The recently discovered antiferromagnetic 1/1 Au-Al-Tb approximant exhibits a whirling spin order oriented along the [111] direction. Here we report an investigation of the atomic structure of the (111) surface by scanning tunneling microscopy and density functional theory (DFT) calculations. Scanning tunneling microscopy reveals a Tb-terminated step-terrace morphology, with step heights appearing to minimize the number of broken bulk icosahedra. The atomic structure of the terraces is bias dependent, with Tb atoms imaged under positive bias and Au/Al atoms under negative bias. The surface is found to reconstruct, with the Au/Al atoms producing a linear row structure, an example of a surface reconstruction in a Tsai-type system. These observations are confirmed by DFT calculations.

We report on detailed structural, electronic, and magnetic studies of ${\mathrm{GdMn}}_{1-x}{\mathrm{Cr}}_x{\mathrm{O}}_3$ for Cr doping levels; $x=0 \le x\le$ 1. X-ray diffraction studies suggest that ${\mathrm{GdMn}}_{0.5}{\mathrm{Cr}}_{0.5}{\mathrm{O}}_3$ has a monoclinic $P{2}_1/b$ structure with alternate arrangements of Mn and Cr atoms along the [001] direction. In the solid solutions, the Jahn-Teller distortion associated with ${\mathrm{Mn}}^{3+}$ ions gives rise to major changes in the $bc$-plane sublattice and also an effective orbital ordering in the $ab$ plane, which persist up to compositions $x\sim$ 0.35. These distinct features in the lattice and orbital degrees of freedom are also correlated with $bc$-plane anisotropy of the local Gd environment. A gradual evolution of electronic states with doping is also clearly seen in O $K$-edge x-ray absorption spectra. Evidence of magnetization reversal in field-cooled-cooling mode for $x\ge$ 0.35 coinciding with the Jahn-Teller crossover suggests a close correlation between magnetic interaction and structural distortion. These observations indicate a strong entanglement between lattice, spin, electronic, and orbital degrees of freedom. The nonmonotonic variation of remnant magnetization can be explained by doping-induced modification of magnetic interactions. Density-functional-theory calculations are consistent with layer-by-layer-type arrangements of Cr ions and Mn ions with ferromagnetic (antiferomagnetic) coupling between Mn (Cr) ions for intermediate compounds ($x=0.5$). For $x=0.25$ compositions, we found alternate layers of Mn and mixed Mn-Cr atoms stacked along the $c$ axis with intralayer ferromagnetic coupling and interlayer antiferromagnetic coupling. For $x=0.75$ compositions, there exists strong antiferomagnetic coupling between half-filled $t_{2g}$ orbitals of in-plane Cr ions along with a feromagnetic Mn-Cr coupling.

We employ mean-field approximation to investigate the interplay between the nontrivial band topology and the formation of excitonic insulator (EI) in a one-dimensional chain of atomic $s\text{−}p$ orbitals in the presence of repulsive interorbital Coulomb interaction. We find that our model, in a noninteracting regime, admits topological and trivial insulator phases, whereas, in strong Coulomb interaction limit, the chiral symmetry is broken and the system undergoes a topological-excitonic insulator phase transition. The latter phase transition stems from an orbital pseudomagnetization and band inversion around $k=0$. Our findings show that contrary to the topological insulator phase, electron-hole bound states do not form exciton condensate in the trivial band insulator phase due to lack of band inversion. Interestingly, the EI phase in low $s\text{−}p$ hybridization limit hosts a Bardeen-Cooper-Schrieffer–Bose-Einstein condensation crossover. Irradiated by a pump pulse, our findings reveal that the oscillations of exciton states strongly depend on the frequency of the laser pulse. We further explore the signatures of dynamics of the exciton condensate in optical measurements.

We present a combined treatment of the nonequilibrium dynamics and transport of electrons and phonons by carrying out ab initio calculations of the fully coupled electron and phonon Boltzmann transport equations. We find that the presence of mutual drag between the two carriers causes the thermopower to be enhanced and dominated by the transport of phonons, rather than electrons as in the traditional semiconductor picture. Drag also strongly boosts the intrinsic electron mobility, thermal conductivity and the Lorenz number. Impurity scattering is seen to suppress the drag enhancement of the thermal and electrical conductivities, while having weak effects on the enhancement of the Lorenz number and thermopower. We demonstrate these effects in n-doped 3C-SiC at room temperature, and explain their origins. This work establishes the roles of microscopic scattering mechanisms in the emergence of strong drag effects in the transport of the interacting electron-phonon gas.

Using Floquet theory, we investigate the effect of light on triplefold fermions. We study a low-energy model as well as a simplified tight-binding model under illumination. We find that the threefold degeneracy remains symmetry protected even after applying light if the original band structure is rotationally symmetric around the degeneracy. Otherwise, light can lift the degeneracy and open up a gap. We further investigate the effect of light on the topological Fermi arcs by means of numerical computations. The changes caused by illumination are reflected in experimentally detectable signatures, such as the anomalous Hall conductivity, which we calculate.

We explore thermalization and quantum dynamics in a one-dimensional disordered SU(2)-symmetric Floquet model, where a many-body localized phase is prohibited by the non-Abelian symmetry. Despite the absence of localization, we find an extended nonergodic regime at strong disorder where the system exhibits nonthermal behaviors. In the strong disorder regime, the level spacing statistics exhibit neither a Wigner-Dyson nor a Poisson distribution, and the spectral form factor does not show a linear-in-time growth at early times characteristic of random matrix theory. The average entanglement entropy of the Floquet eigenstates is subthermal, although violating an area-law scaling with system sizes. We further compute the expectation value of local observables and find strong deviations from the eigenstate thermalization hypothesis. The infinite-temperature spin autocorrelation function decays at long times as $t^{-\beta }$ with $\beta <0.5$, indicating subdiffusive transport at strong disorders.

Motivated by recent experiments in fermionic polar gases, we study the nonequilibrium dynamics of two-component dipolar fermions subject to a quasiperiodic potential. We investigate the localization of charge and spin degrees of freedom time evolving with a long-range spin-SU(2) symmetric fermionic Hamiltonian by calculating experimentally accessible dynamical observables. To study the nonequilibrium dynamics, we start the time evolution with two initial states at half-filling: (i) product state with doublons $|\uparrow \downarrow 0\uparrow \downarrow 0\uparrow \downarrow 0\uparrow \downarrow 0\uparrow \downarrow \rangle$ and (ii) product state with singlons $|\uparrow \downarrow \uparrow \downarrow \uparrow \downarrow \uparrow \downarrow \uparrow \downarrow \rangle$. We carried out the real-time evolution via the fermionic Hamiltonian using exact diagonalization (ED) and the time-dependent variational principle (TDVP) for finite matrix product states (MPS), within experimentally relevant timescales. For the product state with doublons, we observe a delocalized to localized phase transition varying disorder strengths by monitoring the decay of charge imbalance with time. For the long-range interacting Hamiltonian of our focus, and in the presence of strong enough disorder, starting the time evolution with singlons we find a strong reduction in the spin delocalization, contrary to results of previous studies using the disordered short-range (on-site) Hubbard model with SU(2) symmetry. In addition to the quench dynamics, we also demonstrate the localization of charge and spin in the full energy spectrum of the long-range spin-SU(2) symmetric Hamiltonian by monitoring spin and charge autocorrelation functions. Our predictions for localization of both charge and spin should be observable in ultracold experiments with fermionic dipolar atoms subject to a quasiperiodic potential.

Weak hyperfine interactions and spin-orbit coupling (SOC) in organic materials result in long spin lifetimes, which is very promising for spintronics. On the other hand, they also make it challenging to achieve spin polarization, which is of crucial importance for spintronics devices. To overcome this obstacle, we have proposed a physical model for spin-polarized electron transport through a chiral molecule based on the chiral-induced spin selectivity. Because the transport in the chiral molecule is not an isolated one, but rather an electron coupled to its surrounding lattice distortions, namely, a spatial localized polaron, an indispensable polaron effect is incorporated in our model. We show that the polaron transport through the chiral molecule exhibits a spin-momentum-locked feature. Interestingly, no matter what their initial spin state is, all of the polarons could transmit through the molecule with their spins being aligned to the same orientation due to the effective “inverse Faraday effect.” The coexistence of the electron-lattice coupling and SOC results in the spin and lattice being coupled, which leads to a strongly enhanced spin coherence and then a very high spin polarization of $70\%$. In addition, the effects of the helix pitch, polaron size, and drift velocity on spin polarization are also discussed. Our results open the possibility of using chiral molecules in spintronics applications and offer a paradigm for information processing and transmission.

We investigate the spin-$\frac{1}{2}$ Heisenberg model on a rectangular lattice, using the Gutzwiller projected variational wave function known as the staggered flux state. Using Monte Carlo techniques, the variational parameters and instantaneous spin-spin correlation function for different coupling anisotropies $\gamma =J_y/J_x$ are calculated. We observe a gradual evolution of the ground state energy towards a value which is very close to the one-dimensional (1D) estimate provided by the Bethe ansatz and a good agreement between the finite-size scaling of the energies. The spin-spin correlation functions exhibit a power-law decay with varying exponents for different anisotropies. Though the lack of Néel order makes the staggered flux state energetically unfavorable in the symmetric case $\gamma =1$, it appears to capture the essence of the system close to 1D. Hence we believe that the staggered flux state provides an interesting starting point to explore the crossover from quantum disordered chains to the Néel ordered two-dimensional square lattices.

We report an experimental study of the static magnetization $M(H,T)$ and high-field electron spin resonance (ESR) of polycrystalline ${\text{Mg}}_2{\text{Gd}}_3{\text{Sb}}_3{\text{O}}_{14}$, a representative member of the newly discovered class of the so-called tripod kagome antiferromagnets where the isotropic ${\mathrm{Gd}}^{3+}$ spins $(S=7/2)$ form a two-dimensional kagome spin-frustrated lattice. It follows from the analysis of the low-$TM\left(H\right)$ curves that the ${\mathrm{Gd}}^{3+}$ spins are coupled by a small isotropic antiferromagnetic (AFM) exchange interaction, $\left|J\right|\approx 0.3$ K. The $M(H,T)$ dependences measured down to 0.5 K evidence a long-range AFM order at $T_N=1.7\mathrm{K}$ and its rapid suppression at higher fields, $\ge 4$ T. ESR spectra measured in fields up to 15 T are analyzed considering possible effects of demagnetizing fields, single-ion anisotropy, and spin-spin correlations. While the demagnetization effects due to a large sample magnetization in high fields and its shape anisotropy become relevant at low temperatures, the broadening of the ESR line commencing already at $T\lesssim 30\mathrm{K}$ may indicate the onset of the spin-spin correlations far above the ordering temperature due to the geometrical spin frustration in this compound.

We report a detailed study of the ground-state helical magnetic structure in monophosphide FeP by means of ${}^{31}\mathrm{P}$ NMR spectroscopy. We show that the zero-field NMR spectrum of the polycrystalline sample provides strong evidence of an anisotropic distribution of local magnetic fields at the P site with substantially lower anharmonicity than that found at the Fe site by Mössbauer spectroscopy. From field-sweep ${}^{31}\mathrm{P}$ NMR spectra we conclude that a continuous spin-reorientation transition occurs in an external magnetic field range of 4–7 T, which is also confirmed by specific-heat measurements. We observe two pairs of magnetically inequivalent phosphorus positions resulting in a pronounced four-peak structure of the single crystal ${}^{31}\mathrm{P}$ NMR spectra characteristic of an incommensurate helimagnetic ground state. We revealed a spatial redistribution of local fields at the P sites caused by Fe spin-reorientation transition in high fields and developed an effective approach to account for it. We demonstrate that all observed ${}^{31}\mathrm{P}$ spectra can be treated within a model of an isotropic helix of Fe magnetic moments in the ($ab$)-plane with a phase shift of ${36}^{\circ }$ and ${176}^{\circ }$ between Fe1-Fe3 (Fe2-Fe4) and Fe1-Fe2 (Fe3-Fe4) sites, respectively, in accordance with the neutron scattering data.

The weak-coupling limits of the gap and critical temperature computed within Eliashberg theory surprisingly deviate from the BCS theory predictions by a factor of $1/\sqrt{e}$. Interestingly, however, the ratio of these two quantities agrees for both theories. Motivated by this result, here we investigate the weak-coupling thermodynamics of Eliashberg theory, with a central focus on the free energy, specific heat, and the critical magnetic field. In particular, we numerically calculate the difference between the superconducting and normal-state specific heats, and we find that this quantity differs from its BCS counterpart by a factor of $1/\sqrt{e}$, for all temperatures below $T_c$. We find that the dimensionless ratio of the specific-heat discontinuity to the normal-state specific heat reduces to the BCS prediction given by $\mathrm{\Delta }{C}_V\left(T_c\right)/C_{V,n}\left(T_c\right)\approx 1.43$. This gives further evidence to the expectation that all dimensionless ratios tend to their “universal values” in the weak-coupling limit.

Quantum droplets have been realized in experiments on binary boson mixtures and dipolar Bose gases. In these systems, the mean-field energy of the Bose-Einstein condensation is attractive, and the repulsive Lee-Huang-Yang energy is crucial for stability. However, since the Lee-Huang-Yang term is not taken into account when constructing the Bogoliubov Hamiltonian, in the droplet regime one faces the problem of dynamically unstable phonon modes. In this work, we go beyond the Bogoliubov theory to study how the phonon mode is stabilized in the quantum droplet of a binary boson mixture. Similar to Beliaev's approach to a single-component Bose gas, we compute higher-order contributions to the self-energy of the boson propagator. We find that the interaction between spin and phonon excitations is the key for the phonon stability. We obtain the sound velocity which can be tested by measuring the superfluid critical velocity of the droplet in experiments. Beliaev damping of this quantum droplet is also discussed.

We studied the dynamic nuclear spin polarization of nitrogen in negatively charged nitrogen-vacancy (NV) centers in diamond both experimentally and theoretically over a wide range of magnetic fields from 0–1100 G covering both the excited-state level anticrossing and the ground-state level anticrossing magnetic field regions. Special attention was paid to the less studied ground-state level anticrossing region. The nuclear spin polarization was inferred from measurements of the optically detected magnetic resonance signal. These measurements show that a very large (up to $96\pm 2\%$) nuclear spin polarization of nitrogen can be achieved over a very broad range of magnetic field starting from around 400 G up to magnetic field values substantially exceeding the ground-state level anticrossing at 1024 G. We measured the influence of angular deviations of the magnetic field from the NV axis on the nuclear spin polarization efficiency and found that, in the vicinity of the ground-state level anticrossing, the nuclear spin polarization is more sensitive to this angle than in the vicinity of the excited-state level anticrossing. Indeed, an angle as small as a tenth of a degree of arc can destroy almost completely the spin polarization of a nitrogen nucleus. In addition, we investigated theoretically the influence of strain and optical excitation power on the nuclear spin polarization.

The Kitaev model is a rare example of an analytically solvable and physically instantiable Hamiltonian yielding a topological quantum spin liquid ground state. Here we report signatures of Kitaev spin liquid physics in the honeycomb magnet ${\mathrm{Li}}_3{\mathrm{Co}}_2\mathrm{Sb}{\mathrm{O}}_6$, built of high-spin $d^7$ (${\mathrm{Co}}^{2+}$) ions, in contrast to the more typical low-spin $d^5$ electron configurations in the presence of large spin-orbit coupling. Neutron powder diffraction measurements, heat capacity, and magnetization studies support the development of a long-range antiferromagnetic order space group of $C_{C}2/m$, below $T_N=11\mathrm{K}$ at ${\mu }_{0}H=0\mathrm{T}$. The magnetic entropy recovered between $T=2$ and 50 K is estimated to be $0.6R\ln 2$, in good agreement with the value expected for systems close to a Kitaev quantum spin liquid state. The temperature-dependent magnetic order parameter demonstrates a β value of 0.19(3), consistent with XY anisotropy and in-plane ordering, with Ising-like interactions between layers. Further, we observe a spin-flop-driven crossover to ferromagnetic order with space group of $C2/m$ under an applied magnetic field of ${\mu }_{0}H\approx 0.7\mathrm{T}$ at $T=2\mathrm{K}$. Magnetic structure analysis demonstrates these magnetic states are competing at finite applied magnetic fields even below the spin-flop transition. Both the $d^7$ compass model, a quantitative comparison of the specific heat of ${\mathrm{Li}}_3{\mathrm{Co}}_2\mathrm{Sb}{\mathrm{O}}_6$, and related honeycomb cobaltates to the anisotropic Kitaev model further support proximity to a Kitaev spin liquid state. This material demonstrates the rich playground of high-spin $d^7$ systems for spin liquid candidates and complements known $d^5$ Ir- and Ru-based materials.

As a measure of the temperature response of the energy of matter, the heat capacity $C_p$ is a fundamental thermodynamic property. Its dependence on magnetic field, especially at low temperatures, yields insight into the electronic, phononic, and magnetic states of condensed matter. Here, we present a set of paramagnetic and ferromagnetic $(\mathrm{Ti},\mathrm{Zr},\mathrm{Hf})\mathrm{Ni}{\mathrm{Fe}}_x\mathrm{Sn}$ half-Heusler composites that exhibit low-field ($<3$ T) maxima in $C_p$ and higher-field magnetic quenching of the heat capacity at temperatures below 10 K. Using rigorous statistical analysis, we attribute the effect to the existence of paramagnons within the compounds. To explain the lowest-temperature ($<4$ K), low-field declines in $C_p$, we derive a magnon model up to fourth order in dispersion. While the combined paramagnon and magnon model matches the data well, the fit parameters are significantly underdetermined. We provide a qualitative explanation of the secondary effect based on superconducting phases within the composites. Overall, our work highlights the insight of field-dependent heat capacity studies at fixed temperatures that cannot be as easily gleaned from the temperature-dependent heat capacity at fixed magnetic fields.

We study magnon hybridization in a ferrimagnetic heterostructure consisting of ultrathin gadolinium iron garnet and yttrium iron garnet (YIG) layers and show the localized and extended spatial profiles of the magnon modes with different polarizations. These modes are expected to have distinct thermal excitation properties in the presence of a temperature gradient across the heterostructure. From a quantitative analysis of their consequences for the longitudinal spin Seebeck effect, we predict an observable shift of the sign-changing temperature with respect to the one previously observed in gadolinium iron garnet. Moreover, the sign-changing point of the spin Seebeck signal is found to be tunable by YIG thickness. Our results suggest the necessity of taking into account the temperature difference between the magnon modes in ferrimagnetic heterostructures.

In this work, we present a comparative first-principles study of the mechanical properties, electronic structure, phonon dispersion relation, electron-phonon coupling, and magnetism in three isostructural superconductors, namely ThFeAsN, ThCoAsN, and ThNiAsN. Experimentally, ThFeAsN and ThNiAsN show superconducting properties, while ThCoAsN has not been synthesized. Our calculated elastic constants show that all these systems are mechanically stable. Significant differences in the electronic structures of these three compounds in terms of density of states, band structures, and Fermi surfaces are found. Our phonon calculations reveal that all the systems, including ThCoAsN, are dynamically stable. Phonon dispersion relations indicate that the optical modes of all three systems are almost the same, while there are significant variations in the low-frequency manifold consisting of mixed modes. The electron-phonon coupling constants and superconducting transition temperatures calculated based on the Eliashberg formalism predict a rather high $T_c$ of 6.4 K for ThCoAsN and also a $T_c$ of 3.4 K for ThNiAsN, which agrees well with the experimental value of 4.3 K. Nevertheless, we find a $T_c$ of 0.05 K for ThFeAsN, which is much smaller than the experimental $T_c$ of $\sim 30$ K. However, a simple analysis considering the amplifying effects of spin density wave order and out-of-plane soft phonon modes suggests that the $T_c$ could be increased considerably to $\sim 10$ K. Finally, we also discuss the effect of anion As height on the electronic structures, and we study possible magnetic states in these three compounds.