We propose an alternative route to transport experiments for detecting Majorana bound states (MBSs) by combining topological superconductivity with quantum optics in a superconducting $pn$ junction containing a quantum dot (QD). We consider a topological superconductor (TSC) hosting two Majorana bound states at its boundary ($n$ side). Within an effective low-energy model, the MBSs are coherently tunnel-coupled to a spin-split electron level on the QD, which is placed close to one of the MBSs. Holes on the QD are tunnel-coupled to a normal conducting reservoir ($p$ side). Via electron-hole recombination, photons in the optical range are emitted, which have direct information on the MBS properties through the recombined electrons. Using a master equation approach, we calculate the polarization-resolved photon emission intensities (PEIs). In the weak coupling regime between MBSs and QD, we find an analytical expression for the PEI which allows to clearly distinguish the cases of well separated MBSs at zero energy from overlapping MBSs. For separated MBSs, the Majorana spinor polarization is given by the relative widths of the two PEI peaks associated with the two spin states on the QD. For overlapping MBSs, a coupling to the distant (nonlocal) MBS causes a shift of the emission peaks. Additionally, we show that quasiparticle poisoning (QP) influences the PEI drastically and changes its shot noise from super-Poissonian to sub-Poissonian. In the strong coupling regime, more resonances emerge in the PEI due to spin-mixing effects. Finally, we comment on how our proposal could be implemented using a Majorana nanowire.

We develop the theory of single-electron silicon spin qubit relaxation in the presence of a magnetic field gradient. Such field gradients are routinely generated by on-chip micromagnets to allow for electrically controlled quantum gates on spin qubits. We build on a valley-dependent envelope function theory that enables the analysis of the electron wave function in a silicon quantum dot with an arbitrary roughness at the interface. We assume the presence of single-layer atomic steps at a Si/SiGe interface and study how the presence of a gradient field modifies the spin-mixing mechanisms. We show that our theoretical modeling can quantitatively reproduce the results of experimental measurements of qubit relaxation in silicon in the presence of a micromagnet. We further study how a field gradient can modify the EDSR Rabi frequency as well as the quality factor of a silicon spin qubit. We show that this strongly depends on the details of the interface roughness. Interestingly, for a quantum dot with an ideally flat interface, adding a micromagnet can give rise to the reduction of the EDSR frequency within some interval of the external magnetic field strength.

Triple nodal points are degeneracies of energy bands in momentum space at which three Hamiltonian eigenstates coalesce at a single eigenenergy. For spinless particles, the stability of a triple nodal point requires two ingredients: rotational symmetry of order three, four, or six; combined with mirror or space-time-inversion symmetry. However, despite ample studies of their classification, robust boundary signatures of triple nodal points have until now remained elusive. In this work, we first show that pairs of triple nodal points in semimetals and metals can be characterized by Stiefel-Whitney and Euler monopole invariants, of which the first one is known to facilitate higher-order topology. Motivated by this observation, we then combine symmetry indicators for corner charges and for the Stiefel-Whitney invariant in two dimensions with the classification of triple nodal points for spinless systems in three dimensions. The result is a complete higher-order bulk-boundary correspondence, where pairs of triple nodal points are characterized by fractional jumps of the hinge charge. We present minimal models of the various species of triple nodal points carrying higher-order topology, and illustrate the derived correspondence on ${\mathrm{Sc}}_3\mathrm{AlC}$ which becomes a higher-order triple-point metal in applied strain. The generalization to spinful systems, in particular to the WC-type triple-point material class, is briefly outlined.

We demonstrate the pumping of phonons by ferromagnetic resonance in a series of ${[\mathrm{Co}\left(0.8\mathrm{nm}\right)/\mathrm{Pd}\left(1.5\mathrm{nm}\right)]}_{\mathrm{n}}$ multilayers ($n=6$, 11, 15, and 20) with strong magnon-phonon coupling and perpendicular magnetic anisotropy. The effect is shown using broadband ferromagnetic resonance over a range of temperatures (10–300 K), where a resonant damping enhancement is observed at frequencies where the phonons can form standing waves across the multilayer. The strength of this effect is enhanced by approximately a factor of 4 at 10 K compared to room temperature, which is anomalous in the sense that the temperature dependence of the magnon-phonon coupling predicts an enhancement that is less than a factor of 2. We conclude by explaining how the stronger-than-expected temperature dependence highlights the essential role of dynamic magnetization pinning in driving the phonons.

Resonant inelastic x-ray scattering (RIXS) is an evolving tool for investigating the spin dynamics of strongly correlated materials, which complements inelastic neutron scattering. In isotropic spin-$\frac{1}{2}$ Heisenberg antiferromagnetic (HAFM) spin chains, both techniques have observed non-spin-conserving (NSC) excitations confined to the two-spinon phase space. However, a recent O $K$-edge RIXS study of the one-dimensional HAFM ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$ observed spin-conserving (SC) four-spinon excitations outside the two-spinon phase space. Here, we demonstrate that analogous four-spinon excitations can also be accessed at the Cu $L_3$ edge in the related material ${\mathrm{SrCuO}}_2$. Through detailed modeling, we establish that these excitations appear in both the SC and NSC channels of the Cu $L_3$ edge, and are only captured by higher-order terms in the ultrashort core-hole lifetime expansion. Since these terms encode information about spin-spin correlations extending beyond nearest neighbors, our results offer different possibilities for studying nonlocal spin correlations in quantum magnets.

The structurally simplest high-temperature superconductor FeSe exhibits an intriguing superconducting nematic paramagnetic phase with unusual spin excitation spectra that are different from typical spin waves; thus, determining its effective magnetic exchange interactions is challenging. Here we report neutron scattering measurements of spin fluctuations of FeSe in the tetragonal paramagnetic phase. We show that the equal-time magnetic structure factor, $\mathcal{S}\left(\mathbf{Q}\right)$, can be effectively modeled using the self-consistent Gaussian approximation calculation with highly frustrated nearest-neighbor ($J_1$) and next-nearest-neighbor ($J_2$) exchange couplings, and very weak further neighbor exchange interaction. Our results elucidate the frustrated magnetism in FeSe, which provides a natural explanation for the highly tunable superconductivity and nematicity in FeSe and related materials.

The Hubbard and Su-Schrieffer-Heeger (SSH) Hamiltonians are iconic models for understanding the qualitative effects of electron-electron and electron-phonon interactions, respectively. In the two-dimensional square lattice Hubbard model at half filling, the on-site Coulomb repulsion $U$ between up and down electrons induces antiferromagnetic (AFM) order and a Mott insulating phase. On the other hand, for the SSH model, there is an AFM phase when the electron-phonon coupling $\lambda$ is less than a critical value ${\lambda }_c$ and a bond order wave when $\lambda >{\lambda }_c$. In this Letter, we perform numerical studies on the square lattice optical Su-Schrieffer-Heeger-Hubbard Hamiltonian, which combines both interactions. We use the determinant quantum Monte Carlo method which does not suffer from the fermionic sign problem at half filling. We map out the phase diagram and find that it exhibits a direct first-order transition between an antiferromagnetic phase and a bond-ordered wave as $\lambda$ increases. The AFM phase is characterized by two different regions. At smaller $\lambda$ the behavior is similar to that of the pure Hubbard model; the other region, while maintaining long-range AFM order, exhibits larger kinetic energies and double occupancy, i.e., larger quantum fluctuations, similar to the AFM phase found in the pure SSH model.

We recently unveiled that antiferromagnetism (AFM) order can be dominantly induced by the bond Su-Schrieffer-Heeger (SSH) electron-phonon coupling (EPC), which is beyond the conventional wisdom that AFM order is usually driven by strong Coulomb interactions. Nevertheless, many aspects of the interplay between EPC and strong electronic interactions on AFM ordering remain unexplored. Here, we investigate the Su-Schrieffer-Heeger-Hubbard (SSHH) model with bond SSH phonons and on-site Hubbard interactions by large-scale quantum Monte Carlo (QMC) simulations and obtain its ground-state phase diagram for various Hubbard interactions and EPC strength at half filling. Our results show that Hubbard interactions further enhance EPC-induced AFM, especially for small phonon frequency or in adiabatic limit, the regime most relevant to various quantum materials. This could shed further light to understanding the cooperative effect between EPC and electronic correlations in quantum materials.

Kitaev physics has recently attracted attention in condensed matter for its anticipated quantum spin liquid (QSL) state. The thermal transport measurement is crucial for probing the features of charge-neutral quasiparticles. In this letter, we report a significant thermal Hall effect in ${\mathrm{Na}}_2{\mathrm{Co}}_2\mathrm{Te}{\mathrm{O}}_6$ (NCTO), a Kitaev QSL candidate, when the magnetic field is applied along the out-of-plane direction of the honeycomb plane. The thermal conductivity (${\kappa }_{xx}$) and thermal Hall conductivity (${\kappa }_{xy}$) in NCTO reveal distinct magnetic field dependences below and above the Neel temperature $\left(T_{\mathrm{N}}\right)$ of 27 K. For $T>T_{\mathrm{N}}, {\kappa }_{xx}$ has a monotonic decrease in the field dependence, while ${\kappa }_{xy}$ persists up to $T^{*}=150\mathrm{K}$. On the other hand, both ${\kappa }_{xx}$ and ${\kappa }_{xy}$ exhibit complex field dependence for $T<T_{\mathrm{N}}$.

Optical excitation serves as a powerful approach to control the electronic structure of layered van der Waals materials via many-body screening effects, induced by photoexcited free carriers, or via light-driven coherence, such as optical Stark and Bloch-Siegert effects. Although theoretical work has also pointed to an exotic mechanism of renormalizing band structure via excitonic correlations in bound electron-hole pairs (excitons), experimental observation of such exciton-driven band renormalization and the full extent of their implications is still lacking, largely due to the limitations of optical probes and the impact of screening effects. Here, by using extreme-ultraviolet time-resolved angle-resolved photoemission spectroscopy together with excitonic many-body theoretical calculations, we directly unmask the band renormalization effects driven by excitonic correlations in a monolayer semiconductor. We revealed a surprising bandgap opening, increased by 40 meV, and a simultaneous enhancement of band effective mass. Our findings unmask the exciton-driven mechanism toward the band engineering in photoexcited semiconducting materials, opening a playground to manipulate the transient energy states in layered quantum materials via optical controls of excitonic many-body correlations.

The nearest-neighbor level-spacing distributions are a fundamental quantity of disordered systems and are classified into different universality classes. They are the Wigner-Dyson and the Poisson functions for extended and localized states in Hermitian systems, respectively. The distributions follow the Ginibre functions for the non-Hermitian systems whose eigenvalues are complex and away from exceptional points (EPs). However, the level-spacing distributions of disordered non-Hermitian systems near EPs are still unknown, and a corresponding random matrix theory is absent. Here, we show another class of universal level-spacing distributions in the vicinity of EPs of non-Hermitian Hamiltonians. Two distribution functions, $P_{\text{SP}}\left(s\right)$ for the symmetry-preserved phase and $P_{\text{SB}}\left(s\right)$ for the symmetry-broken phase, are needed to describe the nearest-neighbor level-spacing distributions near EPs. Surprisingly, both $P_{\text{SP}}\left(s\right)$ and $P_{\text{SB}}\left(s\right)$ are proportional to $s$ for small $s$, or linear level repulsions, in contrast to cubic level repulsions of the Ginibre ensembles. For disordered non-Hermitian tight-binding Hamiltonians, $P_{\text{SP}}\left(s\right)$ and $P_{\text{SB}}\left(s\right)$ can be well described by a surmise ${\tilde{P}}_{\text{ep}}\left(s\right)={\tilde{c}}_{1}sexp[-{\tilde{c}}_2{s}^{\tilde{\alpha }}]$ in the thermodynamic limit (infinite systems) with a constant $\tilde{\alpha }$ that depends on the localization nature of states at EPs rather than the dimensionality of non-Hermitian systems and the order of EPs.

This Letter theoretically validated that the $\omega =0$ component of a photoexcited Drude peak in one-dimensional Mott insulators is nonzero only when a finite amount of charge fluctuations is included. Here, we define charge fluctuations as annihilations and creations of photoexcited elementary particles, that are, holons (Hs) and doublons (Ds). In contrast, when the electron on-site repulsion, $U$, is infinite, such fluctuations are completely suppressed, and the H and D behave, such as rigid balls, leading to a vanishing $\omega =0$ component. Such behaviors inspire us to name them as “quantum clackers.” Moreover, we demonstrated that the effective broadenings of the finite-frequency components of the Drude peak are affected by charge fluctuations and carrier density. Finally, we discovered that such broadenings can be estimated using the group velocity of the relative motion of an H and a D as would be predicted from a colliding picture of the two particles.

The generation of spatially homogeneous spin polarization by application of electric current is a fundamental manifestation of symmetry-breaking spin-orbit coupling (SOC) in solid-state systems, which underpins a wide range of spintronic applications. Here, we show theoretically that twisted van der Waals heterostructures with proximity-induced SOC are candidates par excellence to realize exotic spin-charge transport phenomena due to their highly tunable momentum-space spin textures. Specifically, we predict that graphene/group-VI dichalcogenide bilayers support room temperature spin-current responses that can be manipulated via twist-angle control. For critical twist angles, the nonequilibrium spin density is pinned parallel to the applied current. This effect is robust against twist-angle disorder, with $\mathrm{graphene}/{\mathrm{WSe}}_2$ possessing a critical angle (purely collinear response) of ${\theta }_c\simeq {14}^{\circ }$. A simple electrical detection scheme to isolate the collinear Edelstein effect is proposed.

The synthesis of the high-temperature superconductor ${\mathrm{LaH}}_{10}$ requires pressures in excess of 100 GPa, wherein it adopts a face-centered cubic structure. Upon decompression, this structure undergoes a distortion, which still supports superconductivity, but with a lower critical temperature. Previous calculations have shown that quantum and anharmonic effects are necessary to stabilize the cubic structure, but have not resolved the low pressure distortion. Using large scale path-integral molecular dynamics enabled by a machine learned potential, we show that a rhombohedral distortion appears at sufficiently low pressures. We also highlight the importance of quantum zero-point motion in stabilizing the cubic structure.

Zintl phases, containing strongly covalently bonded frameworks with separate ionically bonded ions, have emerged as a critical materials family in which to couple magnetism and strong spin-orbit coupling to drive diverse topological phases of matter. Here we report the single-crystal synthesis, magnetic, thermodynamic, transport, and theoretical properties of the Zintl compound ${\mathrm{EuZn}}_2{\mathrm{P}}_2$ that crystallizes in the anti-${\mathrm{La}}_2{\mathrm{O}}_3$ (${\mathrm{CaAl}}_2{\mathrm{Si}}_2$) P-$3m1$ structure, containing triangular layers of ${\mathrm{Eu}}^{2+}$ ions. In-plane resistivity measurements reveal insulating behavior with an estimated activation energy of $E_g=0.11\mathrm{eV}$. Specific heat and magnetization measurements indicate antiferromagnetic ordering at $T_N=23\mathrm{K}$. Curie-Weiss analysis of in-plane and out of plane magnetic susceptibility from $T=150$ to 300 K yields $p_{\mathrm{eff}}=8.61$ for ${\mu }_{0}H\perp c$ and $p_{\mathrm{eff}}=7.74$ for ${\mu }_{0}H//c$, close to the expected values for the $4f^7 J=S=7/2 {\mathrm{Eu}}^{2+}$ ion and indicative of weak anisotropy. Below $T_N$, a significant anisotropy of ${\chi }_{\perp }/{\chi }_{//}\approx 2.3$ develops, consistent with $A$-type magnetic order as observed in isostructural analogs and as predicted by the density functional theory calculations reported herein. The positive Weiss temperatures of ${\theta }_W=19.2\mathrm{K}$ for ${\mu }_{0}H\perp c$ and ${\theta }_W=41.9\mathrm{K}$ for ${\mu }_{0}H//c$ show a similar anisotropy and suggest competing ferromagnetic and antiferromagnetic interactions. Comparing Eu magnetic ordering temperatures across trigonal $\mathrm{Eu}{M}_2{X}_2$ ($M=$ divalent metal, $X=$ pnictide) shows that ${\mathrm{EuZn}}_2{\mathrm{P}}_2$ exhibits the highest ordering temperature, with variations in $T_N$ correlating with changes in expected dipolar interaction strengths within and between layers and independent of the magnitude of electrical conductivity. These results provide experimental validation of the crystochemical intuition that the cation ${\mathrm{Eu}}^{2+}$ layers and the anionic ${\left(M_2{X}_2\right)}^{2–}$ framework can be treated as electronically distinct subunits, enabling further predictive materials design.

We employ an atomistic model using a nearest-neighbor Heisenberg Hamiltonian exchange to study computationally the dependence of the Curie temperature of $L{1}_0$-FePt on finite-size and surface effects in heat-assisted magnetic recording (HAMR) media. We demonstrate the existence of a size threshold at 3.5 nm below which the impact of finite-size effects starts to permeate into the center of the grains and contributes to the reduction of the Curie temperature. We find a correlation between the Curie temperature and the percentage of atomistic bonds lost on the surface as a function of grain size, which can be extended to apply to not only $L{1}_0$-FePt but also generic magnetic systems of any crystal structure. In a recording medium, the inevitable grain size dispersion leads to an irreducible contribution to the dispersion of the Curie temperature. Therefore, our paper gives insight into finite-size effects, which have been predicted to be a serious limitation of HAMR.

Neodymium (Nd) magnets (${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$), known as the strongest permanent magnets, are indispensable in realizing high efficiency in energy conversion devices. To enhance their coercivity at high temperatures, the Nd component is substituted with dysprosium (Dy) in the commercial products of the magnet. Surface Dy-rich shells are considered important for enhancing coercivity. However, the underlying microscopic mechanism has not been studied based on atomistic theories. In this study, first, the features and mechanisms of the enhancement were studied using an atomistic model of the Nd magnet. Subsequently, the threshold field (coercive force) for magnetization reversal and the dynamical features at absolute zero and finite temperatures were investigated. The results show that a change from surface to bulk nucleation occurs when the number of substituted layers increases and the anisotropy energy of Dy is resistant to temperature increase, which significantly enhances coercivity, especially at high temperatures.