Spin valves provide a valuable platform to manipulate spin transport, leading to a change in resistivity by changing the direction of applied magnetic field. Here, we present a combined computational and experimental study of a half-metallic full Heusler alloy (FHA), ${\mathrm{Co}}_2\mathrm{NbAl}$, which exhibits spin valve like magnetoresistance (MR) from low temperatures to room temperature (RT) and beyond. ${\mathrm{Co}}_2\mathrm{NbAl}$ crystallizes in the prototype $L{2}_1$ structure with antisite disorder between Co and Al. Magnetization measurements indicate its ferromagnetic (FM) behavior with a Curie temperature $\left({\mathrm{T}}_{\mathrm{C}}\right)$ above RT, making it attractive for device applications. Longitudinal resistivity data indicate its fully half-metallic ferromagnetic (HMF) nature below 84 K and a reasonably high spin polarization at higher T. Hall measurements confirm intrinsic anomalous contribution to dominate in this system. Ab initio calculations confirm the half-metallic nature of the alloy with a total moment of $2 {\mu }_B/\text{f.u.}$ (larger than measured value of $1.56 {\mu }_B/\text{f.u.}$). Antisite disorder between Co and Al sites explains the discrepancy in the net magnetization obtained between theory and experiment. This also discloses an antiferromagnetic (AFM) correlations mediated by Co atoms distributed in an overall FM matrix. The formation of such AFM-FM interfaces often pins the nearby spins, which is possibly responsible for the spin valve effect.

We report the observation of Rashba splitting in multilayer ${\mathrm{Cs}}_2{\mathrm{PbI}}_2{\mathrm{Cl}}_2$ two-dimensional Ruddlesden-Popper phase perovskite nanocrystals (NCs) induced by chemical strain. Magnetic circular dichroism measurements reveal significant Zeeman splitting, indicating strong spin-orbit coupling. At 10 K, pronounced circular dichroism signals suggest structural asymmetry linked to the Rashba effect. Photoluminescence (PL) peak splitting at low temperatures, supported by polarization-dependent PL measurements showing emission anisotropy below 70 K, confirms the presence of spin selectivity. High-resolution synchrotron x-ray diffraction and temperature-dependent Raman data reveal a transition in unit cell parameters and phonon frequencies around 70 K, respectively, correlating with optical data. The strain induced local asymmetry facilitates these effects. Density functional theory calculations validate the experimental findings, showing clear spin splitting in the valence and conduction bands. This study investigates the influence of chemical strain on asymmetry-induced phenomena, such as the Rashba effect in ${\mathrm{Cs}}_2{\mathrm{PbI}}_2{\mathrm{Cl}}_2$ NCs, highlighting their potential as a promising platform for advanced technologies.

The effects of short-range fermion-fermion interactions on the low-energy properties of rhombohedral trilayer graphene are comprehensively investigated using the momentum-shell renormalization group method. We take into account all one-loop corrections and establish the energy-dependent coupled evolutions of independent fermionic couplings that carry the physical information stemming from the interplay of various fermion-fermion interactions. With detailed numerical analysis, we observe that the ferocious competition among all fermion-fermion interactions drives fermionic couplings to four distinct fixed points, dubbed ${\text{FP}}_1, {\text{FP}}_2, {\text{FP}}_3$, and ${\text{FP}}_4$, in the interaction-parameter space. These fixed points primarily dictate the fate of the system in the low-energy regime and are always associated with some instabilities characterized by specific symmetry breakings, leading to certain phase transitions. To determine the favorable states arising from the potential phase transitions, we introduce a number of fermion-bilinear source terms to characterize the underlying candidate states. By comparing their related susceptibilities, we find that the dominant states correspond to spin-singlet superconductivity, spin-triplet pair-density-waves, and spin-triplet superconductivity for fixed points ${\text{FP}}_{1,3}, {\text{FP}}_2$, and ${\text{FP}}_4$, respectively. These results provide valuable insights into the low-energy properties of rhombohedral trilayer graphene and analogous materials.

We compute the energy per particle of normal liquid ${}^3\mathrm{He}$ in the temperature range $0.15–2$ K using path-integral Monte Carlo simulations, leveraging a recently proposed method to overcome the sign problem—a long-standing challenge in many-body fermionic simulations. This approach is based on introducing a parameter $\xi$ into the partition function, which allows a generalization from bosons ($\xi =1$) to fermions ($\xi =-1$). By simulating systems with $\xi \ge 0$, where the sign problem is absent, one can then extrapolate to the fermionic case at $\xi =-1$. Guided by an independent-particle model that uncovers nonanalytic behavior due to the superfluid transition, which is moderated by finite-size effects, we develop a tailored extrapolation strategy for liquid ${}^3\mathrm{He}$ that departs from the extrapolation schemes shown to be accurate in those cases where quantum degeneracy effects are weak, and enables accurate results in the presence of Bose-Einstein condensation and superfluidity for $\xi >0$. Our approach extends the previously proposed framework and yields energy per particle values in good agreement with experimental data.

$4H_{\mathrm{a}}\text{−}\mathrm{Nb}{\mathrm{Se}}_2$ single crystals were grown by the chemical vapor transport method followed by a quench to room temperature. The temperature-dependent magnetization and in-plane resistance reveal that the sample exhibits both charge-density-wave and superconducting orders, condensing at $T_{\mathrm{CDW}}\sim 43$ K and $T_{\mathrm{c}}\sim 6.5$ K, respectively. The superconductivity was studied by specific heat and lower critical field ($H_{\mathrm{c}1}$) down to 0.5 and 0.4 K, respectively. The electronic specific heat at superconducting state could be well described by an isotropic two-gap BCS model. The superconducting gaps were obtained to be of ${\mathrm{\Delta }}_{\mathrm{L}}=1.1$ meV and ${\mathrm{\Delta }}_{\mathrm{S}}=0.3$ meV with the weighting factors as 0.938 and 0.062, respectively. The in-plane $H_{\text{c}1}^{//ab}\left(T\right)$ and out-of-plane $H_{\text{c}1}^{//c}\left(T\right)$ also support the two-gap $s+s$–wave scenario, which derives $H_{\text{c}1}^{//ab}\left(0\right)$ and $H_{\text{c}1}^{//c}\left(0\right)$ as 78.3 and 149.8 Oe, respectively. The anisotropic parameter at 0 K based on $H_{\mathrm{c}1} \left[{\mathrm{\Gamma }}_{H_{\mathrm{c}1}}\left(0\right)\right]$ was deduced to be ∼1.9. Our results reveal nodeless two-gap pairing symmetry in $4H_{\mathrm{a}}\text{−}\mathrm{Nb}{\mathrm{Se}}_2$, which would gain more insight into the superconductivity of TMDs.

Using a distribution of the Meissner currents over the surface of an infinitely long superconducting slab with a rectangular cross section, the magnetic moment of the slab is calculated, taking into account corrections associated with a small but finite value of the London penetration depth $\lambda$. Since these corrections determine the shift of the resonant frequency in the tunnel-diode oscillator technique, formulas for determining $\lambda$ within this technique are derived for the slab. These formulas are valid for any aspect ratio of its cross section, and they differ from those that are often used in analyzing experimental data. Specifically, it is shown that the sharp edges of the slab can cause a large frequency shift proportional to the change in the value of ${\lambda }^{2/3}$. Although this result complicates the extraction of a temperature dependence of $\lambda$ from the frequency shift, it also opens up additional possibilities for determining the London penetration depth. In particular, under certain conditions, it is possible not only to measure the changes in $\lambda$ with temperature, but also to estimate its absolute value.

We report temperature-dependent Hall effect and low-temperature de Haas–van Alphen (dHvA) effect measurements of the antiferromagnetic heavy-fermion compound ${\mathrm{U}}_2{\mathrm{RhIn}}_8$. Temperature dependence of the Hall resistivity suggests a considerable reduction of the carrier density in the antiferromagnetic phase. The observed angular dependence of the dHvA frequencies suggests the existence of three almost ideally two-dimensional Fermi surfaces one of which is quite large. The measured effective masses range from $2m_0$ to $14m_0$ for the field applied along the $c$ axis. Local density approximation band-structure calculations performed for the paramagnetic ground state reveal more three-dimensional Fermi surfaces than those observed in the experiment. On the other hand, Fermi surfaces obtained for the antiferromagnetic ground state by band folding are more two dimensional. These calculations account reasonably well for the experimental results assuming a slight modification of the calculated Fermi surfaces.

The identification, description, and classification of topological features is an engine of discovery and innovation in several fields of physics. This research encompasses a broad variety of systems, from the integer and fractional Chern insulators in condensed matter, to protected states in complex photonic lattices in optics, and the structure of the QCD vacuum. Here, we introduce another playground for topology: the dissipative dynamics of pseudo-Hermitian many-body quantum systems. For that purpose, we study two different systems, the dissipative Sachdev-Ye-Kitaev (SYK) model, and a quantum chaotic dephasing spin chain. For the two different many-body models, we find the same topological features for a wide range of parameters suggesting that they are universal. In the SYK model, we identify four universality classes, related to pseudo-Hermiticity, characterized by a rectangular block representation of the vectorized Liouvillian that is directly related to the existence of an anomalous trace of the unitary operator implementing fermionic exchange. As a consequence of this rectangularization, we identify a topological index $\nu$ that only depends on symmetry. Another distinct consequence of the rectangularization is the observation, for any coupling to the bath, of purely real topological modes in the Liouvillian. The level statistics of these real modes agree with that of the corresponding random matrix ensemble and therefore can be employed to characterize the four topological symmetry classes. In the limit of weak coupling to the bath, topological modes govern the approach to equilibrium, which may enable a direct path for experimental confirmation of topology in dissipative many-body quantum chaotic systems.

Collective excitations of many-body electron systems can carry internal structure, tied to the quantum geometry of the Hilbert space in which they are embedded. This has been shown explicitly for particle-hole-like excitations, which carry a “quantum geometric dipole” (QGD) that is essentially an electric dipole moment associated with the state. We demonstrate in this work that this property can be formulated in a generic way, which does not require wave functions expressed in terms of single particle-hole states. Our formulation exploits the density matrix associated with a branch of excitations that evolves continuously with its momentum $\mathbf{K}$, from which one may extract single-particle states allowing a construction of the QGD. We demonstrate the formulation using the single-mode approximation for excited states of two quantum Hall systems: the first for an integrally filled Landau level, and the second for a fractional quantum Hall state at filling factor $\nu =1/m$, with $m$ an odd integer. In both cases we obtain the same result for the QGD, which can be attributed to the translational invariance assumed of the system. Our study demonstrates that the QGD is an intrinsic property of collective modes which is valid beyond approximations one might make for their wave functions.

Traditional outgassing models (recombination limited and diffusion limited) depend on hydrogen diffusivity within the bulk and its concentration on the surface. These macroscopic models are incapable of explaining the experimentally observed hydrogen outgassing processes. To study the hydrogen outgassing mechanism, first-principles electronic-structure calculations and molecular dynamics simulations were performed on hydrogen diffusing through FCC Al and Cu (110) surfaces. Orbital hybridization between surface-atom orbitals and hydrogen $s$-orbitals resulted in the formation of stable and sometimes also metastable hydrogen adsorption sites on the metal surfaces. For a systematic and detailed analysis, the outgassing process was divided into four separate stages: (1) transit through bulk, (2) surfacing, (3) diffusion and recombination, and (4) what we call bubble-up recombination and outgassing. For each stage, pertinent properties (diffusion and energetics) were calculated and exhibited excellent agreement with available experimental measurements. These atomistic methods revealed the significant and direct impact of the hydrogen potential energy surface on hydrogen surface mobility and recombination (molecular formation); in particular, stages 2, 3, and 4 can only be understood by analyzing the potential energy surface. The atomistic approach used in this paper can be generalized to any other H outgassing system.

We present a theoretical investigation of the magnetic properties exhibited by twisted bilayer graphene (TBG) systems with small twist angles, where the appearance of flat minibands strongly enhances electron-electron interaction effects. We show that, at three-quarters filling of the conduction miniband, the Stoner mechanism induces a ferromagnetic polarization in the AA-stacking regions, which aligns with recent experimental observations. Our approach models the electronic properties by a tight-binding Hamiltonian combined with a Hubbard mean-field interaction term. We employ a real-space recursion technique to self-consistently calculate the system's local density of states and use our method to investigate the magnetic properties of small-angle TBG at three-quarters filling. The recursion method's $O\left(\mathcal{N}\right)$ efficiency makes it possible to address extremely large superlattices through a full real-space approach. We validate our procedure by comparing it with mean-field momentum-space calculations from the literature, which identify a magnetic phase in charge-neutral TBG.

Bipolar conduction, which is widely observed in various materials, plays a deleterious role in the thermoelectric properties. Traditionally, the single-band model is often applied to understand the transport properties of thermoelectric materials. However, it ignores the contribution from minority carriers and is incapable of quantifying the bipolar conduction. Herein, a general and feasible approach for calculating all the temperature-dependent transport properties based on the two-band model has been developed. In addition to the reduced electron Fermi energy and reduced band-gap energy, band structure asymmetry has been identified to play a significant role in bipolar conduction. Importantly, the effect of band structure asymmetry on bipolar conduction has been highlighted by using the $\mathrm{M}{\mathrm{g}}_3\mathrm{B}{\mathrm{i}}_{2-x}\mathrm{S}{\mathrm{b}}_x$ alloy as a typical example. Our results demonstrate that the band structure asymmetry is of great significance to the materials' thermoelectric performance.

The confinement of a standing wave between perfectly reflecting mirrors represents one of the archetypal mechanisms for the formation of bound states in the continuum (BICs), related to the corresponding Fabry-Pérot (FP) resonance. In the present paper, we study theoretically FP BICs in two-dimensional waveguides with two and three resonators playing the role of the mirrors. We provide an analytical description and numerical evidence for the formation of FP BICs in the waveguide with three mirrors, which do not exhibit Fano resonances in the transmission spectrum when considered separately. Conversely, FP BICs in the waveguide with two mirrors appear only in conjunction with the Fano resonance in the single mirror, as expected. We show that such a counterintuitive phenomenon, which can be related to the manifestation of a virtual (at complex energy) Fano resonance, is quite general and can be observed in both quantum-mechanical and optical waveguides.

Our theoretical study unfolds the topological phase transitions (within bands of the Moiré super-lattice) in small angle twisted double bilayer graphene (tDBLG) under the influence of external gate voltage and intrinsic spin-orbit coupling (SOC) for both AB-AB and AB-BA stacking configurations. Utilizing a low-energy continuum model, we investigate the band structure and perform a comprehensive topological characterization of the system by analyzing the direct band gap closing as well as various Chern numbers. In the absence of SOC, the tDBLG exhibits characteristics of a valley Hall insulator. However, in the presence of SOC, we observe a transition to a quantum spin Hall insulator state, and band topology emerges in the parameter spaces of nontopological regime without SOC. Furthermore, we conduct a comparative analysis between untwisted double bilayer graphene and tDBLG to assess the impact of twisting on the system's properties. Our findings reveal the construction of topological phase diagrams that showcase distinct phases arising from changes in the twist angle compared to the untwisted case. These phase diagrams provide valuable insights into the diverse topological phases achievable in tDBLG with SOC. Our findings contribute to the understanding of the interplay between small twist angles, SOC, and external electric field on the topological band properties of twisted multilayer graphene systems.

Hyperbolic materials are natural or engineered artificial structures that provide means to manipulate and control electromagnetic radiation, leading to a variety of strong light-matter interactions at the nanoscale. In this work, we explore the physical properties of the optical response of two-dimensional (2D) semiconductor-based superlattices, which are engineered with atomic precision and composed of alternating 2D semiconductor monolayers and hexagonal-boron-nitride. We find that such superlattices exhibit a robust hyperbolic response at the visible to near-infrared spectrum, and with dimensions that are an order of magnitude smaller compared to conventional metal-based hyperbolic metamaterials, down to three active monolayers. By employing both analytical and numerical studies, we show that by varying the superlattice configuration we can control and manipulate the nature of the hyperbolic response. Finaly, we propose an optimal structure that will enable the experimental observation of this hyperbolic response in 2D semiconductors superlattices. Such highly compact hyperbolic materials of atomic precision could open the way for deep-subwavelength optoelectronic devices with an extremely small footprint.

We study a one-dimensional antiferromagnetic-elastic model with magnetic ions having spin $S=3/2$. By extensive density matrix renormalization group (DMRG) computations and complementary analytical methods, we uncover a first-order transition from a homogeneous or weakly dimerized phase (a situation that could be similar to the well-known $S=1/2$ spin-Peierls effect) to a highly distorted phase, driven by the spin-phonon coupling $\lambda$. The striking characteristic of the second phase, present at large $\lambda$, is the appearance of weakly ferromagnetic (FM) couplings alternating with strong antiferromagnetic (AFM) ones (we dub it the FM-AFM phase) with a ground state close to a direct-product state of singlet dimers sitting on the AFM bonds. The behavior of the spin gap in both phases is studied by DMRG computation and contrasted with bosonization predictions and perturbation theory around the direct product of dimers. In the FM-AFM phase, robust magnetization plateaus and metamagnetic jumps appear under magnetic fields. The novel phase could be realized in 5d oxides of current interest, with giant spin-phonon coupling. Potential applications of the transition would be associated with the possibility of tuning the transition by external parameters such as striction, magnetic or electric fields, or alloying.

We report on the spin dynamics of two terbium-based molecular nanomagnets, namely $\mathrm{Tb}\left(\mathrm{DTBSQ}\right){\left({\mathrm{HBPz}}_3\right)}_2$ (in short Tb-SQ) and $\mathrm{Tb}\left(\mathrm{Trp}\right){\left({\mathrm{HBPz}}_3\right)}_2$ (in short Tb-Trp), investigated by means of longitudinal muon spin relaxation ($\mu \mathrm{SR}$) measurements as a function of applied field, flanked by ac susceptibility characterization. In the two molecules the terbium(III) ion has an isostructural coordination sphere, but in the former the terbium(III) is coordinated by an organic paramagnetic ligand (3,5 ditertbutylsemiquinonate, SQ), while in the latter is coordinated by a diamagnetic one (tropolonate, Trp). Thus Tb-SQ presents an exchange interaction between the terbium(III) ion and a radical while Tb-Trp does not. Both samples exhibit a muon spin-lattice relaxation rate ${\lambda }_1(T,B_L)$ peak in the temperature range 10–25 K at all applied longitudinal magnetic fields $B_L=50,150,300$ milli-Tesla (mT). In Tb-SQ, ${\lambda }_1(T,B_L)$ displays a BPP-like behavior led by three different correlations times: the first, dominating for $T\ge 15\text{K}$, follows a thermally activated law ${\tau }_c={\tau }_{0}exp({\sigma }_A/k_{B}T)$ with energy barrier ${\sigma }_A/k_B$, while the second and third ones, dominating, respectively, for $8<T<15\mathrm{K}$ and $T<8\mathrm{K}$, follow a power-law-like behavior ${\tau }_c=c_0{T}^{-\alpha }$ with two different values of $c_0$ and $\alpha$. On the other hand, the temperature and field behavior of ${\lambda }_1(T,B_L)$ in Tb-Trp strongly deviates from a BPP law, displaying a strongly anomalous character. Our results indicate that, in the absence of an exchange interaction and maintaining all the other relevant interactions constant, the local spin dynamics of single-ion magnets strongly differ from the one observed in the presence of such interaction. The combination of $\mu \mathrm{SR}$ and ac susceptibility measurements allows us to disentangle the different Orbach, Raman, and direct mechanisms, which are the key ingredients that control the spin dynamics in Tb-SQ, and evidence the potentiality of $\mu \mathrm{SR}$ in elucidating complex spin dynamics.

Current-induced spin torque is widely regarded as a popular and effective method for driving magnetic textures in spintronics. However, attaining fast dynamics requires a large current density, which inevitably results in significant Joule heating. Here, the voltage-controlled magnetic anisotropy gradient, an alternative scheme that accommodates ultrafast dynamics and low-Joule heating, is proposed to excite the magnetization dynamics. We analytically and numerically study the dynamics of an antiferromagnetic skyrmionium driven by the magnetic anisotropy gradients. The numerical simulations demonstrate that antiferromagnetic skyrmioniums can be effectively driven by voltage-controlled magnetic anisotropy gradients, with one order speed-up compared to their ferromagnetic counterparts. In addition, the isotropic deformations of antiferromagnetic skyrmioniums are also identified and investigated, which are distinct from their ferromagnetic counterparts. Furthermore, a structure is designed to achieve the transition from antiferromagnetic skyrmionium to skyrmion. Our results deepen the understanding of antiferromagnetic skyrmionium dynamics and provide ideas for building future antiferromagnetic skyrmionium-based spintronic devices.

Orthorhombic pseudobrookites ${\mathrm{CoTi}}_2{\mathrm{O}}_5$ and ${\mathrm{FeTi}}_2{\mathrm{O}}_5$ have a low-symmetry crystal structure comprising magnetic ${\mathrm{Co}}^{2+}/{\mathrm{Fe}}^{2+}$ ions and nonmagnetic ${\mathrm{Ti}}^{4+}$ ions, where the orbital-nondegenerate ${\mathrm{Co}}^{2+}/{\mathrm{Fe}}^{2+}$ ions form one-dimensional chains running along the orthorhombic $a$ axis. These compounds undergo an antiferromagnetic phase transition at $T_N\sim$ 26 K for ${\mathrm{CoTi}}_2{\mathrm{O}}_5$ and $T_N\sim$ 40 K for ${\mathrm{FeTi}}_2{\mathrm{O}}_5$. Ultrasound velocity measurements on single crystals of ${\mathrm{CoTi}}_2{\mathrm{O}}_5$ and ${\mathrm{FeTi}}_2{\mathrm{O}}_5$ reveal that ${\mathrm{CoTi}}_2{\mathrm{O}}_5$ exhibits unusual elastic softness above $T_N$ in the symmetry-lowering elastic mode of $ac$-plane shear elastic modulus, inconsistent with the structural symmetry breaking caused by antiferromagnetic ordering at $T_N$. This suggests the presence of two distinct types of magnetostructural fluctuations above $T_N$ that should be a precursor to the symmetry-lowering lattice distortion at $T_N$. In contrast, ${\mathrm{FeTi}}_2{\mathrm{O}}_5$ exhibits either negligible or smaller elastic softness, indicating weaker spin-lattice coupling. These findings highlight ${\mathrm{CoTi}}_2{\mathrm{O}}_5$ and ${\mathrm{FeTi}}_2{\mathrm{O}}_5$ as unique spin-latticed-coupled frustrated systems with low crystal symmetry, where, while the exchange interactions are quasi-one-dimensional in nature, the frustration is released by further lowering the crystal symmetry through three-dimensional spin-lattice coupling, which is stronger in ${\mathrm{CoTi}}_2{\mathrm{O}}_5$ than in ${\mathrm{FeTi}}_2{\mathrm{O}}_5$.