Transient response of the spin Peltier effect (SPE) in a Pt/yttrium iron garnet junction system has been investigated by means of a lock-in thermoreflectance method. We applied an alternating charge current to the Pt layer to drive SPE through the spin Hall effect, and measured the AC response of the resultant SPE-induced temperature modulation at frequencies ranging from 10 Hz to 1 MHz. We found that the SPE-induced temperature modulation decreases with increasing the frequency when the frequency is $>1\mathrm{kHz}$. This is a characteristic feature of SPE revealed by the high frequency measurements based on lock-in thermoreflectance, while previous low frequency measurements showed that the SPE signal is independent of the frequency. We attribute the decrease of the temperature modulation to the length scale of the SPE-induced heat current; by comparing the experimental results with one-dimensional heat conduction calculations, the length scale of SPE is estimated to be 0.94 μm.

Semiconductor quantum dots are probably the preferred choice for interfacing anchored matter spin qubits and flying photonic qubits. While full tomography of a flying qubit or light polarization is, in general, straightforward, matter spin tomography is a challenging and resource-consuming task. Here we present an all-optical method for conducting full tomography of quantum dot confined spins. Our method is applicable for electronic spin configurations such as the conduction-band electron and the valence-band hole and for electron-hole pairs such as the bright and dark excitons. We excite the spin qubit using a short, resonantly tuned, polarized optical pulse, which coherently converts the qubit to an excited qubit that decays by emitting a polarized single photon. We perform the tomography by using two different orthogonal, linearly polarized excitations, followed by time-resolved measurements of the degree of circular polarization of the emitted light from the decaying excited qubit. We demonstrate our method on the dark exciton spin state with a fidelity of 0.94, mainly limited by the accuracy of our polarization analyzers.

An insulating ferromagnetic ground state is a fundamental prerequisite for the quantum anomalous Hall (QAH) effect observed in magnetically doped topological insulators such as ${(\text{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$. So far, the QAH effect could only be induced by V and Cr doping, with V resulting in ferromagnetism with a higher $T_C$ and a more robust QAH state. To better understand the difference between the two dopants, we have investigated epitaxial ${\mathrm{V}}_{0.1}{\mathrm{Sb}}_{1.9}{\mathrm{Te}}_3$ and ${\mathrm{Cr}}_{0.1}{\left({\mathrm{Bi}}_{0.1}{\mathrm{Sb}}_{0.9}\right)}_{1.9}{\mathrm{Te}}_3$ films using x-ray absorption spectroscopy and x-ray magnetic circular dichroism. Our analysis of the V and Cr $L_{2,3}$ spectra by multiplet ligand-field theory goes beyond existing studies by allowing several charge-transfer states. We find a strongly covalent ground state, dominated by the superposition of one and two Te-ligand-hole configurations, with a negligible contribution from ionic ${\mathrm{V}}^{3+}$ or ${\mathrm{Cr}}^{3+}$. Crucial for a comparison with theoretical models are the resulting $d$-shell fillings ($n_d^{\text{V}}=3.47$ and $n_d^{\text{Cr}}=4.33$), and spin ($m_{\text{spin}}^{\text{V}}=2.39{\mu }_{\text{B}}$ and $m_{\text{spin}}^{\text{Cr}}=3.22{\mu }_{\text{B}}$) and orbital ($m_{\text{orb}}^{\text{V}}=-0.55{\mu }_{\text{B}}$ and $m_{\text{orb}}^{\text{Cr}}=-0.03{\mu }_{\text{B}}$) magnetic moments, with the total magnetic moments being in good agreement with published magnetometry results. Our findings indicate the importance of the Te $5p$ states for the ferromagnetism in ${(\text{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$ and favor theories involving $pd$-exchange.

We demonstrate that higher-order acoustic phonon oscillations of self-assembled Au nanoparticles can be excited using higher frequency bandwidth picosecond acoustic pulses generated by superdiffusive hot electrons. These modes require directional excitations such as a longitudinal strain pulse, being inaccessible in a direct optical excitation scheme. They are detected using the periodic shift of the surface plasmon resonance and are coherently controlled by a sequence of delayed acoustic pulses. Acoustic mode calculations show that the high-frequency mode corresponds to two higher-order spheroidal modes.

Recently, a Verwey-type transition in the mixed-valence alkali sesquioxide ${\mathrm{Cs}}_4{\mathrm{O}}_6$ was deduced from the charge ordering of molecular peroxide ${\mathrm{O}}_2^{2-}$ and superoxide ${\mathrm{O}}_2^{-}$ anions accompanied by the structural transformation and a dramatic change in electronic conductivity [Adler et al., Sci. Adv. 4, eaap7581 (2018)]. Here, we report that in the sister compound ${\mathrm{Rb}}_4{\mathrm{O}}_6$, a similar Verwey-type charge ordering transition is strongly linked to ${\mathrm{O}}_2^{-}$ orbital and spin dynamics. On cooling, a powder neutron diffraction experiment reveals a charge ordering and a cubic-to-tetragonal transition at $T_{\mathrm{CO}}=290$ K, which is followed by a further structural instability at $T_{\mathrm{s}}=92$ K that involves an additional reorientation of magnetic ${\mathrm{O}}_2^{-}$ anions. Magnetic resonance techniques supported by density functional theory computations suggest the emergence of a peculiar type of ${\pi }^{*}$-orbital ordering of the magnetically active ${\mathrm{O}}_2^{-}$ units, which promotes the formation of a quantum spin state composed of weakly coupled spin dimers. These results reveal that as in $3d$ transition-metal compounds, also in the ${\pi }^{*}$ open-shell alkali sesquioxides the interplay between Jahn-Teller-like electron-lattice coupling and Kugel-Khomskii-type superexchange determines the nature of orbital ordering and the magnetic ground state.

We present a systematic derivation of effective lattice spin Hamiltonians derived from a rotationally invariant multiorbital Hubbard model including a term ensuring Hund's rule coupling. The Hamiltonians are derived down-folding the fermionic degrees of freedom of the Hubbard model into the proper low-energy spin sector using Löwdin partitioning, which will be outlined in detail for the case of two sites and two orbitals at each site. Correcting the ground state systematically up to fourth order in the hopping of electrons, we find, for spin $S\ge 1$, the biquadratic, three-spin, and four-spin interactions beyond the conventional Heisenberg term. Comparing the puzzling energy spectrum of the magnetic states for a single Fe monolayer on Ru(0001), obtained from density functional theory, with the spin Hamiltonians taken at the limit of classical spins, we show that the previously ignored three-spin interaction can be comparable in size to the conventional Heisenberg exchange.

Trapping light in open cavities is a long sought “holy grail” of nanophotonics. Plasmonic materials may offer a unique opportunity in this context, as they may fully suppress the radiation loss and enable the observation of spatially localized light states with an infinite lifetime in an open system. Here, we investigate how the spatial dispersion effects, e.g., caused by the electron-electron interactions in a metal, affect the trapped eigenstates. Heuristically, one may expect that the repulsive-type electron-electron interactions should act against light localization, and thereby that they should have a negative impact on the formation of the embedded eigenstates. Surprisingly, here we find that the nonlocality of the material response creates new degrees of freedom and relaxes the requirements for the observation of trapped light. In particular, a zero-permittivity condition is no longer mandatory and the same resonator shell can potentially suppress the radiation loss at multiple frequencies.

Chiral spin liquids (CSLs) are time-reversal-symmetry-breaking ground states of frustrated quantum magnets that show no long-range magnetic ordering but instead exhibit topological order and fractional excitations. Their realization in simple and tractable microscopic models has, however, remained an open challenge for almost two decades until it was realized that Kitaev models on lattices with odd-length loops are natural hosts for such states, even in the absence of a time-reversal-symmetry-breaking magnetic field. Here we report on the formation of CSLs in a three-dimensional Kitaev model on a hypernonagon lattice composed of nine-site loops, which differ from their widely studied two-dimensional counterparts; namely, they exhibit a crystalline ordering of the ${\mathbb{Z}}_2$ gauge fluxes and thereby break some of the underlying lattice symmetries. We study the formation of these unconventional CSLs via extensive quantum Monte Carlo simulations and demonstrate that they are separated from the featureless paramagnet at high temperatures by a single first-order phase transition at which both time-reversal and lattice symmetries are simultaneously broken. Using variational approaches for the ground state, we explore the effect of varying the Kitaev couplings and find at least five distinct CSL phases, all of which possess crystalline ordering of the ${\mathbb{Z}}_2$ gauge fluxes. For some of these phases, the complementary itinerant Majorana fermions exhibit gapless band structures with topological features such as Weyl nodes or nodal lines in the bulk and Fermi arc or drumhead surface states.

We theoretically study the entanglement between two arbitrary spins in a magnetic material where magnons naturally form a general squeezed coherent state in the presence of an applied magnetic field and axial anisotropies. Employing concurrence as a measure of entanglement, we demonstrate that spins are generally entangled in thermodynamic equilibrium, with the amount of entanglement controlled by the external fields and anisotropies. As a result, the magnetic medium can serve as a resource to store and process quantum information. We furthermore show that the entanglement can jump discontinuously when decreasing the transverse magnetic field. This tunable entanglement can be potentially used as an efficient switch in quantum-information processing tasks.

The discovery of two-dimensional transition metal dichalcogenides (2D TMDs) has promised next-generation photonics and optoelectronics applications, particularly in the realm of nanophotonics. Arguably, the most crucial fundamental processes in these applications are exciton migration and charge transfer in 2D TMDs. However, exciton dynamics in 2D TMDs has yet to be studied on a nanophotonic platform and, more importantly, the control of exciton dynamics by means of nanophotonic structures still needs to be explored. Here, we demonstrate the control of exciton dynamics in $\mathrm{Mo}{\mathrm{S}}_2$ monolayers by introducing a hyperbolic metamaterial (HMM) substrate. We reveal the migration mechanisms of various excitons in $\mathrm{Mo}{\mathrm{S}}_2$ monolayers. Furthermore, we experimentally demonstrate that the Förster radius can be increased by HMMs, which is consistent with the theory we developed on the basis of nonlocal effects of HMM. This study will provide a significant step forward in enabling 2D TMD nanophotonic hybrid devices.

Building on recent results for adiabatic gauge potentials, we propose a variational approach for computing the generator of Schrieffer-Wolff transformations. These transformations consist of block diagonalizing a Hamiltonian through a unitary rotation, which leads to effective dynamics in a computationally tractable reduced Hilbert space. The generators of these rotations are computed variationally and thus go beyond standard perturbative methods, with error controlled by the locality of the variational ansatz. The method is demonstrated on two models. First, in the attractive Fermi-Hubbard model with onsite disorder, we find indications of a lack of observable many-body localization in the thermodynamic limit due to the inevitable mixture of different spinon sectors. Second, in the low-energy sector of the XY spin model with a broken $\text{U}\left(1\right)$ symmetry, we analyze ground-state response functions by combining the variational Schrieffer-Wolf transformation with the truncated spectrum approach.

A quantum tricritical point is shown to exist in coupled time-reversal symmetry (TRS) broken Majorana chains. The tricriticality separates topologically ordered, symmetry-protected topological (SPT), and trivial phases of the system. Here we demonstrate that the breaking of the TRS manifests itself in the emergence of a dimensionless scale, $g=\alpha \left(\xi \right)B\sqrt{N}$, where $N$ is the system size, $B$ is a generic TRS-breaking field, and $\alpha \left(\xi \right)$, with $\alpha \left(0\right)\equiv 1$, is a model-dependent function of the localization length, $\xi$, of boundary Majorana zero modes at the tricriticality. This scale determines the scaling of the finite-size corrections around the tricriticality, which are shown to be universal, and independent of the nature of the breaking of the TRS. We show that the single-variable scaling function, $f\left(w\right),w\propto mN$, where $m$ is the excitation gap, that defines finite-size corrections to the ground-state energy of the system around topological phase transition at $B=0$, becomes double-scaling, $f=f(w,g)$, at finite $B$. We realize TRS breaking through three different methods with completely different lattice details and find the same universal behavior of $f(w,g)$. In the critical regime, $m=0$, the function $f(0,g)$ is nonmonotonic and reproduces the Ising conformal field theory scaling only in limits $g=0$ and $g\to \infty$. The obtained result sets a scale $N\gg 1/{\left(\alpha B\right)}^2$ for the system to reach the thermodynamic limit in the presence of the TRS breaking. We derive the effective low-energy theory describing the tricriticality and analytically find the asymptotic behavior of the finite-size scaling function. Our results show that the boundary entropy around the tricriticality is also a universal function of $g$ at $m=0$.

We report on ultrasound measurements in a single crystal of the antiferromagnet ${\mathrm{U}}_2{\mathrm{Rh}}_2\mathrm{Sn}$ as a function of temperature and magnetic field. We find pronounced anomalies in the sound velocity at the Néel temperature, 25 K, and at the field-induced spin-flop-like transition at 22.5 T, which points to a strong magnetoelastic coupling. Additionally, we find that in the paramagnetic regime the temperature dependence of the magnetic susceptibility and the field dependences of the magnetization and sound velocity of transverse acoustic waves can be well described assuming a localized character of the $5f$ electrons. Using this premise, the crystal-electric-field scheme of ${\mathrm{U}}_2{\mathrm{Rh}}_2\mathrm{Sn}$ has been determined.

We study the transport properties of a voltage-biased contact between two spin-split superconductors separated by an insulating barrier of arbitrary transparency. At low transparency, the contribution of multiple Andreev reflections leads to a subharmonic gap structure that crucially depends on the amplitude and relative angle of the spin-splitting fields of each superconductor. For noncollinear fields, we find an interesting even-odd effect on the bound states within the gap, where the odd order multiple Andreev reflections split, but the even order ones remain at their expected positions. By computing the current-voltage characteristics, we determine the transparency required for the emergence of a subharmonic gap structure and show that the splitting of the odd bound states is associated with different threshold energies of spin-polarized Andreev processes. Our findings provide a tool to experimentally determine the amplitude and alignment of Zeeman fields in spin-split superconductors.

We propose the parent compound of the newly discovered superconducting nickelate ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$ as a self-doped Mott insulator, in which the low-density Nd $5d$ conduction electrons couple to localized Ni $3d_{x^2-y^2}$ electrons to form Kondo spin singlets at low temperatures. This proposal is motivated with our analyses of the reported resistivity and Hall coefficient data in the normal state, showing logarithmic temperature dependence at low temperatures. In the strong Kondo coupling limit, we derive a generalized $t\text{−}J$ model with both Kondo singlets and nickel holons moving through the lattice of an otherwise nickel spin-1/2 background. The antiferromagnetic long-range order is therefore suppressed as observed in experiments. With Sr doping, the number of holons on the nickel sites increases, giving rise to the superconductivity and a strange metal phase analogous to those in superconducting copper oxides.

Superconductivity with a remarkably high $T_c$ has recently been found in Sr-doped ${\mathrm{NdNiO}}_2$ thin films. While this system bears strong similarities to the cuprates, some differences, such as a weaker antiferromagnetic exchange coupling and possible high-spin moments on the doped Ni sites have been pointed out. Here, we investigate the effect of Hund coupling and crystal field splitting in a simple model system and argue that a multiorbital description of nickelate superconductors is warranted, especially in the strongly hole-doped regime. We then look at this system from the viewpoint of the spin-freezing theory of unconventional superconductivity, which provides a unified understanding of unconventional superconductivity in a broad range of compounds. ${\mathrm{Nd}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_2$ falls into a parameter regime influenced by two spin-freezing crossovers; one related to the emergent multiorbital nature in the strongly doped regime and the other related to the single-band character and square lattice geometry in the weakly doped regime.

We generalize the notion of “localization landscape,” introduced by M. Filoche and S. Mayboroda [Proc. Natl. Acad. Sci. USA 109, 14761 (2012)] for the single-particle Schrödinger operator, to a wide class of interacting many-body Hamiltonians. The many-body localization landscape (MBLL) is defined on a graph in the Fock space, whose nodes represent the basis vectors in the Fock space and edges correspond to transitions between the nodes connected by the hopping term in the Hamiltonian. It is shown that in analogy to the single-particle case, the inverse MBLL plays the role of an effective potential in the Fock space. We construct a generalized discrete Agmon metric and prove Agmon inequalities on the Fock-state graph to obtain bounds on the exponential decay of the many-body wave functions in the Fock space. The corresponding construction is motivated by the semiclassical WKB approximation, but the bounds are exact and fully quantum mechanical. We then prove a series of locality theorems which establish where in the Fock space we expect eigenstates to localize. Using these results as well as the locator expansion, we establish evidence for the existence of many-body localized states for a wide class of lattice models in any physical dimension in at least a part of their Hilbert space. The key to this argument is the observation that in sharp contrast to the conventional locator expansion for the Green's function, the locator expansion for the landscape function contains no resonances. For short-range hopping, which limits the connectivity of the Fock-state graph, the locator series is proven to be convergent and bounded by a simple geometric series. This, in combination with the discrete Agmon-type inequalities and the locality theorems, shows that localization for a fraction of the Hilbert space survives weak interactions and weak hopping at least for some realizations of disorder, but cannot prove or rule out localization of the entire Hilbert space. We qualitatively discuss potential breakdown of the locator expansion in the MBLL for long-range hopping and the appearance of a mobility edge in higher-dimensional theories.

Weak coupling theory predicts the critical temperature of a phonon superconductor to be $T_c=1.13e^{-3/2}{\omega }_D{e}^{-1/\lambda }$, where ${\omega }_D$ is the Debye frequency, $\lambda$ is the dimensionless electron-phonon coupling constant, and the factor $e^{-3/2}$ comes from fermionic self-energy and frequency dependence of the interaction. Other corrections are small either in ${\omega }_D/E_F$, by Migdal's theorem, or in $\lambda$. However, this formula assumes that ${\omega }_D\ll E_F$, where $E_F$ is the Fermi energy. We obtain $T_c$ in the dilute regime, when the Fermi energy is smaller than ${\omega }_D$. We argue that in this situation Migdal's theorem is no longer valid, and Kohn-Luttinger-type corrections to the pairing interaction must be included to obtain the correct prefactor for $T_c$.

Gate-induced magnetic switching in bilayer ${\mathrm{CrI}}_3$ has opened new ways for the design of novel low-power magnetic memories based on van der Waals heterostructures. The proposed switching mechanism seems to be fully dominated by electrostatic doping. Here we explain, by first-principles calculations, the ferromagnetic transition in doped bilayer ${\mathrm{CrI}}_3$. For the case of a very small electron doping, our calculations predict the formation of magnetic polarons (“ferrons,” “fluctuons”) where the electron is self-locked in a ferromagnetic droplet in an antiferromagnetic insulating matrix. The self-trapping of holes is impossible, at least within our approximation. Our findings could explain the recently observed asymmetric magnetic switching in dual-gated bilayer ${\mathrm{CrI}}_3$; for electron doping the formation of magnetic polarons facilitates the antiferro-to-ferro phase transition; for holes the transition takes place without any intermediate state requiring a higher doping level.

The wave function of conduction electrons moving in the background of a noncoplanar spin structure can gain a quantal phase—the Berry phase—as if the electrons were moving in a strong fictitious magnetic field. Such an emergent magnetic field effect is approximately proportional to the solid angle subtended by the spin moments on three neighboring spin sites, termed the scalar spin chirality. The entire spin chirality of the crystal, unless macroscopically canceled, causes the geometrical Hall effect of real-space Berry-phase origin, whereas the intrinsic anomalous Hall effect in a conventional metallic ferromagnet is of momentum-space Berry-phase origin induced by relativistic spin-orbit coupling. Here, we report the ordering phenomena of the spin-trimer scalar spin chirality and the consequent large geometrical Hall effect in the breathing kagome lattice compound ${\mathrm{Dy}}_3{\mathrm{Ru}}_4{\mathrm{Al}}_{12}$, where the ${\mathrm{Dy}}^{3+}$ moments form noncoplanar spin trimers with local spin chirality. Using neutron diffraction, we show that the local spin chirality of the spin trimers as well as its ferroic/antiferroic orders can be switched by an external magnetic field, accompanying large changes in the geometrical Hall effect. Our finding reveals that systems composed of tunable spin trimers can be a fertile field to explore large emergent electromagnetic responses arising from real-space topological magnetic orders.