Efficient transfer of the quantum state of ensembles of electronic spins to nuclear spins is at the root of many novel quantum-based technologies. However, accomplishing this transfer requires magnetic fields aligned to specific crystalline axes or cryogenic temperatures, limiting the range of applications. Here, we observe fast Rabi oscillations on electron-nuclear spin exchanging transitions, coherent population trapping, and polarization of nuclear spin ensembles under ambient conditions and using uncritical magnetic field angles. For this, we employ nominally forbidden electron-nuclear spin transitions in ensembles of nitrogen-vacancy (NV) centers in diamond identically coupled to their own ${}^{14}\mathrm{N}$ nuclei. This technique is scalable and could thus push the sensitivity of NV sensors forward. It could also be applied to efficient long-lived storage of microwave photons and to the coupling of nuclear spins to mechanical oscillators in the resolved sideband regime.

Pressure-induced first-order phase transition often involves spatial disruption around the nucleus of a new phase, due to the inherent volume change. Atomic relaxations during this process produce lattice strain that in turn affects the nucleation kinetics and dynamics of transition, especially in nanoparticles. However, it is difficult to experimentally measure the lattice strain of materials, leading to many unsettled questions regarding size-dependent phenomena in nanomaterials at pressure. Here we present a method to determine the lattice strain of nanoparticles during first-order phase transitions using the pressure-volume data. A case study of nano-${\mathrm{Ho}}_2{\mathrm{O}}_3$ with multiple pressure-induced phase transitions is systematically performed to reveal the critical role of lattice strain in the size-dependent phase selection and amorphization. A phenomenological model is also given to describe the metastability of intermediate phases and size-tunable phase evolution during the nucleation of new phases at pressure.

We investigate the coupled spin and orbital textures of the topological surface state in ${\mathrm{Bi}}_2$(Te,${\mathrm{Se})}_3$(0001) across full momentum space using spin- and angle-resolved photoelectron spectroscopy and relativistic one-step photoemission theory. For an approximately isotropic Fermi surface in ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$, the measured intensity and spin momentum distributions, obtained with linearly polarized light, qualitatively reflect the orbital composition and the orbital-projected in-plane spin polarization, respectively. In ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, the in-plane lattice potential induces a hexagonal anisotropy of the Fermi surface, which manifests in an out-of-plane photoelectron spin polarization with a strong dependence on light polarization, excitation energy, and crystallographic direction.

While classical percolation is well understood, percolation effects in randomly packed or jammed structures are much less explored. Here we investigate both experimentally and theoretically the electrical percolation in a binary composite system of disordered spherocylinders, to identify the relation between structural (percolation) and functional properties of nanocomposites. Experimentally, we determine the percolation threshold $p_c$ and the conductivity critical exponent $t$ for composites of conducting ($\mathrm{Cr}{\mathrm{O}}_2$) and insulating (${\mathrm{Cr}}_2{\mathrm{O}}_3$) rodlike nanoparticles that are nominally geometrically identical, yielding $p_c=0.305\pm 0.026$ and $t=2.52\pm 0.03$ respectively. Simulations and modeling are implemented through a combination of the mechanical contraction method and a variant of random walk (de Gennes ant) approach, in which charge diffusion is correlated with the system conductivity via the Nernst-Einstein relation. The percolation threshold and critical exponents identified through finite-size scaling are in good agreement with the experimental values. Curiously, the calculated percolation threshold for spherocylinders with an aspect ratio of 6.5, $p_c=0.312\pm 0.002$, is very close (within numerical errors) to the one found previously in two other distinct systems of disordered jammed spheres and simple cubic lattice, an intriguing and surprising result.

Improper and hybrid-improper ferroelectrics (FEs) host spontaneous polarization ($P$) owing to the anharmonic coupling between polar and nonpolar modes. In general, the unstable zone-boundary nonpolar mode is a primary order parameter that couples to the stable zone-center polar mode to induce $P$. However, in certain FEs, such as the Aurivillius family, the polar mode is also unstable. In such cases, the strength of coupling between polar and nonpolar modes governs whether FE phase transition is a single or double step. Here we investigate a single-step avalanche-like transition in ${\mathrm{SrBi}}_2{\mathrm{Nb}}_2{\mathrm{O}}_9$, an Aurivillius family compound, using inelastic neutron scattering and Raman spectroscopy combined with group theory and first-principles simulations. We find that a strong trilinear coupling constant $\lambda$ between the two nonpolar and one polar instabilities enables their single-step condensation at the FE transition temperature $T_{FE}$ and overcomes the effect of the positive bi-quadratic coupling that works against it. The ratio of $\lambda$ to the leading-order term in the energy invariant polynomial is two to six times larger than in iso-structural ${\mathrm{SrBi}}_2{\mathrm{Ta}}_2{\mathrm{O}}_9$, which undergoes a double-step transition. We identify and track polar and nonpolar instabilities and find their temperature dependence consistent with the theoretical prediction based on the strong trilinear coupling mechanism. From the sign ($\pm$) of bi-quadratic coupling constants in the energy invariant polynomial, we further rule out the triggering mechanism to drive the FE transition. Moreover, we observe little change in phonons across $T_{FE}$ despite a first-order transition. However, a significant phonon broadening occurs even at 300 K, which we rationalize based on a large anharmonic vibrational amplitude of O atoms in the ${\mathrm{NbO}}_6$ octahedra.

$\gamma \text{−}{\mathrm{CoV}}_2{\mathrm{O}}_6$ is a quasi-one-dimensional spin-$\frac{3}{2}$ magnet that possesses two distinct magnetic orders at low temperatures with propagation vectors $k_1$ = ($\frac{1}{2}$, 0, 0) and $k_2$ = ($\frac{1}{4}$ $+$ $\delta h, \delta k, -\frac{1}{4}$ $+$ $\delta l$), respectively. Here we use muon spin relaxation and rotation to reveal the thermodynamics of the magnetic phase separation in this compound. In the paramagnetic (PM) region, short-range correlated spin clusters emerge at $T_{\mathrm{m}}\simeq$ 29 K at the $\mathit{partial}$ expense of the PM volume. Upon further cooling, we show that these emergent clusters become spatially coherent and account for the $k_2$ phase below $T_{\mathrm{N}2}$ = 7.5 K, while the remaining PM spins are driven into the $k_1$ state at $T_{\mathrm{N}1}$ = 6.6 K. These results stress magnetic microphase inhomogeneity as a thermodynamic precursor for the ground-state phase separation in weakly coupled spin-$\frac{3}{2}$ chains.

We explore the mathematical theory to rigorously describe the response of media with linear time-varying, generally dispersive, electromagnetic constitutive parameters. We show that, even when the temporal inhomogeneity takes place on a timescale comparable—or shorter—than the driving fields' time period, one can still define a physically meaningful time-varying dispersion. Accordingly, a generalized set of Kramers-Kronig relations is investigated to link the real and imaginary parts of the time-varying frequency-dispersive spectra characterizing the medium's constitutive response. Among others, we study the case of a Lorentzian dielectric response with time-varying volumetric density of polarizable atoms and present the varying circuital equivalents of the governing differential equation, which in turn allow us to use the notion of generalized time-varying impedances/admittances of a time-dependent resistor, inductor, and capacitor.

We theoretically study spin pumping from a layered van der Waals antiferromagnet in its canted ground state into an adjacent normal metal. We find that the resulting dc spin pumping current bears contributions along all spin directions. Our analysis allows for detecting intra- and cross-sublattice spin-mixing conductances via measuring the two in-plane spin current components. We further show that sublattice symmetry-breaking Gilbert damping can be realized via interface engineering and induces a dissipative coupling between the optical and acoustic magnon modes. This realizes magnon level attraction and exceptional points in the system. Furthermore, the dissipative coupling and cross-sublattice spin pumping contrive to produce an unconventional spin current in the out-of-plane direction. Our findings provide a route to extract the spin-mixing conductance matrix and uncovers the unique opportunities, such as level attraction, offered by van der Waals antiferromagnet–normal metal hybrids.

We calculate the quasiparticle dispersion and spectral weight of the quasiparticle that results when a hole is added to an antiferromagnetically ordered $\mathrm{Cu}{\mathrm{O}}_2$ plane of a cuprate superconductor. We also calculate the magnon contribution to the quasiparticle spectral function. We start from a multiband model for the cuprates considered previously [Ebrahimnejad et al., Nat. Phys. 10, 951 (2014)]. We map this model and the operator for creation of an O hole to an effective one-band generalized $t-J$ model, without free parameters. The effective model is solved using the state-of-the-art self-consistent Born approximation. Our results reproduce all the main features of experiments. They also reproduce qualitatively the dispersion of the multiband model, giving better results for the intensity near wave vector $(\pi ,\pi )$, in comparison with the experiments. In contrast to what was claimed in Ebrahimnejad et al., we find that spin fluctuations play an essential role in the dynamics of the quasiparticle and hence in both its weight and dispersion.

We analyze the two-orbital (with $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ orbitals) analog of the $t-J-U$ model as applied to the description of the copper-oxide monolayer deposited on the ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ substrate ($\mathrm{Cu}{\mathrm{O}}_2$/BSCCO). We show that an extended $s$-wave superconducting dome appears in the heavily hole-overdoped regime of the model, with dominant contribution to the pairing coming from the $d_{3z^2-r^2}$ orbitals. Also, the lower critical doping for the appearance of the SC state concurs with the Lifshitz transition after which the hole-like Fermi pockets are created around the M points in the Brillouin zone. The obtained results are in accord with the recent experimental result [cf. Zhong et al., Sci. Bull. 61, 1239 (2016)]. An analogous two-band description is also analyzed in the context of the ${\mathrm{Ba}}_2\mathrm{Cu}{\mathrm{O}}_{4-y}$ bulk compound, where the heavily hole-overdoped situation is believed to be reached [cf. Li et al., Proc. Natl. Acad. Sci. USA 116, 12156 (2019)]. As we show, a two-dome structure can be realized in such system with the $d$- and $s$-wave symmetries of the gap, corresponding to the low hole-doping and heavily hole-overdoped cases, respectively.

We study the spin diffusion and spin conductivity in the square lattice Hubbard model by using the finite-temperature Lanczos method. We show that the spin diffusion behaves differently from the charge diffusion and has a nonmonotonic $T$ dependence. This is due to a progressive liberation of charges that contribute to spin transport and enhance it beyond that active at low temperature due to the Heisenberg exchange. We further show that going away from half filling and zero magnetization increases the spin diffusion, but that the increase is insufficient to reconcile the difference between the model calculations and the recent measurements on cold atoms.

We obtain the most general forms of rank-2 and rank-3 tensors allowed by the crystal symmetries of the honeycomb lattice of edge-sharing octahedra for crystals belonging to different crystallographic point groups, including the monoclinic point group $2/m$ and the trigonal (or rhombohedral) point group $\overline{3}$. Our results are relevant for two-dimensional materials, such as $\alpha -{\mathrm{RuCl}}_3, {\mathrm{CrI}}_3$, and the honeycomb iridates. We focus on the magnetic-field-dependent thermal conductivity tensor ${\kappa }_{ij}\left(\mathbf{H}\right)$, which describes a system's longitudinal and thermal Hall responses, for the cases when the magnetic field is applied along high-symmetry directions, perpendicular to the plane and in the plane. We highlight some unexpected results, such as the equality of fully longitudinal components to partially transverse components in rank-3 tensors for systems with threefold rotational symmetry, and make testable predictions for the thermal conductivity tensor.

The true topological nature of the Kondo insulator $\mathrm{Sm}{\mathrm{B}}_6$ remains to be unveiled. Our previous tunneling study not only found evidence for the existence of surface Dirac fermions, but it also uncovered that they inherently interact with the spin excitons, collective excitations in the bulk. We have extended such a spectroscopic investigation into crystals containing a Sm deficiency. The bulk hybridization gap is found to be insensitive to the deficiency up to 1% studied here, but the surface states in Sm-deficient crystals exhibit quite different temperature evolutions from those in stoichiometric ones. We attribute this to the topological surface states remaining incoherent down to the lowest measurement temperature due to their continued interaction with the spin excitons that remain uncondensed. This result shows that the detailed topological nature of $\mathrm{Sm}{\mathrm{B}}_6$ could vary drastically in the presence of disorder in the lattice. This sensitiveness to disorder is seemingly contradictory to the celebrated topological protection, but it can be understood as being due to the intimate interplay between strong correlations and topological effects.

The quasi-one-dimensional organic conductors (${\mathrm{TMTTF})}_{2}X$ with noncentrosymmetric anions commonly undergo charge- and anion-order transitions upon cooling. While for compounds with tetrahedral anions ($X={\mathrm{BF}}_4^{-}, {\mathrm{ReO}}_4^{-}$, and ${\mathrm{ClO}}_4^{-}$) the charge-ordered phase is rather well understood, the situation is less clear in the case of planar triangular anions, such as (${\mathrm{TMTTF})}_2{\mathrm{NO}}_3$. Here we explore the electronic and structural transitions by transport experiments, optical, and magnetic spectroscopy. This way we analyze the temperature dependence of the charge imbalance $2\delta$ and an activated behavior of $\rho \left(T\right)$ with ${\mathrm{\Delta }}_{\mathrm{CO}}\approx 530\mathrm{K}$ below $T_{\mathrm{CO}}=250\mathrm{K}$. Since (${\mathrm{TMTTF})}_2{\mathrm{NO}}_3$ follows the universal relation between charge imbalance $2\delta$ and size of the gap ${\mathrm{\Delta }}_{\mathrm{CO}}$, our findings suggest that charge order is determined by TMTTF stacks with little influence of the anions. Clear signatures of anion ordering are detected at $T_{\mathrm{AO}}=50\mathrm{K}$. The tetramerization affects the dc transport, the vibrational features of donors and acceptors, and leads to formation of spin singlets.

We test the valley-filtering capabilities of a quantum dot inscribed by locally straining an $\alpha \text{−}{\mathcal{T}}_3$ lattice. Specifically, we consider an out-of-plane Gaussian bump in the center of a four-terminal configuration and calculate the generated pseudomagnetic field having an opposite direction for electrons originating from different valleys, the resulting valley-polarized currents, and the conductance between the injector and collector situated opposite one another. Depending on the quantum dot's width and width-to-height ratio, we detect different transport regimes with and without valley filtering for both the $\alpha \text{−}{\mathcal{T}}_3$ and dice lattice structures. In addition, we analyze the essence of the conductance resonances with a high valley polarization in terms of related (pseudo-) Landau levels, the spatial distribution of the local density of states, and the local current densities. The observed local charge and current density patterns reflect the local inversion symmetry breaking by the strain, besides the global inversion symmetry breaking due to the scaling parameter $\alpha$. By this way we can also filter out different sublattices.

We study the ground state of the one-dimensional “$t\text{−}{J}_s\text{−}{J}_{\tau }$ model,” which is a variant of the $t\text{−}J$ model with an additional channel degree of freedom. The model is not only a generalization of the $t\text{−}J$ model but also an effective model of the two-channel Kondo lattice model in the strong-coupling region. The low-energy excitations and correlation functions are systematically calculated by the density matrix renormalization group method, and the ground-state phase diagram at quarter filling consisting of a Tomonaga-Luttinger liquid, spin-gap state, channel-gap state, insulator, and phase separation is determined. We find that weak channel fluctuations stabilize the spin-gap state, while strong channel fluctuations lead to the transition to the insulator.

Proposing a theoretical model of the core level photoemission of metals, we investigate the plasmon-driven inelastic photoemission delays based on a nonperturbative treatment of many-electron responses due to the long-range Coulomb potential. Being irrelevant to the plasmon coupling strength as well as the plasmon frequency, the emission delays $\mathrm{\Delta }{\tau }_n$ of the $n\mathrm{th}$-order plasmon satellites ($n=1,2,3,...$) are found to be universal order by order among the metals in the core level photoemission where the recoil-less approximation would be valid. In particular, for a main line with its weight $e^{-\gamma }$, where $\gamma$ quantifies the plasmon coupling strength, the average inelastic photoemission delay $\langle \mathrm{\Delta }\tau \rangle$ is found to be $\gamma {\left(1-e^{-\gamma }\right)}^{-1}\mathrm{\Delta }{\tau }_1$ and thus is simply scaled by a universal time delay $\mathrm{\Delta }{\tau }_1$. This finding is sharply contrasted with the emission delays under the localized potential, which indicates a fundamental difference in the emission process between extended and localized screenings.

The time-dependent exact-diagonalization method is used to study the light-induced phase transition of magnetic orders in the anisotropic triangular-lattice Hubbard model. Calculating the spin correlation function, we confirm that the phase transition from the ${120}^{\circ }$ order to the Néel order can take place due to high-frequency periodic fields. We show that the effective Heisenberg-model Hamiltonian derived from the high-frequency expansion by the Floquet theory describes the present system very well and that the ratio of the exchange interactions expressed in terms of the frequency and amplitude of the external field determines the type of the magnetic orders. Our results demonstrate the controllability of the magnetic orders by tuning the external field.

We observe minima of the longitudinal resistance corresponding to the quantum Hall effect of composite fermions at quantum numbers $p=1$, 2, 3, 4, and 6 in an ultraclean strongly interacting bivalley SiGe/Si/SiGe two-dimensional electron system. The minima at $p=3$ disappear below a certain electron density, although the surrounding minima at $p=2$ and $p=4$ survive at significantly lower densities. Furthermore, the onset for the resistance minimum at a filling factor $\nu =3/5$ is found to be independent of the tilt angle of the magnetic field. These surprising results indicate the intersection or merging of the quantum levels of composite fermions with different valley indices, which reveals the valley effect on fractions.