We have performed ${}^{13}\mathrm{C}$ NMR measurements on $\lambda \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{STF}\right)}_2{\mathrm{Fe}}_x{\mathrm{Ga}}_{1-x}{\mathrm{Cl}}_4$ and evaluated Fe content dependence of the magnetic properties. We found that the angle dependence of the Knight shift of $\lambda \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{STF}\right)}_2{\mathrm{Fe}}_x{\mathrm{Ga}}_{1-x}{\mathrm{Cl}}_4$ changes depending on the Fe content, which is explained by the exchange fields through the $\pi \text{−}d$ interaction and dipole fields. An antiferromagnetic ordering was observed in all Fe content samples except for $\lambda \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{STF}\right)}_2{\mathrm{GaCl}}_4$ ($x=0$). This indicates that the antiferromagnetic ground state is stabilized by the interaction between $\pi$ and $3d$ spins. From the linewidths of the NMR spectra measured at the lowest temperature, we confirmed that the size of the ordered $\pi$ spin moment is independent of the Fe content. Based on the results, we discuss the role of $\pi \text{−}d$ interaction for the stabilization of the antiferromagnetic ordering and the mechanism of the unconventional magnetism of $3d$ spins observed in $\pi \text{−}d$ systems.

The highly unusual divalent silver in silver difluoride (${\mathrm{AgF}}_2$) features a nearly square lattice of ${\mathrm{Ag}}^{+2}$ bridged by fluorides. As a structural and electronic analog of cuprates, its superconducting properties are yet to be examined. Our first-principles electronic structure calculations reveal a striking resemblance between ${\mathrm{AgF}}_2$ and the cuprates. Computed spin susceptibility shows a magnetic instability consistent with the experimentally observed antiferromagnetic transition. A linearized Eliashberg theory in the fluctuation-exchange approximation shows an unconventional singlet $d$-wave superconducting pairing for bulk ${\mathrm{AgF}}_2$ at an optimal electron doping. The pairing is found to strengthen with decreasing interlayer coupling, highlighting the importance of the quasi-two-dimensional nature of the crystal structure. These findings place ${\mathrm{AgF}}_2$ in the category of unconventional high-$T_{\text{C}}$ superconductors, and its chemical uniqueness may help shed light on the high-$T_{\text{C}}$ phenomena.

Clapping modes, which are relative amplitude and phase modes between two chiral components of Cooper pairs, are bosonic collective modes inherent to chiral superconductors. These modes behave as long-lived bosons with masses smaller than the threshold energy, $2\mathrm{\Delta }$, for decay into unbound fermion pairs. Here, we clarify that the real/imaginary clapping modes in chiral superconductors directly couple to acoustic wave propagation when the weak particle-hole asymmetry of the normal state quasiparticle dispersion is taken into account. The clapping modes driven by an acoustic wave generate an alternating electric current, that is, the acoustoelectric effect in superconductors. Significantly, the clapping modes give rise to a transverse electric current. When the sound velocity is comparable to the Fermi velocity, as in heavy fermion compounds, the transverse current is resonantly enhanced at energy below the threshold for continuum excitations. This resonance provides smoking-gun evidence of chiral superconductivity.

We study the emergence of quasicrystal configurations in the ground-state phase diagram of bosonic systems interacting through pair potentials of Lifshitz's type. By using a variational mean-field approach, we determine the relevant features of the corresponding potential interaction kind stabilizing such quasicrystalline states in two dimensions. Unlike their classical counterpart, in which the interplay between only two wave vectors determines the resulting symmetries of the solutions, the quantum picture relates in a more complex way to the instabilities of the excitation spectrum. Moreover, the quantum quasicrystal patterns are found to emerge as the ground state with no need of moderate thermal fluctuations. The study extends to the exploration of the excitation properties and the possible existence of superquasicrystals, i.e., supersolidlike quasicrystalline states in which the long-range nonperiodic density profile coexists with a nonzero superfluid fraction. Our calculations suggest that, in an intermediate region between the homogeneous superfluid and the normal quasicrystal phases, these exotic states indeed exist at zero temperature. Comparison with full numerical simulations provides a solid verification of the variational approach adopted in this paper.

Angle-dispersive x-ray powder diffraction experiments have been performed on praseodymium metal compressed in a soft pressure-transmitting medium at ambient temperature up to 185 GPa. We observe the previously reported high-pressure structural transition sequence up to 20 GPa and the coexistence of body-centered orthorhombic (bco) Pr and α-U Pr from $\sim 20$ up to $\sim 38\mathrm{GPa}$. The α-U structure of Pr is stable from 20 to 185 GPa, and no evidence of the proposed transition to a primitive orthorhombic phase $>147\mathrm{GPa}$ was observed. With density functional theory (DFT), we calculated the lattice parameters and $y$ coordinate for α-U Pr and found good agreement between our calculations and experimental measurements. The obtained DFT energies of the proposed primitive orthorhombic $\left(P{2}_1{2}_1{2}_1\right)$ and the α-uranium phases at $\sim 150\mathrm{GPa}$ show that the α-uranium phase is lower in energy. Hence, neither our experimental data nor our DFT results support the transition to a primitive orthorhombic phase $>150\mathrm{GPa}$. DFT suggests, however, that Pr may transform to the $P{2}_1{2}_1{2}_1$ phase above $\sim 220\mathrm{GPa}$. We also compare the axial ratios and lattice parameters of praseodymium to α-uranium structured Nd, Ce, and U.

Shape-memory properties of equiatomic NiTi rely on the thermal- or stress-induced reversible martensitic phase transformation between the $B2$ and $B{19}^{\prime }$ phases. Irradiation defects suppress thermal-induced transformations, but their effects on stress-induced transformations are poorly understood. We use molecular dynamics to investigate the effect of vacancies, vacancy clusters, interstitials, and antisite defects on stress-induced transformations. All defects suppress the transformation, but vacancy clusters do so to the greatest extent, while antisite defects do so to the least extent. The responsible mechanisms are grain boundary pinning and chemical disordering.

Auto-oscillations of the magnetization in ferromagnetic hybrid nanostructures driven by spin currents produced by a variety of processes have attracted attention for their challenging physics and possible applications, such as microwave assisted magnetic recording, neuromorphic computing, and chip to chip wireless communications. Of particular interest are applications in which the spin current is produced by a thermal gradient in the configuration of the spin Seebeck effect, because it makes it possible to harvest the thermal energy generated in nanodevices. A few years ago, it was demonstrated experimentally that in a simple bilayer made of a thin film of the insulating ferrimagnet yttrium iron garnet in contact with a platinum layer, the application of a temperature difference across the bilayer produced a coherent microwave auto-oscillation. This device was called a spin caloritronic nano-oscillator. Here we show that these experiments are explained quantitatively by a theory based on a mechanism in which one magnon in the spin current splits into two magnons, one of them being the magnon mode resonating at the nanostructure. The theoretical value of the critical temperature gradient necessary to overcome the magnetic damping to produce auto-oscillations is in good agreement with the one employed in the experiments.

We present a fast, matrix-based semianalytical method to calculate the stray magnetic field and its derivative above spin textures with cylindrical symmetry. A magnetostatic Green's function approach is used to obtain accurate fields and magnetic forces, which are confirmed using micromagnetic simulations. The developed method can be used to quickly analyze and fit experimental measurements of the stray magnetic field (measured, for example, via nitrogen vacancies in diamond) or magnetic forces (measured via magnetic force microscopy) above a thin film or patterned element. Calculations for magnetic skyrmions show that these techniques have the potential to distinguish between Bloch and Néel walls for skyrmions in single-layer as well as in multilayer thin films where the wall structure evolves with depth.

One of the signatures of the face-centered cubic (fcc) antiferromagnet as a typical example of a geometrically frustrated system is the large ground state degeneracy of the classical nearest-neighbor and next-nearest-neighbor Heisenberg (isotropic) model on this lattice. In particular, collinear states are degenerate with noncollinear and noncoplanar ones: this degeneracy is accidental and is expected to be lifted by anisotropic exchange interactions. In this work, we derive the most general nearest and next-nearest-neighbor exchange model allowed by the space-group symmetry of the noncentrosymmetric half-Heusler compounds, which includes three anisotropic terms: the so-called Kitaev, Gamma, and Dzyaloshinskii-Moriya interactions—most notably, the latter is allowed by the breaking of inversion symmetry in these materials and has not been previously been studied in the context of the fcc lattice. We compute the resulting phase diagram, show how the different terms lift the ground state degeneracy of the isotropic model, and lay emphasis on finding regimes where multi-$q$ (noncollinear/noncoplanar) states are selected by anisotropy. We then discuss the role of quantum fluctuations and the coupling to a magnetic field in the ground state selection and show that these effects can stabilize noncoplanar (triple-$q$) states. These results suggest that some half-Heusler antiferromagnets might host rare noncollinear/noncoplanar orders, which may, in turn, explain the unusual transport properties detected in these semimetals.

We propose a scheme for enhancing the optomechanical coupling between microwave and mechanical resonators by up to seven orders of magnitude to the ultrastrong coupling limit in a circuit optomechanical setting. The tripartite system considered here consists of a Josephson junction Cooper-pair box that mediates the coupling between the microwave cavity and the mechanical resonator. The optomechanical coupling can be modified by tuning the gate charge and the magnetic flux bias of the Cooper-pair box which in turn affect the Josephson capacitance of the Cooper-pair box. We additionally show that with a suitable choice of tuning parameters, the optomechanical coupling vanishes and the system purely exhibits a cross-Kerr type of nonlinearity between the cavity and the mechanical resonator. This allows the system to be used for phonon counting.

We show that several distinct broken-symmetry phases in a spin-$\frac{1}{2}$ antiferromagnet on a square lattice with easy-plane anisotropy, including valence bond solid, chiral spin liquid, and the XY-ordered state, can all be accessed by perturbing a multicritical point with two massless Dirac fermions coupled to a level-one Chern-Simons gauge field. This allows for a unified description of these phases, as well as the phase transitions between them. In a specific phase transition, our analysis provides a lattice realization of one of the recently proposed fermion-boson dualities, thus lending support to it. We also briefly discuss the relation between our paper and the long-sought deconfined criticality in such systems.

The state-of-the-art theoretical description of magnetic materials relies on solving effective Heisenberg spin problems or their generalizations to relativistic or multi-spin-interaction cases that explicitly assume the presence of local magnetic moments in the system. We start with a general interacting fermionic model that is often obtained in ab initio electronic structure calculations and show that the corresponding spin problem can be introduced even in the paramagnetic regime, which is characterized by a zero average value of the magnetization. Further, we derive a physical criterion for the formation of the local magnetic moment and confirm that the latter exists already at high temperatures well above the transition to the ordered magnetic state. The use of path-integral techniques allows us to disentangle spin and electronic degrees of freedom and to carefully separate rotational dynamics of the local magnetic moment from Higgs fluctuations of its absolute value. It also allows us to accurately derive the topological Berry phase and relate it to a physical bosonic variable that describes dynamics of the spin degrees of freedom. As the result, we demonstrate that the equation of motion in the case of a large magnetic moment takes a conventional Landau-Lifshitz form that explicitly accounts for the Gilbert damping due to itinerant nature of the original electronic model.

We report on the observation of the circular transversal terahertz photoconductivity in monolayer graphene supplied by a back gate. The photoconductivity response is caused by the free carrier absorption and reverses its sign upon switching the radiation helicity. The observed dc Hall effect manifests the time inversion symmetry breaking induced by circularly polarized terahertz radiation in the absence of a magnetic field. For low gate voltages, the photosignal is found to be proportional to the radiation intensity and can be ascribed to the alignment of electron momenta by the combined action of THz and static electric fields as well as by the dynamic heating and cooling of the electron gas. Strikingly, at high gate voltages, we observe that the linear-in-intensity Hall photoconductivity vanishes; the photoresponse at low intensities becomes superlinear and varies with the square of the radiation intensity. We attribute this behavior to the interplay of the second- and fourth-order effects in the radiation electric field which has not been addressed theoretically so far and requires additional studies.

Graphene-based sensors are exceptionally sensitive with high carrier mobility and low intrinsic noise, and have been intensively investigated in the past decade. The detection of individual gas molecules has been reported, albeit the underlying sensing mechanism is not yet well understood. We focus on the adsorption of ${\mathrm{NO}}_2$, ${\mathrm{H}}_2\mathrm{O}$, and ${\mathrm{NH}}_3$ on a molecular junction with a pyrene core, which can be considered as a minimal graphenelike unit. We systematically investigate the chemiresistive response within the framework of density functional theory and nonequilibrium Greens functions. We highlight the fundamental role of quantum interference (QI) in the sensing process, and we propose it as a paradigmatic mechanism for sensing. Owing to the open-shell character of ${\mathrm{NO}}_2$, its interaction with pyrene gives rise to a Fano resonance thereby triggering the strongest chemiresistive response, while the weaker interactions with ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{NH}}_3$ result in lower sensitivity. We demonstrate that by exploiting destructive QI arising in the meta-substituted pyrene, it is possible to calibrate the sensor to enhance both its sensitivity and chemical selectivity by almost two orders of magnitude so that individual molecules can be detected and distinguished. These results provide a fundamental strategy to design high-performance chemical sensors with graphene functional blocks.

Widely recognized as a thermally activated process, atomic stick-slip friction has been typically explained by Prandtl-Tomlinson model with thermal activation. Despite the limited success, theoretical predictions from the classic model are primarily based on a one-dimensional (1D) assumption, which is generally not compatible with real experiments that are two-dimensional (2D) in nature. In this letter, a theoretical model based on 2D transition state theory has been derived and confirmed to be able to capture the 2D slip kinetics in atomic-scale friction experiments on crystalline surface with a hexagonal energy landscape. Moreover, we propose a reduced scheme that enables extraction of intrinsic interfacial parameters from 2D experiments approximately using the traditional 1D model. The 2D model provides a theoretical tool for understanding the rich kinetics of atomic-scale friction or other phenomena involving higher dimensional transitions.

The discovery of the topological states has become a key topic in condensed matter physics with the focus evolving from the Dirac or Weyl points to high-dimensional topological states of the nodal lines and nodal surfaces. For a topological material to manifest its quantum properties and become useful in applications, the topological states need to be genuine and clean so that they lie close to the Fermi level without other trivial bands existing at the Fermi level. While a number of high-dimensional topological materials are predicted, only a few of them have been synthesized and confirmed and the genuine and clean ones are especially scarce. Here, we report the realization of genuine clean multiple high-dimensional topological states in NaAlSi. By performing high-resolution angle-resolved photoemission measurements and band-structure calculations, we have observed two sets of nodal surfaces and the formation of two homocentric nodal ring states in NaAlSi. The observed nodal rings are distinct in that the inner one is a type-I nodal ring while the outer one is a type-I nodal ring embedded with four type-III nodal points. All the bands involved in the nodal rings lie very close to the Fermi level with no other trivial bands coexisting at the Fermi level. These observations make NaAlSi a desirable topological material to explore for novel quantum states and exotic properties.

The optical transitions between spin-polarized bands of the quasi-two-dimensional Rashba system BiTeI are investigated using polarization-resolved resonant Raman spectroscopy. We detect chiral excitations between states with opposite helicity and compare spectra to calculations within a three-band model. Using the resonant Raman excitation profile, we deduce the Rashba parameters and band gaps of the higher conduction bands near the Fermi level, and compare the parameters to values obtained by ab initio density functional theory.

We theoretically study potential unconventional superconductivity in doped $AB$-type IV-VI semiconductors, based on a minimal effective model with interaction up to the next-nearest neighbors. According to the experimental implications, we focus on the spin-triplet channels and obtain the superconducting phase diagram with respect to the anisotropy of the Fermi surfaces and the interaction strength. Abundant nodal and nodeless states with different symmetry breaking appear in the phase diagram, and all the states are time-reversal invariant and topologically nontrivial. Specifically, the various nodal superconducting ground states, dubbed as the topological Dirac superconductors, are featured by Dirac nodes in the bulk and Majorana arcs on the surface; among the full-gap states, there exist a mirror-symmetry-protected second-order topological superconductor state favoring helical Majorana hinge cones, and different first-order topological superconductor states supporting four surface Majorana cones. The experimental verification of the different kinds of superconducting ground states is also discussed.

We present a study of the basic microscopic model of a $s$-wave superconductor with frustrated interband interaction. When frustration is strong, such an interaction gives raise to a $s+is$ state. This is a $s$-wave superconductor that spontaneously breaks time reversal symmetry. We show that in addition to the known $s+is$ state, there is additional phase where the system's bulk is a conventional $s$-wave state, but superconducting surface states break time reversal symmetry. Furthermore, we show that $s+is$ superconductors can have spontaneous boundary currents and spontaneous magnetic fields. These arise at lower-dimensional boundaries, namely, the corners in two-dimensional samples. This demonstrates that boundary currents effects in superconductors can arise in states which are not topological and not chiral according to the modern classification.