CeRhSn with a quasikagome lattice of Ce atoms in the hexagonal $c$ plane has been expected to be in close vicinity to a zero-field quantum criticality derived from magnetic frustration. We have studied how the ground state changes with substitution of Pd for Rh in $\mathrm{CeR}{\mathrm{h}}_{1-x}\mathrm{P}{\mathrm{d}}_x\mathrm{Sn}$ $\left(x\le 0.75\right)$ by measuring the specific heat $C$, magnetic susceptibilities ${\chi }_{\mathrm{dc}}$ and ${\chi }_{\mathrm{ac}}$, magnetization $M$, electrical resistivity ρ, and magnetoresistance. For $x=0$, the field dependence of ${\chi }_{\mathrm{ac}}$ at $T=0.03\mathrm{K}$ shows a peak at $B\parallel a=3.5\mathrm{T}$, confirming the spin-flop crossover in the field applied along the hard axis. The temperature dependence of ${\chi }_{\mathrm{ac}}$ shows a broad maximum at 0.1 K whereas $C/T$ continues to increase down to 0.08 K. For $x\geqq 0.1,\rho \left(T\right)$ is dominated by incoherent Kondo scattering and both $C/T$ and ${\chi }_{\mathrm{ac}}\left(T\right)$ exhibit peaks, indicating the development of an antiferromagnetic order. The ordering temperature rises to 2.5 K as $x$ is increased to 0.75. Our results indicate that the ground state in the quasikagome Kondo lattice $\mathrm{CeR}{\mathrm{h}}_{1-x}\mathrm{P}{\mathrm{d}}_x\mathrm{Sn}$ leaves the quantum critical point at $x=0$ with increasing $x$ as a consequence of suppression of both the magnetic frustration and Kondo effect.

Topological semimetals have been at the forefront of experimental and theoretical attention in condensed-matter physics. Among these, recently discovered Weyl semimetals have a dispersion described by a three-dimensional Dirac cone, which is at the root of exotic physics such as the chiral anomaly in magnetotransport. In a time-reversal symmetric (TRS) Weyl semimetal film, the confinement gap gives the quasiparticles a mass, while TRS is preserved by having an even number of valleys with opposite masses. The film can be tuned through a topological phase transition by a gate electric field. In this work, we present a theoretical study of the quantum corrections to the conductivity of a topological semimetal thin film, which is governed by the complex interplay of the chiral band structure, mass term, and scalar and spin-orbit scattering. We study scalar and spin-orbit scattering mechanisms on the same footing, demonstrating that they have a strong qualitative and quantitative impact on the conductivity correction. We show that, due to the spin structure of the matrix Green's functions, terms linear in the extrinsic spin-orbit scattering are present in the Bloch and momentum relaxation times, whereas previous works had identified corrections starting from the second order. In the limit of small quasiparticle mass, the terms linear in the impurity spin-orbit coupling lead to a potentially observable density dependence in the weak antilocalization correction. Moreover, when the mass term is around 30% of the linear Dirac terms, the system is in the unitary symmetry class with zero quantum correction, and switching the extrinsic spin-orbit scattering drives the system to the weak antilocalization. We discuss the crossover between the weak localization and weak antilocalization regimes in terms of the singlet and triplet Cooperon channels, and analyze this transition as a function of the mass and spin-orbit scattering strength. Experimental schemes to detect this transition are discussed.

Quantum entanglement is a stunning consequence of the superposition principle. This universal property of quantum systems has been intensively explored with photons, atoms, ions, and electrons. Collective excitations such as surface plasmons exhibit quantum behaviors. We report the remote preparation of a single plasmon state through the projective measurement of a photon entangled with the plasmon. We achieved photon-plasmon entanglement by converting one photon of an entangled photon pair into a surface plasmon. The plasmon is tested on a plasmonic platform in a Mach-Zehnder interferometer. A projective measurement on the polarization of the photon allows the remote preparation of the interference state of the plasmon. Entanglement between particles of various nature paves the way to the design of hybrid systems in quantum information networks.

We investigate the heat transport through a rare earth multilayer system composed of yttrium (Y), dysprosium (Dy), and niobium (Nb) by ultrafast x-ray diffraction. This is an example of a complex heat flow problem on the nanoscale, where several different quasiparticles carry the heat and conserve a nonequilibrium for more than 10 ns. The Bragg peak positions of each layer represent layer-specific thermometers that measure the energy flow through the sample after excitation of the Y top layer with fs-laser pulses. In an experiment-based analytic solution to the nonequilibrium heat transport problem, we derive the individual contributions of the spins and the coupled electron-lattice system to the heat conduction. The full characterization of the spatiotemporal energy flow at different starting temperatures reveals that the spin excitations of antiferromagnetic Dy speed up the heat transport into the Dy layer at low temperatures, whereas the heat transport through this layer and further into the Y and Nb layers underneath is slowed down. The experimental findings are compared to the solution of the heat equation using macroscopic temperature-dependent material parameters without separation of spin and phonon contributions to the heat. We explain why the simulated energy density matches our experiment-based derivation of the heat transport, although the simulated thermoelastic strain in this simulation is not even in qualitative agreement.

The electronic structure of Mn doped GaAs and GaN have been examined within a multiband Hubbard model. By virtue of the positioning of the Mn $d$ states, Mn doped GaAs is found to belong to the $p-d$ metal regime of the Zaanen-Sawatzky-Allen phase diagram and its variants, while Mn doping in GaN belongs to the covalent insulator regime. Their location in the phase diagram also determines how they would behave under quantum confinement which would increase the charge transfer energy. The ferromagnetic stability of Mn doped GaAs, we find, increases with confinement therefore providing a route to higher ferromagnetic transition temperatures.

Quantum magnetic phases near the magnetic saturation of triangular-lattice antiferromagnets with XXZ anisotropy have been attracting renewed interest since it has been suggested that a nontrivial coplanar phase, called the $\pi$-coplanar or $\mathrm{\Psi }$ phase, could be stabilized by quantum effects in a certain range of anisotropy parameter $J/J_z$ besides the well-known 0-coplanar (known also as $V)$ and umbrella phases. Recently, Sellmann et al. [Phys. Rev. B 91, 081104(R) (2015)] claimed that the $\pi$-coplanar phase is absent for $S=1/2$ from an exact-diagonalization analysis in the sector of the Hilbert space with only three down-spins (three magnons). We first reconsider and improve this analysis by taking into account several low-lying eigenvalues and the associated eigenstates as a function of $J/J_z$ and by sensibly increasing the system sizes (up to 1296 spins). A careful identification analysis shows that the lowest eigenstate is a chirally antisymmetric combination of finite-size umbrella states for $J/J_z\gtrsim 2.218$ while it corresponds to a coplanar phase for $J/J_z\lesssim 2.218$. However, we demonstrate that the distinction between 0-coplanar and $\pi$-coplanar phases in the latter region is fundamentally impossible from the symmetry-preserving finite-size calculations with fixed magnon number. Therefore, we also perform a cluster mean-field plus scaling analysis for small spins $S\le 3/2$. The obtained results, together with the previous large-$S$ analysis, indicate that the $\pi$-coplanar phase exists for any $S$ except for the classical limit $\left(S\to \infty \right)$ and the existence range in $J/J_z$ is largest in the most quantum case of $S=1/2$.

We consider a two-leg boson ladder in an artificial U(1) gauge field and show that, in the presence of interleg attractive interaction, the flux induced vortex state can be melted by dislocations. For increasing flux, instead of the Meissner to vortex transition in the commensurate-incommensurate universality class, first, an Ising transition from the Meissner state to a charge density wave takes place, then, at higher flux, the melted vortex phase is established via a disorder point where incommensuration develops in the rung current correlation function and in momentum distribution. Finally, the quasi-long-range ordered vortex phase is recovered for sufficiently small interaction. Our predictions for the observables, such as the spin current and the static structure factor, could be tested in current experiments with cold atoms in bosonic ladders.

A prominent feature of quantum spin liquids is fractionalization of the spin degree of freedom. Fractionalized excitations have their own dynamics in different energy scales, and hence, affect finite-temperature ($T$) properties in a peculiar manner even in the paramagnetic state harboring the quantum spin liquid state. We here present a comprehensive theoretical study of the spin dynamics in a wide $T$ range for the Kitaev model on a honeycomb lattice, whose ground state is a quantum spin liquid. In this model, the fractionalization occurs to break up quantum spins into itinerant matter fermions and localized $Z_2$ fluxes, which results in two crossovers at very different $T$ scales. Extending the previous study for the isotropic coupling case [J. Yoshitake, J. Nasu, and Y. Motome, Phys. Rev. Lett. 117, 157203 (2016)], we calculate the dynamical spin structure factor $S(\mathbf{q},\omega )$, the NMR relaxation rate $1/T_1$, and the magnetic susceptibility $\chi$ while changing the anisotropy in the exchange coupling constants, by using the dynamical mean-field theory based on a Majorana fermion representation. We describe the details of the methodology including the continuous-time quantum Monte Carlo method for computing dynamical spin correlations and the maximum entropy method for analytic continuation. We confirm that the combined method provides accurate results in a wide $T$ range including the region where the spins are fractionalized. We find that also in the anisotropic cases the system exhibits peculiar behaviors below the high-$T$ crossover whose temperature is comparable to the average of the exchange constants: $S(\mathbf{q},\omega )$ shows an inelastic response at the energy scale of the averaged exchange constant, $1/T_1$ continues to grow even though the equal-time spin correlations are saturated and almost $T$ independent, and $\chi$ deviates from the Curie-Weiss behavior. In particular, when the exchange interaction in one direction is stronger than the other two, the dynamical quantities exhibit qualitatively different $T$ dependences from the isotropic case at low $T$, reflecting the opposite parity between the flux-free ground state and the flux-excited state, and a larger energy cost for flipping a spin in the strong interaction direction. On the other hand, when the exchange anisotropy is in the opposite way, the results are qualitatively similar to those in the isotropic case. All these behaviors manifest the spin fractionalization in the paramagnetic region. Among them, the dichotomy between the static and dynamical spin correlations is unusual behavior hardly seen in conventional magnets. We discuss the relation between the dichotomy and the spatial configuration of the $Z_2$ gauge fields. Our results could stimulate further experimental and theoretical analyses of candidate materials for the Kitaev quantum spin liquids.

${\mathrm{BaCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ consists of Co chains in which a ${\mathrm{Co}}^{2+}$ ion carries a fictitious spin $\frac{1}{2}$ with Ising anisotropy. We performed elastic and inelastic neutron scattering experiments in ${\mathrm{BaCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ in a magnetic field perpendicular to the $c$ axis which is the chain direction. With applying magnetic field along the $a$ axis at 3.5 K, the antiferromagnetic order with the easy axis along the $c$ axis, observed in zero magnetic field, is completely suppressed at 8 T, while the magnetic field gradually induces an antiferromagnetic order with the spin component along the $b$ axis. We also studied magnetic excitations as a function of transverse magnetic field. The lower boundary of the spinon excitations splits gradually with increasing magnetic field. The overall feature of the magnetic excitation spectra in the magnetic field is reproduced by the theoretical calculation based on the spin $\frac{1}{2} XXZ$ antiferromagnetic chain model, which predicts that the dynamic magnetic structure factor of the spin component along the chain direction is enhanced and that along the field direction has clear incommensurate correlations.

Room-temperature dilute ferromagnetism has been reported for many semiconducting or insulating materials, which are usually in the forms of bulk or thin film. Here, we successfully fabricated dilute ferromagnetic nanowires by using dislocations—one-dimensional lattice defects—embedded between optically transparent, nonmagnetic $\mathrm{SrTi}{\mathrm{O}}_3$ single crystals. At the dislocation cores, we have both locally codoped magnetic $\mathrm{M}{\mathrm{n}}^{2+}$ ions and electron donors. The structure, chemistry, and ferromagnetism of dislocations were studied by atomic-resolution scanning transmission electron microscopy combined with magnetic force microscopy. We discuss the origin of dilute ferromagnetism at the dislocations in terms of the percolation of bound magnetic polarons along the dislocation cores, where antiferromagnetic coupling between the high spin state of $\mathrm{M}{\mathrm{n}}^{2+}$ ions and electron donors leads to the long-range Mn-Mn ferromagnetic exchange interaction.

We point out that a face-centered-cubic (fcc) optical lattice, which can be realized by a simple scheme using three lasers, provides one a highly controllable platform for creating Weyl points and topological nodal superfluids in ultracold atoms. In noninteracting systems, Weyl points automatically arise in the Floquet band structure when shaking such fcc lattices, and sophisticated design of the tunneling is not required. More interestingly, in the presence of attractive interaction between two hyperfine spin states, which experience the same shaken fcc lattice, a three-dimensional topological nodal superfluid emerges, and Weyl points show up as the gapless points in the quasiparticle spectrum. One could either create a double Weyl point of charge 2, or split it into two Weyl points of charge 1, which can be moved in the momentum space by tuning the interactions. Correspondingly, the Fermi arcs at the surface may be linked with each other or separated as individual ones.

Nitrogen-vacancy centers in diamond allow for coherent spin-state manipulation at room temperature, which could bring dramatic advances to nanoscale sensing and quantum information technology. We introduce a method for the optical measurement of the spin contrast in dense nitrogen-vacancy (NV) ensembles. This method brings insight into the interplay between the spin contrast and fluorescence lifetime. We show that for improving the spin readout sensitivity in NV ensembles, one should aim at modifying the far-field radiation pattern rather than enhancing the emission rate.

Model-independent compact representations of imaginary-time data are presented in terms of the intermediate representation (IR) of analytical continuation. We demonstrate the efficiency of the IR through continuous-time quantum Monte Carlo calculations of an Anderson impurity model. We find that the IR yields a significantly compact form of various types of correlation functions. This allows the direct quantum Monte Carlo measurement of Green's functions in a compressed form, which considerably reduces the computational cost and memory usage. Furthermore, the present framework will provide general ways to boost the power of cutting-edge diagrammatic/quantum Monte Carlo treatments of many-body systems.

We consider the quantum lifetime derived from low-field Shubnikov–de Haas oscillations as a metric of the quality of the two-dimensional electron gas in GaAs quantum wells that expresses large excitation gaps of the $\nu =\frac{5}{2}$ fractional quantum Hall state in the $N=1$ Landau level. In high quality samples, small density inhomogeneities dramatically impact the amplitude of Shubnikov–de Haas oscillations such that the canonical method [cf. Coleridge, Phys. Rev. B 44, 3793 (1991)] for determination of the quantum lifetime substantially underestimates ${\tau }_{\text{q}}$ unless density inhomogeneity is explicitly considered. We have developed a method that can be used to determine density inhomogeneity and extract the intrinsic ${\tau }_{\text{q}}$ by analyzing the Shubnikov–de Haas oscillations. However, even after accounting for inhomogeneity, ${\tau }_{\text{q}}$ does not correlate well with sample quality as measured by ${\mathrm{\Delta }}_{5/2}$, the excitation gap of the fractional quantum Hall state at 5/2 filling.

Artificial molecular states of double quantum dots defined in bilayer graphene are studied with the atomistic tight-binding method and its low-energy continuum approximation. We indicate that the extended electron wave functions have opposite parities on sublattices of the layers and that the ground-state wave-function components change from bonding to antibonding with the interdot distance. In the weak-coupling limit, the one most relevant for quantum dots defined electrostatically, the signatures of the interdot coupling include, for the two-electron ground state, formation of states with symmetric or antisymmetric spatial wave functions split by the exchange energy. In the high-energy part of the spectrum the states with both electrons in the same dot are found with the splitting of energy levels corresponding to simultaneous tunneling of the electron pair from one dot to the other.

Employing ab initio calculations, we characterize the interfaces formed between graphene, a much discussed two-dimensional material, and MXene, another two-dimensional material of recent interest. Our study considering the specific case of ${\mathrm{Ti}}_2\mathrm{C}$, a member of the MXene family, shows the formation of chemical bonds between Ti atoms and C atoms of graphene. This results in reconstruction of the electronic structure at the interface, making the interface metallic, though graphene is a zero-gap semiconductor and ${\mathrm{Ti}}_2\mathrm{C}$ is an antiferromagnetic insulator in their respective native form. The optical and phonon properties of the interfaces are found to be strongly dependent on the stacking arrangement, driven by the nature of chemical-bond formation. Consideration of O-passivated ${\mathrm{Ti}}_2\mathrm{C}$ is found to weaken the interaction between graphene and ${\mathrm{Ti}}_2\mathrm{C}$ substantially, making it a physisorption process rather than chemisorption in the unpassivated situation. Our first-principles study is expected to motivate future experimental investigation.

Strong coupling between the electronic states of monolayer transition metal dichalcogenides (TMDCs) such as ${\mathrm{MoS}}_2,{\mathrm{MoSe}}_2,{\mathrm{WS}}_2$, or ${\mathrm{WSe}}_2$, and a single in-plane optical cavity mode gives rise to valley- and spin-dependent cavity-QED effects. The Dirac Hamiltonian for this two-dimensional gapped semiconductor with large spin-orbit coupling facilitates pure Jaynes-Cummings-type coupling with spin-valley locking—providing an additional handle for spintronics using circularly polarized light. Besides being an on-chip light source, the strong cavity coupling causes the TMDC monolayer to act as a spontaneous spin oscillator. In addition, this system can be a sensitive magnetic field sensor for an in-plane magnetic field. It also displays unusual persistent Rabi oscillations between different conduction-band states that are insensitive to small magnetic field variations. Our analysis for dissipation due to finite exciton relaxation times and cavity losses suggests that these effects are observable.

Photonic crystal topological insulators host protected states at their edges. In the band structure these edge states appear as continuous bands crossing the photonic band gap. They allow light to propagate unidirectionally and without scattering. In practice it is essential to make devices relying on these effects as miniature as possible. Here we study photonic topological insulator particles (finite crystals). In such particles the edge state frequencies are discrete. Nevertheless, the discrete states support pseudospin-dependent unidirectional propagation. They allow light to bend around sharp corners similarly to the continuous edge states and act as topologically protected whispering gallery modes, which can store and filter light as well as manipulate its angular momentum. Though we consider a particular all-dielectric realization that does not require a magnetic field, the results in the findings are general, explaining multiple experimental observations of discrete transmission peaks in photonic topological insulators.