When the Fermi Hubbard model was first introduced 60 years ago, one of the original motivations was to understand the correlation effects in itinerant ferromagnetism. In the past two decades, ultracold Fermi gas in an optical lattice has been used to study the Fermi Hubbard model. However, the metallic ferromagnetic correlation was observed only in a recent experiment using frustrated lattices, and its underlying mechanism is not yet clear. In this paper, we point out that, under the particle-hole transformation, the single-particle ground state can exhibit double degeneracy in such a frustrated lattice. Therefore, the low-energy state exhibits valley degeneracy, reminiscent of multiorbit physics in ferromagnetic transition metals. The local repulsive interaction leads to the valley Hund's rule, responsible for the observed ferromagnetism. We generalize this mechanism to distorted honeycomb lattices and square lattices with flux. This mechanism was first discussed by Müller-Hartmann in a simpler one-dimensional model. However, this mechanism has not been widely discussed and still needs to be related to experimental observations. Hence, our study not only explains the experimental findings but also enriches our understanding of itinerant ferromagnetism.

We explore the stability of certain many-body quantum states which may exist at zero or finite temperatures, may lack long-range order and even topological order, and still are thermodynamically distinct from uncorrelated disordered phases. We sharply characterize such states by the conservation of topological charge, or equivalently confinement of instantons, using a generalization of the Wilson loop and the correlation length of an emergent gauge field. Our main conclusions are (i) topological orders can exist at finite temperatures, (ii) relativistic liquids of topological defects can also exist as stable phases at finite temperatures, and (iii) there are two universality classes of instanton suppression. We also relate the instanton dynamics to the problem of the pseudogap state in underdoped cuprates. A universal experimental signature of the instanton deconfinement transition is a change of the quantum noise spectrum, which can perhaps be measured in some situations, for example, via a quantum anomaly, or indirectly detected with a specific heat jump. The method of analysis is a functional renormalization group that generalizes the Coulomb gas treatment of Kosterlitz and Thouless to arbitrary interactions and dimensions. In particular, we construct an exact nonperturbative technique for confining interactions between instantons that introduce irreparable infrared divergences in the standard perturbative approaches.

Lead phthalocyanine (PbPc), a nonplanar molecule, is studied on Pb(100) by scanning tunneling spectroscopy. A rigid shift of the molecular orbitals is found between molecules with the central Pb ion pointing toward $(\mathrm{PbPc}\downarrow )$ or away from $(\mathrm{PbPc}\uparrow )$ the substrate and is understood from the interaction between the molecules and their image charges. Within the superconducting energy gap, Yu-Shiba-Rusinov (YSR) resonances are observed for $\mathrm{PbPc}\uparrow$ molecules in islands, indicating the presence of a magnetic moment. Such bound states are not present on $\mathrm{PbPc}\downarrow$ molecules, nor on isolated $\mathrm{PbPc}\uparrow$ or molecules that have lost the Pb ion during deposition $\left({\mathrm{H}}_0\mathrm{Pc}\right)$. The YSR energies vary depending on the orientation and type of the molecular neighbors. We analyze the role of the out-of-plane electrostatic dipole moment of PbPc.

We study the effect of a Rashba spin-orbit coupling on the nodal superconducting phase of an Ising superconductor. Such nodal phase was predicted to occur when applying an in-plane field beyond the Pauli limit to a superconducting monolayer transition metal dichalcogenides (TMD). Generically, Rashba spin orbit is known to lift the chiral symmetry that protects the nodal points, resulting in a fully gapped phase. However, when the magnetic field is applied along the $x$ direction, a residual vertical mirror symmetry protects a nodal crystalline phase. We study a single-band tight-binding model that captures the low-energy physics around the $\mathrm{\Gamma }$ pocket of monolayer TMD. We calculate the topological properties, the edge state structure, and the current phase relation in a Josephson junction geometry of the nodal crystalline phase. We show that while the nodal crystalline phase is characterized by localized edge modes on non-self-reflecting boundaries, the current phase relation exhibits a trivial $2\pi$ periodicity in the presence of Rashba spin-orbit coupling.

Antiferromagnets (AFMs) have great benefits for spintronic applications such as high frequencies (up to THz), high speeds (up to tens of km/s) of magnetic excitations, and field-free operation. Advanced devices will require high-speed propagating spin waves (SWs) as signal carriers, i.e., SWs with high $k$ vectors, the excitation of which remains challenging. We show that a domain wall (DW) in anisotropic AFM driven by the spin current can be a source of such propagating SWs with high frequencies and group velocities. In the proposed generator, the spin current, with polarization directed along the easy anisotropy axis, excites the precession of the Néel vector within the DW. The threshold current is defined by the value of the anisotropy in the hard plane, and the frequency of the DW precession is tunable by the strength of the spin current. We show that the above precession of spins inside the DW leads to robust emission of high-frequency propagating SWs into the AFM strip with very short wavelengths comparable to the exchange length, which is hard to achieve by any other method.

Pyrochlore materials are characterized by their hallmark network of corner-sharing rare-earth tetrahedra, which can produce a wide array of complex magnetic ground states. Ferromagnetic Ising pyrochlores often obey the “two-in-two-out” spin ice rules, which can lead to a highly degenerate spin structure. Large moment systems, such as ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, tend to host a classical spin ice state with low-temperature spin freezing and emergent magnetic monopoles. Systems with smaller effective moments, such as ${\mathrm{Pr}}^{3+}$-based pyrochlores, have been proposed as excellent candidates for hosting a “quantum spin ice” characterized by entanglement and a slew of exotic quasiparticle excitations. However, experimental evidence for a quantum spin ice state has remained elusive. Here, we show that the low-temperature magnetic properties of ${\mathrm{Pr}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$ satisfy several important criteria for continued consideration as a quantum spin ice. We find that ${\mathrm{Pr}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$ exhibits two distinct spin-correlation time scales of $\tau \ge {10}^{-4}$ and $\sim {10}^{-10}$ s in the spin ice regime. Our comprehensive bulk characterization and neutron scattering measurements enable us to map out the magnetic field-temperature phase diagram, producing results consistent with expectations for a ferromagnetic Ising pyrochlore. We identify key hallmarks of spin ice physics and show that the application of small magnetic fields (${\mu }_0{H}_c\sim 0.5$ T) suppresses the spin ice state and induces a field-polarized, ordered spin-ice phase. Together, our work clarifies the current state of ${\mathrm{Pr}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$ and encourages future studies aimed at exploring the potential for a quantum spin ice ground state in this system.

Spin Hall nano-oscillators (SHNOs) have shown applications in unconventional computing schemes and broadband frequency generation in the presence of applied external magnetic field. However, under external magnetic field free conditions, the oscillation characteristics of nanoconstriction-based SHNOs exhibits a notable reliance on the effective field, which can be adjusted by varying the constriction width. In this study, we explore how the width of the nanoconstriction affects the magnetization dynamics in field free SHNOs assisted by anisotropy. In uniaxial anisotropy-based field free SHNOs, either the anisotropy field $\left(B_{\mathrm{anis}}\right)$ or the demagnetization field $\left(B_{\mathrm{demag}}\right)$ dominates the magnetization dynamics depending on the constriction width. Our findings reveal distinct auto-oscillation characteristics in narrower constrictions with 20- and 30-nm constriction width compared to their wider counterpart with 100-nm width. The observed frequency shift variations with input current ($I_{\mathrm{dc}}$) and constriction widths stem from the inherent nonlinearity of the system. The interplay between the $B_{\mathrm{demag}}$ and $B_{\mathrm{anis}}$, controlled by constriction width, yields rich dynamics, and offers control over frequency and amplitude of auto-oscillations, and the threshold current needed for auto-oscillations. Notably, the spatial configuration of spin-wave wells within the constriction undergoes transformations in response to changes in both constriction width and anisotropy. The findings highlight the significant influence of competing internal fields at the constriction on the field free auto-oscillations of SHNOs, with the impact intensifying as the constriction width is varied.

We present a study on the structural and magnetic properties of thin films of the high entropy perovskite $\mathrm{Dy}\left({\mathrm{Fe}}_{0.2}{\mathrm{Mn}}_{0.2}{\mathrm{Co}}_{0.2}{\mathrm{Cr}}_{0.2}{\mathrm{Ni}}_{0.2}\right){\mathrm{O}}_3$ (Dy5BO). An element-sensitive investigation was performed using synchrotron-based x-ray absorption spectroscopy, employing x-ray magnetic circular and linear dichroism. The results reveal that the moments residing on the $3d$ transition metal ions and the rare-earth $\mathrm{Dy}$ ions produce a saturation magnetization one order of magnitude larger than any previously studied high-entropy oxide perovskite. Employing temperature-dependent x-ray magnetic linear dichroism, we see clear features of a spin reorientation at the $\mathrm{Mn}$ and $\mathrm{Fe}$ sites, occurring around 18 K, likely driven by the interaction of the corresponding transition metal ions and $\mathrm{Dy}$.

We demonstrate that the well-known $(k\uparrow ,-k\downarrow )$ Bardeen-Cooper-Schrieffer interaction, when considered in real space, is equivalent to an infinite-range Penson-Kolb pairing mechanism coexisting with an attractive Hubbard term. Driven by this discovery and aiming at exploring the conduction properties, we investigate the dynamics of fermionic particles confined in a ring-shaped lattice. We assume that fermions are simultaneously influenced by the pairing interaction and by an Aharonov-Bohm electromagnetic phase, which is incorporated into the model in a highly nontrivial manner. Remarkably, the aforementioned model shows Richardson integrability for both integer and half-integer values of the applied magnetic flux $\mathrm{\Phi }/{\mathrm{\Phi }}_0$, thus permitting the exact solution of a genuine many-body problem. We discuss the ground-state properties of both two-particle and many-particle systems, drawing comparisons with results from the attractive Hubbard model. Our approach combines exact diagonalization, density matrix renormalization group techniques, and numerical solution of the Richardson equations. This comprehensive analysis allows us to study various key metrics, including the system's conductivity as a function of the interaction strength. In this way, the BCS-BEC transition is investigated in a continuous manner, thus permitting to shed light on fundamental aspects of superconducting pairing. Our findings can be experimentally tested in a condensed matter context or, with greater level of control, using atomtronics platforms.

Transition metal disulfides $4H_b$-Ta(S, ${\mathrm{Se})}_2$ with natural heterostructure of $1T$- and $1H$-Ta(S, ${\mathrm{Se})}_2$ layers have became the focus of correlated materials their unique combinations of Mott physics and possible topological superconductivity. In this work, we study the upper critical fields ${\mu }_0{H}_{c2}$ of $4H_b\text{−}{\mathrm{TaS}}_2$ and $4H_b\text{−}{\mathrm{TaS}}_{1.99}{\mathrm{Se}}_{0.01}$ single crystals systematically. Transport measurements up to 35 T show that both of $ab$-plane and $c$-axis upper critical fields (${\mu }_0{H}_{c2,ab}$ and ${\mu }_0{H}_{c2,c}$) for $4H_b\text{−}{\mathrm{TaS}}_2$ and $4H_b\text{−}{\mathrm{TaS}}_{1.99}{\mathrm{Se}}_{0.01}$ exhibit a linear temperature dependent behavior down to 0.3 K, suggesting the three-dimensional superconductivity with dominant orbital depairing mechanism in bulk $4H_b$-Ta(S, ${\mathrm{Se})}_2$. However, the zero-temperature ${\mu }_0{H}_{c2,ab}$(0) for both crystals are far beyond the Pauli paramagnetic limit ${\mu }_{0}H{}_{\mathrm{P}}$. It could be explained by the effects of spin-momentum locking in $1H$-Ta(S, ${\mathrm{Se})}_2$ layers with local inversion symmetry broken and the relatively weak intersublattice interaction between $1H$ layers due to the existence of $1T$ layers.

In cosmology, the axion is a hypothetical particle that is currently considered as candidate for dark matter. In condensed matter, a counterpart of the axion (the “axion quasiparticle”) has been predicted to emerge in magnetoelectric insulators with fluctuating magnetic order and in charge-ordered Weyl semimetals. To date, both the cosmological and condensed-matter axions remain experimentally elusive or unconfirmed. Here, we show theoretically that ordinary lattice vibrations can form an axion quasiparticle in Dirac insulators with broken time- and space-inversion symmetries, even in the absence of magnetic fluctuations. The physical manifestation of the phononic axion is a magnetic-field-induced phonon effective charge, which can be probed in optical spectroscopy. By replacing magnetic fluctuations with lattice vibrations, our theory widens the scope for the observability of the axion quasiparticle in condensed matter.

We investigate the impact of crystallinity on the generation of orbital torques in Ni, which is predicted to exhibit the strongest orbital response among conventional $3d$ ferromagnetic metals. We show that the current-induced torques in Hf/Ni bilayers are primarily dominated by the orbital torque, arising from the orbital Hall effect in the Hf layer. We find that the orbital torque efficiency is enhanced by a factor of 2 when the stacking order of the Hf/Ni bilayer is altered. Through the examination of bulk and interfacial structural properties of the Ni and Hf layers, and by quantifying the torque efficiency in a symmetric Hf/Ni/Hf trilayer, we show that the orbital torque efficiency is strongly dependent on the crystallinity of the Ni layer. This dependence is in stark contrast to conventional spin-orbit torques, which arise from the spin Hall effect and are typically insensitive to the crystallinity of the ferromagnetic layer. These findings highlight the significant role of the crystalline structure of the ferromagnetic layer in its orbital response and illustrate the potential of crystal structure engineering in optimizing orbital torques.

We show that the momentum-dependent scaling exponents of the holographic fermion self-energy of the conformal-to-${\mathrm{AdS}}_2$ Gubser-Rocha model can describe new findings from angle-resolved photoemission spectroscopy experiments on a single-layer (Pb,Bi)${}_2{\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CuO}}_{6+\delta }$ copper oxide. In particular, it was recently observed in high-precision measurements on constant energy cuts along the nodal direction that the spectral function departs from the Lorentzian line shape that is expected from the power-law-liquid model of a nodal self-energy, with an imaginary part featureless in momentum as ${\mathrm{\Sigma }}_{\text{PLL}}^{\prime \prime }\left(\omega \right)\propto {\left({\omega }^2\right)}^{\alpha }$. By direct comparison with experimental results, we provide evidence that this departure from either a Fermi liquid or the power-law liquid, resulting in an asymmetry of the spectral function as a function of momentum around the central peak, is captured at low temperature and all dopings by a semiholographic model that predicts a momentum-dependent scaling exponent in the electron self-energy as $\mathrm{\Sigma }(\omega ,k)\propto \omega {(-{\omega }^2)}^{\alpha (1-(k-k_F)/k_F)-1/2}$, with $\hslash k_F$ the Fermi momentum.

Te is a naturally $p$-doped semiconductor with a chiral structure, where an electrical current causes the conduction electrons to become spin-polarized parallel to the transport direction. In this paper, we present a comprehensive theoretical study of this effect, named current-induced spin polarization (CISP), by employing density functional theory (DFT) combined with the nonequilibrium Green's function (NEGF) technique for quantum transport. CISP can be quantitatively described in terms of the nonequilibrium spin density, which we show to be localized around the atoms. We then compute the atomic magnetic moments as a function of doping and electronic temperature, obtaining results overall consistent with those of previous theoretical studies. Beyond that, our DFT + NEGF calculations show that CISP also leads to a spin current, which is found to be quite a large fraction of the charge current, indicating that Te can be an efficient material for spin transport. We further predict that the resistance along a ballistic Te wire changes when an external magnetic field is applied parallel or antiparallel to the direction of the charge current. The computed magnetoresistance ratio is, however, quite small ($\sim 0.025\%$). Finally, we conclude by arguing that CISP, as treated within the DFT + NEGF framework, coincides with the phenomenon called chiral-induced spin selectivity, recently reported in several nanojunctions.

Weyl semimetal, which does not require any symmetry except translation for protection, is a robust gapless state of quantum matter in three dimensions. When translation symmetry is preserved, the only way to destroy a Weyl semimetal state is to bring two Weyl nodes of opposite chirality close to each other to annihilate pairwise. An external magnetic field can destroy a pair of Weyl nodes (which are separated by a momentum space distance $2k_0$) of opposite chirality, when the magnetic length $l_B$ becomes close to or smaller than the inverse separation $1/2k_0$. In this work, we investigate pairwise annihilation of Weyl nodes induced by an external magnetic field which ranges from a small to a very large value in the Hofstadter regime. We show that this pairwise annihilation in a Weyl semimetal featuring two Weyl nodes leads to the emergence of either a normal insulator or a layered Chern insulator. In the case of a Weyl semimetal with multiple Weyl nodes, the potential for generating a variety of states through external magnetic fields emerges. Our study introduces a straightforward and intuitive representation of the pairwise annihilation process induced by magnetic fields, enabling accurate predictions of the phases that may appear after pairwise annihilation of Weyl nodes.

Recently, symmetry-broken polaritons within low-symmetry crystals have triggered extensive research interest since they present enhanced directionality of polariton propagation for nanoscale manipulation and steering of photons. The latest discovery of hyperbolic shear polaritons (HShPs) in low-symmetry Bravais crystals provides great promise for innovating valleytronics. Herein, we theoretically demonstrate the coherent manipulation of the valley degree of freedom in a two-dimensional valleytronic material interfaced with monoclinic $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ and $\mathrm{CdW}{\mathrm{O}}_4$ crystals. Robust and wideband tunable valley interference values are achieved in the mid- to far-infrared wavelengths. By virtue of stronger shear effect in monoclinic $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$, the valley quantum interference fringes modulated by $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ crystal are more than those tuned via $\mathrm{CdW}{\mathrm{O}}_4$ crystal. After the monoclinic $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ crystal is doped by free charge carriers, the number of HShP modes gradually decreases accompanied by the blueshifts and broadening of some hyperbolic dispersion bands as the doping concentration increases. In consequence, main fringes of valley quantum interference are broadened and shift toward the short wavelengths. Additionally, the doping increases the optical losses which limit the effective propagation of shear polaritons in monoclinic crystals. Therefore, the valley quantum interference is gradually reduced to a smaller and smaller negative value range as the doping concentration increases in the monoclinic crystal. Finally, the azimuthal dispersion of the HShP propagation direction gives rise to symmetry-broken valley quantum interference patterns when tuning the azimuth and twist angles of monoclinic hybrid structures. The azimuth angles of quantum interference fringes are susceptive to the variation of lattice displacement direction induced via the doping concentration. Thus, the valley quantum interference has great potential in estimating the doping concentration and the propagation directions of HShPs in monoclinic crystals.

Achieving large tunneling electroresistance (TER) is the key to the application of two-dimensional (2D) ferroelectric tunnel junctions (FTJs) in high-density memory devices. In this work, we show that an aided layer of 2D metal materials with ultrahigh work function can lead to significantly enhanced TER effect in 2D FTJs. To implement the idea, we design a 2D van der Waals heterostructure consisting of a metallic ${\mathrm{NbS}}_2$ monolayer and a ${\mathrm{Sc}}_2{\mathrm{CO}}_2$ ferroelectric, and propose a FTJ based on this heterostructure. By performing density functional theory and nonequilibrium Green's function method calculations, we find that the type of interfacial contact in the ${\mathrm{NbS}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$ heterostructure can be flexibly switched between Ohmic and Schottky contact by altering the ferroelectric polarization. Accordingly, the ${\mathrm{NbS}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$-based FTJ exhibit giant TER ratio of up to ${10}^{13}\%$, and its TER ratio increases with the tunneling barrier length. These findings not only provide a feasible strategy for realizing large TER effect in 2D FTJs, but also offer promising candidates to designing nonvolatile memories.

Hole qubits in germanium quantum dots are promising candidates for coherent control and manipulation of the spin degree of freedom through electric dipole spin resonance. We theoretically study the time dynamics of a single heavy-hole qubit in a laser-driven planar germanium quantum dot confined laterally by a harmonic potential in the presence of linear and cubic Rashba spin-orbit couplings and an out-of-plane magnetic field. We obtain an approximate analytical formula of the Rabi frequency using a Schrieffer-Wolff transformation and establish a connection of our model with the ESDR results obtained for this system. For stronger beams, we employ different methods such as unitary transformation and Floquet theory to study the time evolution numerically. We observe that high radiation intensity is not suitable for the qubit rotation due to the presence of high-frequency noise superimposed on the Rabi oscillations. We display the Floquet spectrum and highlight the quasienergy levels responsible for the Rabi oscillations in the Floquet picture. We study the interplay of both the types of Rashba couplings and show that the Rabi oscillations, which are brought about by the linear Rashba coupling, vanish for typical values of the cubic Rashba coupling in this system.

We have considered theoretically nonlinear transport phenomena known as bilinear magnetoresistance (BMR) and nonlinear planar Hall effect (NPHE) within the effective model describing surface states of a 3D topological insulator. Both phenomena can occur in nonmagnetic materials with strong spin-orbit interaction and reveal a term that depends linearly on the charge current density (external electric field) and in-plane magnetic field. In earlier studies, the physical mechanism of BMR and NPHE was related to scattering on spin-momentum locking inhomogeneities or to the hexagonal warping of Dirac cones. Here, we focus on another mechanism related to scattering on impurities that inherently contain spin-orbit coupling. Using the Green's function formalism and diagramatic method, we have derived analytical results for diagonal and transverse conductivities and determined nonlinear signals. The analytical and numerical results on BMR and NPHE indicate the possibility of determining the material constants, such as the Fermi wave vector and spin-orbit coupling parameter, by simple magnetotransport measurements.

We investigate the topological phases of Weyl semimetals possessing a pair of multifold ($N$-fold) Weyl points (WPs) irradiated by an off-resonant circularly polarized light in the frame of Floquet theory. The irradiation splits each $N$-fold WP into two singlet WPs and one residual WP of $N-2$ fold in $k$ space. When the irradiation intensity increases, the two singlet WP pairs move faster and are gapped out first; therefore, a phase transition from $N$-fold to $(N-2)$-fold Weyl semimetal occurs. The pair of remaining $(N-2)$-fold WPs are annihilated later and a sequential phase transition to insulator happens. The irradiation influences the system by means of two mechanisms—the time average of the perturbation and the emission and reabsorption of photons, with the latter usually regarded as responsible for the phase transitions of matter and the former sometimes omitted due to its trivial effect. However, we find that these phase transitions are caused by the former, not by the latter, and the latter only affects how the WPs in the same pair approach each other, except for the case of $N=1$ in which both mechanisms take effect. The most common Weyl semimetal, say $N=1$, has the most special response. The irradiation can induce not only the transition from Weyl semimetal to insulator, but also the inverse transition, while the latter cannot happen in Weyl semimetals with $N\ge 2$.