Motivated by the difference between the dynamics of magnetization textures in ferromagnets and antiferromagnets, the Landau-Lifshitz equation of motion is explored. A typical one-dimensional domain wall in a bulk ferromagnet with biaxial magnetic anisotropy is considered. In the framework of Walker-type solutions of steady-state ferromagnetic domain wall motion, the reduction of the nonlinear Landau-Lifshitz equation to a Lorentz-invariant sine-Gordon equation typical for antiferromagnets is formally possible for velocities lower than a critical velocity of the topological soliton. The velocity dependence of the domain wall energy and the domain wall width are expressed in the relativistic-like form in the limit of large ratio of the easy-plane/easy-axis anisotropy constants. It is shown that the mapping of the Landau-Lifshitz equation of motion to the sine-Gordon equation can be performed only by going beyond the steady-motion Walker-type solutions.

In organic-inorganic perovskite semiconductor methylammonium lead iodide the occurrence of magnetism has been confirmed from magnetization and ${}^1\mathrm{H}$ nuclear magnetic resonance (NMR) measurements. From room temperature down to 4 K, the magnetization is a combination of weak ferromagnetism and paramagnetism. ${}^1\mathrm{H}$ NMR spectra at low temperatures also reveal two components, one with a small chemical shift characteristic of ${}^1\mathrm{H}$ resonance in nonmetals, and the other with a much larger shift of $\sim -100$ ppm that is ascribed to the ferromagnetism of the material. The analysis of NMR spectra as well as that of ${}^1\mathrm{H}$ spin-lattice relaxation rate ($1/T_1$) indicates that disorder produces an inhomogeneous distribution of magnetic moments that order ferromagnetically where they are dense enough and otherwise behave as random paramagnetic impurities. It is shown that magnetism arises from iodine and lead vacancies present in the orthorhombic and cubic crystal phases of the material.

The heterometallic ludwigite system ${\mathrm{Fe}}_{3-x}{\mathrm{Mn}}_x{\mathrm{BO}}_5$, recently investigated for its ferroelectric properties, has been studied using synchrotron and neutron diffraction, combined with x-ray absorption near-edge structure and magnetization measurements. The results show that the Pbam crystal structure is preserved with little structural distortions up to $x=1.5$, and that divalent Mn is substituted preferentially on the 3LL2 sublattice unit. As $x$ increases, and up to ${\mathrm{Fe}}_2{\mathrm{MnBO}}_5$, the decoupled magnetic sublattice character of ${\mathrm{Fe}}_3{\mathrm{BO}}_5$ is preserved: magnetic order on 3LL1 [${\mathbf{k}}_1=(00\frac{1}{2}$), moments along $b$] survives with reduced magnetic moments, while the correlation length of the magnetic order on 3LL2 [${\mathbf{k}}_2=\left(000\right)$, moments along $a$] decreases. In contrast, for $x=1.5$, a $\mathbf{k}$ = (0 0 0) magnetic ordering, coupling both sublattices, is observed, with all moments aligned along $c$. These results provide insight on the physical properties of the system, which are discussed in terms of three main parameters : (i) nonlinear evolution of the substitution on each sublattice, (ii) changes in the direct-exchange and superexchange couplings as ${\mathrm{Mn}}^{2+}$ ($3d^5$, isoelectronic with ${\mathrm{Fe}}^{3+}$, is introduced in the structure), and (iii) competing easy-axis anisotropy and magnetic exchanges along the 3LL legs in the decoupled sub-lattice regime. These three parameters are at the origin of an extremely rich (x, T) magnetic phase diagram in the ${\mathrm{Fe}}_{3-x}{\mathrm{Mn}}_x{\mathrm{BO}}_5$ system.

Topological magnon is a vibrant research field gaining more and more attention in the past few years. Among many theoretical proposals and limited experimental studies, ferromagnetic kagome lattice emerges as one of the most elucidating systems. Here we report neutron scattering studies of ${\mathrm{YMn}}_6{\mathrm{Sn}}_6$, a metallic system consisting of ferromagnetic kagome planes. This system undergoes a commensurate-to-incommensurate antiferromagnetic phase transition upon cooling with the incommensurability along the out-of-plane direction. We observe magnon band gap opening at the symmetry-protected $K$ points and ascribe this feature to the antisymmetric Dzyaloshinskii-Moriya (DM) interactions. Our observation supports the existence of topological Dirac magnons in both the commensurate collinear and incommensurate coplanar magnetic orders, which is further corroborated by symmetry analysis. This finding places ${\mathrm{YMn}}_6{\mathrm{Sn}}_6$ as a promising candidate for room-temperature magnon spintronics applications.

Recent dynamic compression experiments [M. D. Knudson et al., Science 348, 1455 (2015); P. M. Celliers et al., Science 361, 677 (2018)] have observed the insulator-metal transition in dense liquid deuterium, but with an approximately 95-GPa difference in the quoted pressures for the transition at comparable estimated temperatures. It was claimed in the latter of these two papers that a very large latent heat effect on the temperature was overlooked in the first, requiring correction of those temperatures downward by a factor of 2, thereby putting both experiments on the same theoretical phase boundary and reconciling the pressure discrepancy. We have performed extensive path-integral molecular dynamics calculations with density functional theory to directly calculate the isentropic temperature drop due to latent heat in the insulator-metal transition for dense liquid deuterium and show that this large temperature drop is not consistent with the underlying thermodynamics.

While a pure quantum state may accumulate both the Berry phase and dynamic phase as it undergoes a cyclic path in the parameter space, the situation is more complicated when mixed quantum states are considered. From the Ulhmann bundle, a mixed quantum state can accumulate the Ulhmann phase if the parallel-transport condition is satisfied. However, we show that the Ulhmann process is in general not compatible with the evolution equation of the density matrix governed by the Hamiltonian. Thus a mixed quantum state usually accumulates a dynamic phase during its time evolution. We present the expression of the dynamic phase for mixed quantum states. In examples of one-dimensional two-band models and simple harmonic oscillator, the dynamic phase can take multiple discrete values in cyclic, quasistatic processes at infinitely high temperature due to the resonant points. However, the behavior differs if the energy spectrum is continuous without a band gap. Moreover, there is no natural analog of the dynamic phase in classical systems.

This study is focused on the quantum dynamics of a nitrogen-vacancy (NV) center coupled to a nonlinear, periodically driven mechanical oscillator. For a continuous periodic driving that depends on the position of the oscillator, the mechanical motion is described by Mathieu elliptic functions. This solution is employed to study the dynamics of the quantum spin system including environmental effects and to evaluate the purity and the von Neumann entropy of the NV spin. The unitary generation of coherence is addressed. We observe that the production of coherence through a unitary transformation depends on whether the system is prepared initially in the mixed state. Production of coherence is efficient when the system initially is prepared in the region of the separatrix (i.e., the region where classical systems exhibit dynamical chaos). From the theory of dynamical chaos, we know that phase trajectories of the system passing through the homoclinic tangle have limited memory, and therefore the information about the initial conditions is lost. We proved that quantum chaos and diminishing of information about the mixed initial state favors the generation of quantum coherence through the unitary evolution. We introduced quantum distance from the homoclinic tangle and proved that for the initial states permitting efficient generation of coherence, this distance is minimal.

We present a first-principles study of spin properties of the triplet ${\mathit{TM}}_3{\mathrm{O}}_3^{0/+}\left(\mathit{TM}=\mathrm{Fe},\mathrm{Co},\mathrm{Ni}\right)$ clusters and their laser-induced ultrafast spin dynamics. The differences in geometries, infrared spectra, and level distributions of the three structures are discussed. It is found that the spin localization of the clusters is both structurally and magnetic-field dependent. Specifically, within the same magnetic field, the number of the spin-localized states decreases when the magnetic center changes from Fe via Co to Ni. Even for the same cluster, the spin-localized states may differ under different field directions, due to the fact that spin magnitudes and spin directions of the states change with magnetic field. For the spin dynamics, the results indicate that the system which possesses more spin-localized states normally exhibits rich spin functionalities. Among the various achieved laser-induced ultrafast spin-transfer scenarios, a spin-transfer cycle in ${\mathrm{Fe}}_3{\mathrm{O}}_3$, a partial demagnetization process in ${\mathrm{Co}}_3{\mathrm{O}}_3^{+}$, and a spin bifurcation scenario in ${\mathrm{Ni}}_3{\mathrm{O}}_3$ are presented and analyzed, respectively. These results provide added information for the varieties of ultrafast optical control of magnetism in transition-metal oxide systems, and pave the way toward future related device design and molecular spintronics applications.

We study the effect of short-range interactions in three-dimensional nodal-line semimetals with linear band crossings. We analyze the Yukawa theories for gapped instabilities in the charge, spin, and superconducting channels using the Wilsonian renormalization group framework, employing a large number of fermion flavors $N_f$ for analytical control. We obtain stable nontrivial fixed points and provide a unified description of the critical exponents for the ordering transitions in terms of the number of order parameter components $N_b$ systematically to order $1/N_f$. We show that in all cases, the dynamical exponent $z=1$ in one loop, whereas $1/N_f$ corrections to various exponents follow from the anomalous dimension of the bosonic fields only.

Band engineering in layered transition metal dichalcogenides (TMDs) leads to a variety of emergent phenomena and has obtained considerable attention recently. Transition metal ditelluride ${\mathrm{NiTe}}_2$ has been discovered experimentally to be a type-II Dirac semimetal at ambient pressure, and was predicted to display superconductivity in the monolayer limit. Here we systematically investigate the structural and electronic properties of type-II Dirac semimetal ${\mathrm{NiTe}}_2$ under high pressure. Room-temperature synchrotron x-ray diffraction and Raman scattering measurements reveal the stability of the pristine hexagonal phase up to 52.2 GPa, whereas both the pressure coefficient and linewidth of Raman mode $E_g$ exhibit anomalies at a critical pressure $P_{\mathrm{c}}\sim 16\mathrm{GPa}$. Meantime, Hall resistivity measurement indicates that the hole-dominated behavior maintains up to 15.6 GPa and transforms into electron-dominated behavior at higher pressures. Our findings consistently demonstrate a pressure-induced Lifshitz transition in type-II Dirac semimetal ${\mathrm{NiTe}}_2$.

The coherence of collective modes, such as phonons and polarons, and their modulation of electronic states is long sought in complex systems, which is a crosscutting issue in photovoltaics and quantum electronics. In photovoltaic cells and lasers based on metal halide perovskites, the presence of polarons, i.e., photocarriers dressed by the macroscopic motion of charged lattice, assisted by terahertz (THz) longitudinal-optical (LO) phonons, has been intensely studied yet is still debated. This may be key for explaining the remarkable properties of the perovskite materials, e.g., defect tolerance, long charge lifetimes, and diffusion lengths. Here we use the intense single-cycle THz pulse with peak electric field up to $E_{\mathrm{THz}}=1000$ kV/cm to drive coherent polaronic band-edge oscillations at room temperature in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3\left({\mathrm{MAPbI}}_3\right)$. We reveal the oscillatory behavior is dominated by a specific quantized lattice vibration mode at ${\omega }_{\mathrm{LO}}\sim 4\mathrm{THz}$, which is both dipole and momentum forbidden. THz-driven coherent polaron dynamics exhibits distinguishing features: room temperature coherent oscillations at ${\omega }_{\mathrm{LO}}$ longer than 1 ps in both single crystals and thin films, mode-selective modulation of different band-edge states assisted by electron-phonon interaction, and dynamic mode splitting at low temperature due to entropy and anharmonicity of organic cations. Our results demonstrate intense THz-driven coherent band-edge modulation is a powerful probe of electron-lattice coupling phenomena and polaronic quantum control in perovskites.

We study the role of charge density wave fluctuations on the temperature dependence of Seebeck coefficient in quasi-one-dimensional conductors with a Peierls instability. The description of low-dimensional incommensurate charge density wave fluctuations as obtained by a generalized Ginzburg-Landau approach for arrays of weakly coupled chains is embodied in the numerical solution of the semiclassical Boltzmann transport equation. The energy and temperature dependence of the scattering time of electrons on fluctuations can then be extracted and its influence on the Seebeck coefficient calculated. The connection between theory and experiments carried out on molecular conductors is presented and critically discussed.

We study electron-phonon interaction and related transport properties of nodal-line semimetal ZrSiS using first-principles calculations. We find that ZrSiS is characterized by a weak electron-phonon coupling on the order of 0.1, which is almost energy independent. The main contribution to the electron-phonon coupling originates from long-wavelength optical phonons, causing no significant renormalization of the electron spectral function. At the charge neutrality point, we find that electrons and holes provide a comparable contribution to the scattering rate. The phonon-limited resistivity calculated within the Boltzmann transport theory is found to be strongly direction-dependent with the ratio between out-of-plane and in-plane directions being ${\rho }_{zz}/{\rho }_{xx}\sim 7.5$, mainly determined by the anisotropy of carrier velocities. We estimate zero-field resistivity to be ${\rho }_{xx}\approx 12\mu \mathrm{\Omega }\mathrm{cm}$ at 300 K, which is in good agreement with experimental data. Relatively small resistivity in ZrSiS can be attributed to a combination of weak electron-phonon coupling and high carrier velocities.

The existence of robust chiral edge states in a finite topologically nontrivial Chern insulator is a consequence of the bulk-boundary correspondence. In this paper, we present a theoretical framework based on lattice Green's function to study the scattering of such chiral edge electrons by a single localized impurity. To this end, in the first step, we consider the standard topological Haldane model on a honeycomb lattice with strip geometry. We obtain analytical expressions for the wave functions and their corresponding energy dispersion of the low-energy chiral states localized at the edge of the ribbon. Then, we employ the $T$-matrix Lippmann-Schwinger approach to explicitly show the robustness of chiral edge states against the impurity scattering. This backscattering-free process has an interesting property that the transmitted wave function acquires an additional phase factor. Although this additional phase factor does not affect quantum transport through the chiral channel, it can carry quantum information. As an example of such quantum information transport, we investigate the entanglement of two magnetic impurities in a Chern insulator through the dissipationless scattering of chiral electrons.

Impurities are unavoidable during the preparation of graphene samples and play an important role in graphene's electronic properties when they are adsorbed on the graphene surface. In this work, we study the electronic structures and transport properties of a two-terminal zigzag graphene nanoribbon (ZGNR) device whose scattering region is covered by various adsorbates within the framework of the tight-binding approximation, by taking into account the coupling strength $\gamma$ between adsorbates and carbon atoms, the adsorbate concentration $n_i$, and the on-site energy disorder of adsorbates. Our results indicate that when the scattering region is fully covered by homogeneous adsorbates, i.e., $n_i=1$, a transmission gap opens around the Dirac point and its width is almost proportional to ${\gamma }^2$. In particular, two conductance plateaus of $G=2e^2/h$ appear in the vicinity of the electron energy $E=\pm \gamma$. When the scattering region is partially covered by homogeneous adsorbates ($0<n_i<1$), the transmission gap still survives around the Dirac point even at low $n_i$, and its width is firstly increased by $n_i$ and then declined by further increasing $n_i$; whereas the conductance decreases with $n_i$ in the regime of low $n_i$ and increases with $n_i$ in the regime of high $n_i$. While in the presence of disordered adsorbates whose on-site energies are random variables characterized by the disorder degree, the transmission gap disappears at low $n_i$ and reappears at relatively high $n_i$. Furthermore, the transmission ability of the ZGNR device can be enhanced by the adsorbate disorder when the disorder degree surpasses a critical value, contrary to the localization picture that the conduction of a nanowire becomes poorer with increasing the disorder degree. The physics underlying these transport characteristics is discussed. Our results are in good agreement with experiments and may help for engineering graphene devices.

In previous studies, the topological invariants of one-dimensional non-Hermitian systems have been defined in open boundary condition (OBC) to satisfy the bulk-boundary correspondence. The extreme sensitivity of bulk energy spectra to boundary conditions has been attributed to the breakdown of the conventional bulk-boundary correspondence based on the topological invariants defined under periodic boundary condition (PBC). Here we propose non-Hermitian many-body polarization as a topological invariant for 1D non-Hermitian systems defined in PBC, which satisfies the bulk-boundary correspondence. Employing many-body methodology in the non-Hermitian Su-Schrieffer-Heeger model for fermions, we show the absence of non-Hermitian skin effect due to the Pauli exclusion principle and demonstrate the bulk-boundary correspondence using the invariant defined under PBC. Moreover, we show that the bulk topological invariant is quantized in the presence of chiral or generalized inversion symmetry. Our study suggests the existence of generalized crystalline symmetries in non-Hermitian systems, which give quantized topological invariants that capture the symmetry-protected topology of non-Hermitian systems.

Motivated by recent proposals of experimental realization of fast scramblers, we study a family of solvable variants of the ($q=4$) Sachdev-Ye-Kitaev model in which the rank and eigenvalue distribution of the coupling matrix $J_{ij,kl}$ are tuneable. When the rank is proportional to the number of fermions, the low temperature behavior is sensitive to the eigenvalue distribution. We obtain a complete classification of the possible non-Fermi liquid quantum phases. These include two previously studied phases whose fermion scaling dimension depends continuously on the rank; we show that they are maximally chaotic, but necessitate an extensively degenerate or negative semidefinite coupling matrix. More generic distributions give rise to “almost Fermi liquids” with a scaling dimension $\mathrm{\Delta }=1/2$, but which differ from a genuine Fermi liquid in quasiparticle decay rate, quantum Lyapunov exponent, and/or specific heat.

A system of confined charged electrons interacting via the long-range Coulomb force can form a Wigner crystal due to their mutual repulsion. This happens when the potential energy of the system dominates over its kinetic energy, i.e., at low temperatures for a classical system and at low densities for a quantum one. At $T=0$, the system is governed by quantum mechanics, and hence the spatial density peaks associated with crystalline charge localization are sharpened for a lower average density. Conversely, in the classical limit of high temperatures, the crystalline spatial density peaks are suppressed (recovered) at a lower (higher) average density. In this paper, we study those two limits separately using an exact diagonalization of small one-dimensional (1D) systems containing few ($<10$) electrons and propose an approximate method to connect them into a unified effective phase diagram for Wigner few-electron crystallization. The result is a qualitative quantum-classical crossover phase diagram of an effective 1D Wigner crystal. We show that although such a 1D system is at best an effective crystal with no true long-range order (and thus no real phase transition), the spatial density peaks associated with the quasicrystallization should be experimentally observable in a few-electron 1D system. We find that the effective crystalline structure slowly disappears with both the crossover average density and crossover temperature for crystallization decreasing with increasing particle number, consistent with the absence of any true long-range 1D order. Thus, an effective few-electron 1D Wigner crystal may be construed either as existing at all densities (manifesting short-range order) or as nonexisting at all densities (not manifesting any long-range order). Within one unified description, we show through exact theoretical calculations how a small 1D system interacting through the long-range Coulomb interaction could manifest effective Wigner solid behavior both in classical and quantum regimes. In fact, one peculiar aspect of the effective finite-size nature of 1D Wigner crystallization we find is that even a short-range interaction would lead to a finite-size 1D crystal, except that the crystalline order vanishes much faster with increasing system size in the short-range interacting system compared with the long-range interacting one.