We present a method to compute the phonon scattering matrix of a given lattice that uses molecular dynamics simulations and is therefore able to capture perturbation terms in the Hamiltonian to all orders. This allows the computation of the relaxons' eigenmodes associated with the system as well as their properties with regard to the flow of heat in the system. Applying this method to the one-dimensional Fermi-Pasta-Ulam-$\beta$ ($\mathrm{FPU}\beta$) lattice, we show that there is an approximate one-to-one mapping from the phonon eigenmodes to the relaxon eigenmodes. For every temperature under study, which span the transition from the weakly stochastic to the strongly stochastic regime, we find that a single relaxon mode with a very long lifetime plays a very important role in the heat flow dynamics. Other relaxons have relaxation times similar to that of the phonons. These conclusions are fully consistent through the various anharmonic regimes associated with the $\mathrm{FPU}\beta$ model.

We calculate the electric charge current flowing through a vibrating molecular nanojunction, which is driven by an ac voltage, in its regime of nonlinear oscillations. Without loss of generality, we model the junction by a vibrating molecule, which is doubly clamped to two metallic leads which are biased by time-periodic ac voltages. Dressed-electron tunneling between the leads and the molecule drives the mechanical degree of freedom out of equilibrium. In the deep quantum regime, where only a few vibrational quanta are excited, the formation of coherent vibrational resonances affects the dressed-electron tunneling. In turn, back action modifies the electronic ac current passing through the junction. The concert of nonlinear vibrations and ac driving induces quantum transport currents, which are antiresonant to the applied ac voltage. Quantum back action on the flowing nonequilibriun current allows us to obtain rather sharp spectroscopic information on the population of the mechanical vibrational states.

We investigate electrical transport properties of micrometer-sized samples of the rare-earth-based monoaxial chiral magnetic crystal ${\mathrm{YbNi}}_3{\mathrm{Al}}_9$ in order to characterize chiral magnetic structures. The magnetoresistance (MR) data exhibit discrete changes, supporting the formation of the chiral spin soliton lattice (CSL) in ${\mathrm{YbNi}}_3{\mathrm{Al}}_9$. Reversal and lock-in behavior of the MR signal are significant features during the CSL formation, which contrast with hysteresis behavior reported in a prototype monoaxial chiral magnetic crystal of ${\mathrm{CrNb}}_3{\mathrm{S}}_6$. The magnetic phase diagrams are provided based on the electrical transport properties in micrometer-sized ${\mathrm{YbNi}}_3{\mathrm{Al}}_9$ crystals.

A density functional theory based two-terminal scattering formalism that includes spin-orbit coupling and spin noncollinearity is described. An implementation using tight-binding muffin-tin orbitals combined with extensive use of sparse matrix techniques allows a wide variety of inhomogeneous structures to be flexibly modelled with various types of disorder including temperature induced lattice and spin disorder. The methodology is illustrated with calculations of the temperature dependent resistivity and magnetization damping for the important substitutional disordered magnetic alloy permalloy (Py), ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$. Comparison of calculated results with recent experimental measurements of the damping (including its temperature dependence) indicates that the scattering approach captures the most important contributions to this important property.

We present an alternative, simpler expression for the Hamiltonian of the partially isotropic (XXZ-like) version of the Haldane-Shastry model, which was derived by D. Uglov over two decades ago in an apparently little-known preprint. While resembling the pairwise long-range form of the Haldane-Shastry model, our formula accounts for the multispin interactions obtained by Uglov. Our expression is physically meaningful, makes hermiticity manifest, and is computationally more efficient. We discuss the model's properties, including its limits and (ordinary and quantum-affine) symmetries, and review the model's exact spectrum found by Uglov for finite spin-chain length, which parallels the isotropic case up to level splitting due to the anisotropy. We also extend the partially isotropic model to higher rank, with $SU\left(n\right)$ “spins,” for which the spectrum is determined by ${\mathfrak{sl}}_n$ motifs.

We investigate the magnetization reversal of a single-domain magnetic nanoparticle driven by a linear down-chirp microwave magnetic field pulse. Numerical simulations based on the Landau-Lifshitz-Gilbert equation reveal that a down-chirp microwave pulse is solely capable of inducing subnanosecond magnetization reversal. With a certain range of initial frequency and chirp rate, the required field amplitude is much smaller than that of a constant-frequency microwave field. The fast reversal is due to the fact that the down-chirp microwave field pulse triggers stimulated microwave absorptions (emissions) by (from) the spin before (after) it crosses over the energy barrier. Applying a spin-polarized current additively to the system further reduces the microwave field amplitude. Our findings provide a way to realize low-cost and fast magnetization reversal.

Magnetic excitations and the spin Hamiltonian of the spin-1 trimer chain compound $\mathrm{CaN}{\mathrm{i}}_3{\mathrm{P}}_4{\mathrm{O}}_{14}$ have been investigated by inelastic neutron scattering. The trimer spin chains in $\mathrm{CaN}{\mathrm{i}}_3{\mathrm{P}}_4{\mathrm{O}}_{14}$ result from the crystal structure that provides a special periodicity of the exchange interactions ($J_1\text{−}{J}_1\text{−}{J}_2$) comprising intratrimer ($J_1$) and intertrimer ($J_2$) exchange interactions along the crystallographic $b$ axis. Experimental data reveal gapped dispersive spin-wave excitations in the 3D long-range ordered magnetic state ($T_C=16\mathrm{K}$), and gapless magnetic excitations above the $T_C$ due to the low-dimensional spin-spin correlations within chains. Simulated magnetic excitations, by using the linear spin-wave theory, for a model of coupled trimer spin chains provide a good description of the observed experimental data. The analysis reveals both ferromagnetic $J_1$ and $J_2$ interactions within the chains, and an antiferromagnetic interchain interaction $J_3$ between chains. The strengths of the $J_1$ and $J_2$ are found to be closer ($J_2/J_1\sim 0.81$), and $J_3$ is determined to be weaker ($|J_3/J_1|\sim 0.69$), which is consistent with the spin-chain-type crystal structure. Presence of a weak single-ion anisotropy ($D/J=0.19$) is also revealed. The strengths and signs of exchange interactions explain why the 1/3 magnetization is absent in the studied spin-1 compound $\mathrm{CaN}{\mathrm{i}}_3{\mathrm{P}}_4{\mathrm{O}}_{14}$ in contrast to its $S=5/2$ counterpart Mn-based isostructural compound. The signs of the exchange interactions are in agreement with that obtained from the reported density-functional theory calculations, whereas their strengths are found to be significantly different. The relatively strong value of the $J_3$ in $\mathrm{CaN}{\mathrm{i}}_3{\mathrm{P}}_4{\mathrm{O}}_{14}$ gives a conventional 3D-type magnetic ordering behavior below the $T_C$ with full ordered moment at 1.5 K, however, retains its 1D character above the $T_C$. The present experimental study also reveals a sharp change of single-ion anisotropy across the $T_C$, indicating that the stability of the 3D magnetic ordering in $\mathrm{CaN}{\mathrm{i}}_3{\mathrm{P}}_4{\mathrm{O}}_{14}$ is ascribed to the local magnetic anisotropy in addition to interchain interactions. The present study divulges the importance of full knowledge of the exchange interactions in trimer spin-chain compounds to understand their exotic magnetic properties, such as 1/3 magnetization plateau. The importance of the observed gapless magnetic excitations of the strongly correlated spin chains above the $T_C$ is also discussed.

The emerging field of superconductor (SC) spintronics has attracted intensive attentions recently. Many fantastic spin-dependent properties in SCs have been discovered, including large magnetoresistance, long spin lifetimes, and the giant spin Hall effect, etc. Regarding the spin dynamics in superconducting thin films, few studies have been reported yet. Here, we report the investigation of the spin dynamics in an $s$-wave superconducting NbN film via spin pumping from an adjacent insulating ferromagnet GdN film. A profound coherence peak of the Gilbert damping of GdN is observed slightly below the superconducting critical temperature of the NbN, which agrees well with recent theoretical prediction for $s$-wave SCs in the presence of impurity spin-orbit scattering. This observation is also a manifestation of the dynamic spin injection into superconducting NbN thin film. Our results demonstrate that spin pumping could be used to probe the dynamic spin susceptibility of superconducting thin films, thus pave the way for future investigation of spin dynamics of interfacial and two-dimensional crystalline SCs.

A substantial influence of the proximity and pair breaking effects on the range of internal phase differences is shown to take place in symmetric double Josephson junctions with closely spaced interfaces and to affect the evolution of the supercurrent $j$ with the changing central lead's length $L$. If the phase difference $\varphi$ between the external leads is controlled and $L$ exceeds a few coherence lengths, the regime of interchanging modes is established. The range of the phase differences across the two individual interfaces is reduced with decreasing $L$, and the states of the higher energy mode are gradually eliminated. With a further decrease of $L$ the regime of interchanging modes is destroyed along with the asymmetric mode. The conventional single junction current-phase relation $j\left(\varphi \right)$ is eventually established and the condensate states' doubling is fully removed at very small $L$.

Magnetism induced by external pressure ($p$) was studied in a FeSe crystal sample by means of muon-spin rotation. The magnetic transition changes from second order to first order for pressures exceeding the critical value $p_{\mathrm{c}}\simeq 2.4–2.5$ GPa. The magnetic ordering temperature ($T_{\mathrm{N}}$) and the value of the magnetic moment per Fe site ($m_{\mathrm{Fe}}$) increase continuously with increasing pressure, reaching $T_{\mathrm{N}}\simeq 50$ K and $m_{\mathrm{Fe}}\simeq 0.25{\mu }_{\mathrm{B}}$ at $p\simeq 2.6$ GPa, respectively. No pronounced features at both $T_{\mathrm{N}}\left(p\right)$ and $m_{\mathrm{Fe}}\left(p\right)$ are detected at $p\simeq p_{\mathrm{c}}$, thus suggesting that the stripe-type magnetic order in FeSe remains unchanged above and below the critical pressure $p_{\mathrm{c}}$. A phenomenological model for the $(p,T)$ phase diagram of FeSe reveals that these observations are consistent with a scenario where the nematic transitions of FeSe at low and high pressures are driven by different mechanisms.

We have studied the half-Heusler compound TbPdBi through resistivity, magnetization, Hall effect, and heat capacity measurements. A semimetal behavior is observed in its normal-state transport properties, which is characterized by a large negative magnetoresistance below 100 K. Notably, we find the coexistence of superconductivity and antiferromagnetism in this compound. The superconducting transition appears at 1.7 K, while the antiferromagnetic phase transition takes place at 5.5 K. The upper critical field $H_{c2}$ shows an unusual linear temperature dependence, implying unconventional superconductivity. Moreover, when the superconductivity is suppressed by magnetic field, its resistivity shows plateau behavior, a signature often seen in topological insulators/semimetals. These findings establish TbPdBi as a platform for the study of the interplay between superconductivity, magnetism, and nontrivial band topology.

Single-layer FeSe films grown on ${\mathrm{SrTiO}}_3$ (STO) substrates have attracted much attention because of their record high superconducting critical temperature $T{}_C$. It is generally believed that the composition of the epitaxially grown single-layer FeSe/STO films is stoichiometric; that is, the ratio of Fe to Se is 1:1. Here we report the identification of a large amount of excess Fe in superconducting single-layer FeSe/STO films. By depositing Se onto superconducting single-layer FeSe/STO films, we find by in situ scanning tunneling microscopy the formation of second-layer FeSe islands on top of the first layer during the annealing process at a surprisingly low temperature ($\sim 150{}^{\circ }\mathrm{C}$) which is much lower than the usual growth temperature ($\sim 490{}^{\circ }\mathrm{C}$). This observation is used to detect excess Fe and estimate its quantity in single-layer FeSe/STO films. The amount of excess Fe detected is at least 20%, which is surprisingly high for superconducting single-layer FeSe/STO films. The discovery of such a large amount of excess Fe should be taken into account in understanding the high-$T{}_C$ superconductivity and points to a likely route to further enhance $T{}_C$ in superconducting single-layer FeSe/STO films.

We investigate the three-dimensional electronic structure of the seminal charge-density-wave (CDW) material $2H-\mathrm{NbS}{\mathrm{e}}_2$ by soft x-ray angle-resolved photoelectron spectroscopy and density functional theory. Our results reveal the pronounced 3D character of the electronic structure formed in the quasi-two-dimensional layered crystal structure. In particular, we find a strong dispersion along $k_z$ excluding a nesting-driven CDW formation based on experimental data. The 3D-like band structure of $2H-\mathrm{NbS}{\mathrm{e}}_2$ has strong implications for the intriguing phase competition of CDW order with superconductivity.

We designed a spin-caloritronics device based on armchair graphene nanoribbons (AGNRs). We theoretically show that a trapezoidal-shaped graphene flake consisting of four zigzag edges bridged between two AGNRs becomes a spin semiconductor with a tunable spin-dependent transmission gap. The results indicate that the appearance of spin semiconducting properties with spin-dependent localized transmission peaks around the Fermi level could produce the spin-caloritronics effects of AGNRs. Interestingly, the values of the spin Seebeck coefficient $S_S$ are comparable to values obtained for zigzag graphene nanoribbons, and also $S_S$ sensitively increases as the transmission gap increases. Furthermore, by engineering the position and orientation of a triangular antidot only in the scattering region, both $S_S$ and the spin figure of merit could be separately enhanced at room temperature.

We study the effect of electron-phonon interactions on the electrical conductance of a helical edge state of a two-dimensional topological insulator. We show that the edge deformation caused by bulk acoustic phonons modifies the spin texture of the edge state, and that the resulting spin-phonon coupling leads to inelastic backscattering which makes the transport diffusive. Using a semiclassical Boltzmann equation we compute the electrical conductivity and show that it exhibits a metallic Bloch-Grüneisen law. At temperatures on the order of the Debye temperature of the host material, spin-phonon scattering thus drastically lowers the conductivity of the edge state. Transport remains ballistic only for short enough edges, and in this case the correction to the quantized conductance vanishes as $\delta G\propto T^5$ at low temperatures. Relying only on parallel transport of the helical spin texture along the deformed edge, the coupling strength is determined by the host material's density and sound velocity. Our results impose fundamental limits for the finite-temperature conductivity of a helical edge channel.

The continuous quantum phase transition and the nature of the quantum critical point (QCP) in a modified Kondo lattice model with Ising anisotropic exchange interactions are studied within the density-matrix renormalization group algorithm. We investigate the effect of quantum fluctuations on critical Kondo destruction QCP by probing static and dynamic properties of the magnetic order and the Kondo effect. In particular, we identify that local Kondo physics itself becomes critical at the magnetic phase transition point, providing unbiased evidences for local quantum criticality between two insulators without involving the Fermi surface.

The thermal conductivity of heavy-fermion superconductor ${\mathrm{Ce}}_{0.95}{\mathrm{Nd}}_{0.05}{\mathrm{CoIn}}_5$ was measured with a magnetic field rotating in the tetragonal $a\text{−}b$ plane. The thermal conductivity exhibits a step within the high-field low-temperature (HFLT) phase, which exists above 8 Tesla within the superconducting state, when field is rotated through the antinodal [100] direction of the superconducting $d$-wave order parameter. This anomaly indicates the presence of a third order parameter within the HFLT phase, similar to that of pure ${\mathrm{CeCoIn}}_5$. Therefore, the HFLT state with triply intertwined orders, i.e., superconducting $d$-wave, magnetic spin-density wave (SDW), and a putative superconducting $p$-wave pair-density wave (PDW), is robust against 5% Nd doping. The second magnetic phase within the superconducting state below 8 Tesla, a low-field SDW, displays a hysteresis in thermal conductivity as a function of field magnitude, which is also revealing the existence of a third order consistent with a $p$-wave PDW. Given different origins and properties of the HFLT and low-field SDW states, the presence of three intertwined orders in both phases is remarkable and of far-reaching consequences.