHAD data, making it possible to gain a deep insight into the various contributions to the Gibbs free energy. The present study provides an atomic-level description of the thermodynamic quantities controlling the limited mutual solubility of Co and Cu and highlights the importance of high-temperature magnetism.

The magnetic excitations in $\mathrm{CoP}{\mathrm{S}}_3$, a two-dimensional van der Waals (vdW) antiferromagnet with ${\mathrm{Co}}^{2+}$ ion on a honeycomb lattice, have been measured using powder inelastic neutron scattering. The absence of spin-orbit exciton around 30 meV indicates that ${\mathrm{Co}}^{2+}$ ions in $\mathrm{CoP}{\mathrm{S}}_3$ have an $S=3/2$ state rather than a spin-orbital entangled $J_{\mathrm{eff}}=1/2$ ground state. And, clear dispersive spin waves are observed with a large spin gap of ∼13 meV. The magnon spectra were fitted using an XXZ-type $J_1\text{−}{J}_2\text{−}{J}_3$ Heisenberg Hamiltonian with single-ion anisotropy assuming no magnetic exchange interaction between the honeycomb layers. The best-fit parameters show ferromagnetic exchange interactions $J_1=-2.08\mathrm{meV}$ and $J_2=-0.26\mathrm{meV}$ for the nearest- and second-nearest neighbors and a sizable antiferromagnetic exchange interaction $J_3=4.21\mathrm{meV}$ for the third-nearest neighbor with the strong easy-axis anisotropy $K=-2.06\mathrm{meV}$. The anisotropic XXZ-type Hamiltonian could only achieve a suitable fitting. The exchange interaction for the out-of-plane spin component is smaller than that for the in-plane one by a ratio $\alpha =J_z/J_x=0.6$. Our result directly shows that $\mathrm{CoP}{\mathrm{S}}_3$ is an experimental realization of the XXZ model with a honeycomb lattice in two-dimensional vdW magnets.

Motivated by the realization of Bose-Einstein condensates in noncubic lattices [J. Struck et al., Science 333, 996 (2011)], in this work we study the phases and collective excitation of bosons with nearest-neighbor interaction in a triangular lattice at finite temperature, using mean field (MF) and cluster mean field (CMF) theory. We compute the finite-temperature phase diagram both for hardcore and softcore bosons, as well analyze the effect of correlation arising due to geometric frustration of a triangular lattice and interaction systematically using the CMF method. A semianalytic estimate of the transition temperatures between different phases is derived within the framework of MF Landau theory, particularly for hardcore bosons. Apart from the usual phases such as density waves and superfluid, we also characterize different supersolids. These phases and their transitions at finite temperature are identified from the collective modes. The low-lying excitations, particularly Goldstone and Higgs modes of the supersolid, can be detected in the ongoing cold atom experiments.

We investigate in detail the low-temperature structural and physical properties of the trilayer nickelates $R_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ ($R=\mathrm{La}$, Pr, and Nd), which crystallize with a monoclinic symmetry (space group $P{2}_{1}a,Z=4$) and undergo a metal-to-metal transition (MMT) near $T_{\mathrm{MMT}}$= $135\mathrm{K}\left(\mathrm{La}\right), 156\mathrm{K}$ (Pr), and $160\mathrm{K}$ (Nd). Using a high-resolution synchrotron powder x-ray diffraction technique, we show that the lattice parameters in all cases exhibit an anomalous behavior at $T_{\mathrm{MMT}}$, however, without any sign of change in the lattice symmetry. Unambiguous signature of MMT is also observed in the magnetic and transport data, suggesting a strong coupling between the electronic, magnetic, and structural degrees of freedom. Analysis of thermal expansion yields hydrostatic pressure dependence of MMT in close agreement with previous high-pressure experiments. In ${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$, the ${\mathrm{Pr}}^{3+}$ ions located in the rocksalt (RS) layers order magnetically near $5\mathrm{K}$, which is significantly suppressed compared to ${\theta }_p\sim -36\mathrm{K}$. In contrast, ${\mathrm{Pr}}^{3+}$ ions in the perovskite-block (PB) layers exhibit a crystal field (CF) induced nonmagnetic singlet ground state. In ${\mathrm{Nd}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$, on the other hand, the CF ground state of ${\mathrm{Nd}}^{3+}$ ions in both RS and PB layers is a Kramers doublet. The heat capacity of ${\mathrm{Nd}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ shows a pronounced Schottky-like anomaly near $40\mathrm{K}$, and a sharp upturn indicating short-range correlations between the Nd-moments below $10\mathrm{K}$. However, no signs of long-range ordering of Nd-moments could be found down to $2\mathrm{K}$ despite a sizable value of ${\theta }_p\sim -40\mathrm{K}$. The strongly suppressed magnetic long-range ordering in both $R=\mathrm{Pr}$ and Nd suggests the presence of strong magnetic frustration in these compounds. In the presence of an overwhelming Schottky contribution, the electronic term in the specific heat of ${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ and ${\mathrm{Nd}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ appears highly inflated, which can be falsely interpreted as a sign of heavy fermion behavior as is done in a recent study on ${\mathrm{Nd}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$. Accordingly, the low-temperature resistivity of these compounds is found to follow a $-T^{0.5}$ rather than a $-\ln T$ dependence.

Quasi-one-dimensional (Q1D) systems, i.e., three- and two-dimensional (3D/2D) arrays composed of weakly coupled one-dimensional lattices of interacting quantum particles, exhibit rich and fascinating physics. They are studied across various areas of condensed matter and ultracold atomic lattice-gas physics, and are often marked by dimensional crossover as the coupling between one-dimensional systems is increased or temperature decreased, i.e., the Q1D system goes from appearing largely 1D to largely 3D. Phase transitions occurring along the crossover can strongly enhance this effect. Understanding these crossovers and associated phase transitions can be challenging due to the very different elementary excitations of 1D systems compared to higher-dimensional ones. In the present work, we combine numerical matrix product state (MPS) methods with mean-field (MF) theory to study paradigmatic cases of dimensional crossovers and the associated phase transitions in systems of both hard-core and soft-core lattice bosons, with relevance to both condensed matter physics and ultracold atomic gases. We show that the superfluid-to-insulator transition is a first order one, as opposed to the isotropic cases, and calculate transition temperatures for the superfluid states, finding excellent agreement with analytical theory. At the same time, our $\mathrm{MPS}+\mathrm{MF}$ approach keeps functioning well where the current analytical framework cannot be applied. We further confirm the qualitative and quantitative reliability of our approach by comparison to exact quantum Monte Carlo calculations for the full 3D arrays.

Color centers in diamond have emerged as leading solid-state “artificial atoms” for a range of quantum technologies, from quantum sensing to quantum networks. Concerted research activities are now underway to identify new color centers that combine stable spin and optical properties of the nitrogen vacancy (${\mathrm{NV}}^{-}$) with the spectral stability of the silicon vacancy (${\mathrm{SiV}}^{-}$) centers in diamond, with recent research identifying other group-IV color centers with superior properties. In this paper, we investigate a class of diamond quantum emitters from first principles, the group-III color centers, which we show to be thermodynamically stable in a spin-1, electric-field-insensitive structure. From ab initio electronic structure methods, we characterize the product Jahn-Teller (pJT) effect present in the excited-state manifold of these group-III color centers, where we capture symmetry-breaking distortions associated with strong electron-phonon coupling. These predictions can guide experimental identification of group-III vacancy centers and their use in applications in quantum information science and technology.

We theoretically investigate the electronic states in monolayer $\mathrm{Nb}{\mathrm{Se}}_2$ and develop continuous models to describe these states in Fermi pockets. In $1H$-type metallic transition metal dichalcogenides (TMDCs), the Fermi surface consists of three pockets enclosing the $\mathrm{\Gamma },$ $K,$ and $K^{\prime }$ points. We reveal that the conventional effective model used for semiconducting TMDCs is not sufficient to describe the electronic states in metallic TMDCs and thus introduce a scheme to construct the effective model from the first-principle results. All models can be represented by a $3\times 3$ Hamiltonian and well reproduce electronic states around the Fermi energy in terms of the orbital composition and the phase factor. We also show that the $p$ orbitals in chalcogen atoms, which are ignored in the conventional $2\times 2$ model, play a crucial role in metallic TMDCs. Although the aim of these models is to reproduce electronic states, they can well describe states near the high-symmetry points and the profile of Berry curvature in wave-vector space. The continuous model can be a handleable tool to describe the electronic states and to analyze the transport phenomena in metallic TMDCs.

We investigate the equilibrium state and the collective modes of an excitonic insulator (EI) in a Fabry-Pérot cavity. In an EI, two bands of a semiconductor or semimetal spontaneously hybridize due to the Coulomb interaction between electrons and holes, leading to the opening of a gap. The coupling to the electromagnetic field reduces the symmetry of the system with respect to phase rotations of the excitonic order parameter from $U\left(1\right)$ to $Z_2$. While the reduction to a discrete symmetry would, in general, lead to a gapped phase mode and enhance the stability of the ordered phase, the coupling to the cavity leaves the mean-field ground state unaffected. Its energy remains invariant under $U\left(1\right)$ phase rotations, in spite of the lower $Z_2$ symmetry imposed by the cavity. In a dipolar gauge, this can be traced back to the balancing of the linear light-matter coupling and the dipolar self-interaction at zero frequency. At nonzero frequency, however, the collective excitations do reflect the lower $Z_2$ symmetry. While our model is studied using a mean-field decoupling of the light-matter approximation (which is exact in the single-mode limit), the results show that fluctuations beyond mean field could play a crucial role in finding the true phase at finite temperature.

Using first-principles calculations, we analyze the evolution of the electronic structure and magnetic properties of infinite-layer nickelates $R{\mathrm{NiO}}_2$ ($R=\text{rare}\text{earth}$) as $R$ changes across the lanthanide series from La to Lu. By correlating these changes with in-plane and out-of-plane lattice parameter reductions, we conclude that the in-plane Ni-O distance is the relevant control parameter in infinite-layer nickelates. An antiferromagnetic ground state is obtained for all $R{\mathrm{NiO}}_2$ ($R=\text{La-Lu}$). This antiferromagnetic state remains metallic across the lanthanide series and is defined by a multi-orbital picture with low-energy relevance of a flat Ni-$d_{z^2}$ band pinned at the Fermi level, in contrast with cuprates. Other non-cuprate-like properties such as the involvement of $R\text{−}d$ bands at the Fermi level and a large charge-transfer energy are robust for all $R{\mathrm{NiO}}_2$ materials.

We study the effect of magnetic field on the properties of a high-mobility gated two-dimensional electron gas in a field-effect transistor with the Hall bar geometry. When approaching the current saturation when the drain side of the channel becomes strongly depleted, we see a number of unusual effects related to the magnetic field induced redistribution of the electron density in the conducting channel. The experimental results obtained in the nonlinear regime have been interpreted based on the results obtained in the linear regime by a simple theoretical model, which describes quite well our observations.

We present systematic investigations of the electrically induced breakdown of the integer quantum Hall effect (QHE) in (Al,Ga)As-based Hall bars of different widths ranging from 7 to 70 $\mu \mathrm{m}$. It is striking that, for the narrow Hall bars, the threshold values for the applied voltage that induces the breakdown of the longitudinal zero-resistance state differ appreciably between the upper and lower sides of a quantum Hall (QH) plateau. When moving from the low magnetic field side of the QH plateau to higher magnetic fields, the threshold rises in a strongly superlinear manner until—at the expected integer value for the Landau level (LL) filling factor in the bulk of the two-dimensional electron system (2DES)—the threshold value abruptly drops to low values for the rest of the plateau. With increasing widths of the Hall bars, the zero-resistance state extends slightly further to higher magnetic field values. The threshold values on the low-magnetic field side are almost independent of the Hall bar width, whereas the threshold values on the high magnetic field side scale linearly with the Hall bar width. These observations correspond perfectly with the microscope picture of the QHE where the biased current flows in a dissipationless manner in electrically incompressible regions of the 2DES of locally the same LL filling factor, driven by the respective drop of the Hall voltage over the width of these incompressible regions. Owing to the self-consistent evolution of the electrically incompressible/compressible landscape within the 2DES as a function of magnetic field, a transition from an edge- to a bulk-dominated QH regime is described within a QH plateau. This microscopic picture was derived from sophisticated, long-lasting scanning probe experiments measuring Hall potential profiles on typically 15-$\mu$m-wide Hall bars. Conversely, performing systematic electrically induced breakdown measurements like those presented here would allow us to identify the presence of edge- and bulk-dominated regimes of the QHE in a wider variety of samples.

Recent experiments on the flow of helium-4 fluid through nanopores with tunable pore radius provide a platform for studying the quasi-one-dimensional (quasi-1D) superfluid behaviors. In the extreme 1D limit, the helium atoms are localized by disordered small variations in the substrate potential provided by the pore walls. In the limit of wide pore radius, a solid layer of helium-4 is expected to coat the pore walls smoothing out the substrate potential, and superfluidity is observed in the central region. Building on earlier quantum Monte Carlo results, we propose a scenario for this crossover using a shell model of coupled Luttinger liquids. We find that a small-radius pore will always localize the helium atoms, but above a critical radius, a single 1D channel flows through the pore and can be described by Luttinger liquid theory.

The structures of solid hydrogen under high pressures have been revisited by using first-principles electronic structure calculations. Focusing on the stability, the intramolecular bond strength of hydrogen molecules in solid hydrogen, and the cell volume, we made a comparison study on seven structures including $P21/c\text{−}24$, $C2/c\text{−}24$, $P6122\text{−}36$, $Pc\text{−}48$, $Pca21\text{−}48$, $Cmca\text{−}4$, and $Cmca\text{−}12$ with the recently proposed strongly constrained and appropriately normed (SCAN) functional and three conventional exchange-correlation functionals, namely Perdew-Burke-Ernzerhof (PBE), Becke-Lee-Yang-Parr (BLYP), and van der Waals-density functional (vdW-DF). On one hand, with the SCAN functional the $C2/c$ structure takes the minimum static lattice enthalpies in the pressure range from 150 to 450 GPa, which agrees with the recent synchrotron infrared spectroscopic study and is similar to the result of the BLYP functional. On the other hand, the vibration frequencies of hydrogen molecules calculated by the SCAN functional match well with the experimental characteristic IR and Raman peaks, which indicates that the SCAN functional gives a reasonable estimation of the bond strength or bond length of hydrogen molecules in solid hydrogen, while the BLYP and vdW-DF functionals overestimate the bond strength. All SCAN, PBE, BLYP, and vdW-DF functionals give a good estimation of the unit cell volumes. Thus, the structures optimized with the SCAN functional can be an excellent starting point for advanced works on solid hydrogen.