The Mott transition from a dipolar excitonic to an electron-hole plasma state is demonstrated in a wide GaN/(Al,Ga)N quantum well at $T=7$ K by means of spatially resolved magnetophotoluminescence spectroscopy. Increasing optical excitation density, we drive the system from the excitonic state, characterized by a diamagnetic behavior and thus a quadratic energy dependence on the magnetic field, to the unbound electron-hole state, characterized by a linear shift of the emission energy with the magnetic field. The complexity of the system requires taking into account both the density dependence of the exciton binding energy and the exciton-exciton interaction and correlation energy that are of the same order of magnitude. We estimate the carrier density at Mott transition as $n_{\mathrm{Mott}}\approx 2\times {10}^{11} {\mathrm{cm}}^{-2}$ and address the role played by excitonic correlations in this process. Our results strongly rely on the spatial resolution of the photoluminescence and the assessment of the carrier transport. We show that in contrast to GaAs/(Al,Ga)As systems, where transport of dipolar magnetoexcitons is strongly quenched by the magnetic field due to exciton mass enhancement, in GaN/(Al,Ga)N the band parameters are such that the transport is preserved up to 9 T.

The fluctuations of the heat current in a quantum dot coupled to electron reservoirs are calculated at finite frequency, voltage, and temperature using the nonequilibrium Green function technique. The nonsymmetrized heat noise is expressed as an integral on energy containing four contributions, each of which includes transmission amplitudes, electron-hole pair distribution functions, and energy difference factors. The effect of the asymmetry of the couplings between the quantum dot and the reservoirs is studied. Features of the heat noise are highlighted and discussed for an equilibrium and an out-of-equilibrium quantum dot. In the latter case and within the high transmission limit, the heat noise is closely related to the radiative power spectrum, leading to an out-of-equilibrium Planck's law. Proposals for the measurement of the heat noise are discussed.

Deep defects in wide band gap semiconductors have emerged as leading qubit candidates for realizing quantum sensing and information applications. Due to the spatial localization of the defect states, these deep defects can be considered as artificial atoms/molecules in a solid state matrix. Here we show that unlike single-particle treatments, the multiconfigurational quantum chemistry methods, traditionally reserved for atoms/molecules, accurately describe the many-body characteristics of the electronic states of these defect centers and correctly predict properties that single-particle treatments fail to obtain. We choose the negatively charged nitrogen-vacancy (${\mathrm{NV}}^{-}$) center in diamond as the prototype defect to study with these techniques due to its importance for quantum information applications and because its properties are well known, which makes it an ideal benchmark system. By properly accounting for electron correlations and including spin-orbit coupling and dipolar spin-spin coupling in the quantum chemistry calculations, for the ${\mathrm{NV}}^{-}$ center in diamond clusters, we are able to: (i) show the correct splitting of the ground (first-excited) spin-triplet state into two levels (four levels), (ii) calculate zero-field splitting values of the ground and excited spin-triplet states, in good agreement with experiment, (iii) determine many-body configurations of the spin-singlet states, and (iv) calculate the energy differences between the ground and exited spin-triplet and spin-singlet states, as well as their ordering, which are also found to be in good agreement with recent experimental data. The numerical procedure we have developed is general, and it can screen other color centers whose properties are not well known but promising for applications.

We propose a general analytic method to study the localization transition in one-dimensional quasicrystals with parity-time ($\mathcal{PT}$) symmetry, described by complex quasiperiodic mosaic lattice models. By applying Avila's global theory of quasiperiodic Schrödinger operators, we obtain exact mobility edges and prove that the mobility edge is identical to the boundary of $\mathcal{PT}$-symmetry breaking, which also proves the existence of correspondence between extended (localized) states and $\mathcal{PT}$-symmetry ($\mathcal{PT}$-symmetry-broken) states. Furthermore, we generalize the models to more general cases with nonreciprocal hopping, which breaks $\mathcal{PT}$ symmetry and generally induces skin effect, and obtain a general and analytical expression of mobility edges. While the localized states are not sensitive to the boundary conditions, the extended states become skin states when the periodic boundary condition is changed to open boundary condition. This indicates that the skin states and localized states can coexist with their boundary determined by the mobility edges.

Electron transport in magnetic orders and the magnetic order dynamics have a mutual dependence, which provides the key mechanisms in spin-dependent phenomena. Recently, antiferromagnetic orders have been focused on as the magnetic order, where current-induced spin-transfer torques, a typical effect of electron transport on the magnetic order, have been debatable, mainly because of the lack of an analytic derivation based on quantum field theory. Here, we construct the microscopic theory of spin-transfer torques on the slowly varying staggered magnetization in antiferromagnets with weak canting. In our theory, the electron is captured by bonding/antibonding states, each of which is the eigenstate of the system, is doubly degenerate, and spatially spreads to sublattices because of electron hopping. The spin of the eigenstates depends on the momentum in general, and a nontrivial spin-momentum locking arises for the case with no site inversion symmetry, without considering any spin-orbit couplings. The spin current of the eigenstates includes an anomalous component proportional to a kind of gauge field defined by derivatives in momentum space and induces the adiabatic spin-transfer torques on the magnetization. Unexpectedly, we find that one of the nonadiabatic torques has the same form as the adiabatic spin-transfer torque, while the obtained forms for the adiabatic and nonadiabatic spin-transfer torques agree with the phenomenological derivation based on the symmetry consideration. This finding suggests that the conventional explanation for the spin-transfer torques in antiferromagnets should be changed. Our microscopic theory provides a fundamental understanding of spin-related physics in antiferromagnets.

We report a comprehensive investigation of the triple perovskite iridate ${\mathrm{Ba}}_3\mathrm{Co}{\mathrm{Ir}}_2{\mathrm{O}}_9$. Stabilizing in the hexagonal $P{6}_3/mmc$ symmetry at room temperature, this system transforms to a monoclinic $C2/c$ symmetry at the magnetic phase transition. On further reduction in temperature, the system partially distorts to an even lower symmetry ($P2/c$), with both these structurally disparate phases coexisting down to the lowest measured temperatures. The magnetic structure as determined from neutron diffraction data indicates a weakly canted antiferromagnetic structure, which is also supported by first-principles calculations. Theory indicates that the ${\mathrm{Ir}}^{5+}$ carries a finite magnetic moment, which is also consistent with the neutron data. This suggests that the putative $J=0$ state is avoided. Measurements of heat capacity, electrical resistance noise, and dielectric susceptibility all point toward the stabilization of a highly correlated ground state in the ${\mathrm{Ba}}_3\mathrm{Co}{\mathrm{Ir}}_2{\mathrm{O}}_9$ system.

Concerning the magnetic anisotropy and magnetic moments of the $M^{3+}$ $(M=\mathrm{V}, \mathrm{Cr})$ ions in ferromagnetic $\left(\mathrm{FM}\right){M\mathrm{I}}_3$ monolayers, which have a honeycomb pattern of edge-sharing ${M\mathrm{I}}_6$ octahedra, conflicting observations have been reported in experimental and theoretical studies. We resolve these conflicts by determining the magnetic anisotropy energies for the $M^{3+}$ ions of ${M\mathrm{I}}_3$ monolayers, by analyzing their preferred spin orientations in terms of the selection rules based on the highest occupied molecular orbital–lowest unoccupied molecular orbital interactions of the ${M\mathrm{I}}_6$ octahedra, and by discussing whether or not the $M^{3+}$ ions are uniaxial. Here we show that the FM monolayer ${\mathrm{VI}}_3$ is uniaxial, but that of ${\mathrm{CrI}}_3$ is not. The magnetic anisotropy energy for the ${\mathrm{V}}^{3+}\left(d^2,S=1\right)$ ion of ${\mathrm{VI}}_3$ is greater than that for the ${\mathrm{Cr}}^{3+}$ $(d^3,$ $S=3/2)$ ion of ${\mathrm{CrI}}_3$ by more than an order of magnitude (i.e., ∼8 vs ∼0.6 meV). The ${\mathrm{V}}^{3+}$ ion exhibits uniaxial magnetism because its orbital quantum number L is not zero $\left(L=1\right)$, in contrast to the ${\mathrm{Cr}}^{3+}$ ion $\left(L=0\right)$.

The magneto-optical (MO) effects are a powerful probe of magnetism and electronic structure of magnetic solids but also have valuable applications in high-density data-storage technology. Yttrium iron garnet (${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$) (YIG) and bismuth iron garnet (${\mathrm{Bi}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$) (BIG) are two widely used magnetic semiconductors with significant magneto-optical effects. In this paper, we present a thorough theoretical investigation on magnetism, electronic, optical, and MO properties of YIG and BIG, based on the density functional theory with the generalized gradient approximation plus on-site Coulomb repulsion. We find that YIG exhibits significant MO Kerr and Faraday effects in UV frequency range that are comparable to ferromagnetic iron. Moreover, BIG shows gigantic MO effects in visible frequency region that are several times larger than YIG. This is because the calculated MO conductivity (${\sigma }_{xy}$) of BIG is nearly ten times larger than that of YIG. Interestingly, the calculated band structures reveal that BIG is a single spin semiconductor. They also show that in YIG, Y $sd$ orbitals mix mainly with the high-lying conduction bands, leaving Fe $d$ orbital dominated lower conduction bands almost unaffected by the spin-orbit coupling (SOC) on the Y atom. In contrast, Bi $p$ orbitals in BIG hybridize significantly with Fe $d$ orbitals in the lower conduction bands, leading to large SOC-induced band splitting in the bands. Consequently, the MO transitions between the upper valence bands and lower conduction bands are greatly enhanced when Y is replaced by heavier Bi. Our calculated Kerr and Faraday rotation angles of YIG as well as Faraday rotation angles for BIG agree well with the available experimental values. Thus, we believe that our predicted giant MO Kerr effect in BIG will stimulate further MOKE experiments on high-quality BIG crystals. This work thus shows that the iron garnets not only offer a useful platform for exploring the interplay of spin current, magnetism, and optics degrees of freedom, but also have promising applications in high-density MO data-storage and low-power consumption spintronic nanodevices.

We study the magnetic coupling in a thin film bilayer exchange-spring magnet composed of an amorphous ${\mathrm{Tb}}_{10}{\mathrm{Co}}_{90}$ layer with a large perpendicular anisotropy and an amorphous ${\mathrm{Co}}_{85}{\left({\mathrm{Al}}_{70}{\mathrm{Zr}}_{30}\right)}_{15}$ layer with uniaxial in-plane anisotropy. The CoAlZr is directly exchange coupled to the TbCo with an interface region of at least 7.5 nm within which the CoAlZr magnetization is perpendicular to the plane and switches with the underlying TbCo. The influence of the coupling extends up to 30 nm into the CoAlZr resulting in an effective tilted anisotropy. The coercivity of the CoAlZr is greatly enhanced due to the coupling and is tuneable over a large range by varying its thickness.

Selected members of the $\mathrm{La}\left(\mathrm{Fe},\mathrm{Mn}\right)\mathrm{As}\left({\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}\right)$ system were analyzed using high-resolution synchrotron x-ray powder diffraction. The tetragonal to orthorhombic structural transition is progressively recovered in the optimally electron-doped $\mathrm{LaFeAs}\left({\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}\right)$ phase by very light Mn substitution; at the same time, superconductivity is suppressed whereas magnetic ordering is restored. Distinct incommensurate satellite peaks develop within different thermal ranges and mark the occurrence of charge density waves characterized by distinct propagation wave vectors, as well as multiple incommensurate structural transitions; in particular, some of them arise in conjunction with the structural transformation process, disappearing after the completion of the dissymmetrization. The thermal evolution of satellite reflections observed at $\mathbf{Q}\sim 1.93{\AA }^{–1}$ indicates a strong competition between the charge density waves and the superconductive state. A phase diagram of the $\mathrm{La}\left(\mathrm{Fe},\mathrm{Mn}\right)\mathrm{As}\left({\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}\right)$ system is drawn on the basis of the structural, magnetic, and electronic properties of the analyzed samples.

Electron-phonon coupling in $\mathrm{Zr}{\mathrm{Co}}_2\mathrm{Sn}$ is investigated within the density-functional theory and linear-response approach in the mixed-basis pseudopotential representation. Phonon-induced scattering is analyzed for excited electrons (holes) in two majority bands, which form the topological Weyl state. Although the electron-phonon coupling in the energy range under consideration demonstrates some dependence on the available phase space, the strength of the phonon-mediated scattering, $\lambda$, changes rather weakly with the hole (electron) energy and momentum, varying between 0.1 and 0.3. The momentum-averaged value of $\lambda$ ranges from 0.25 to 0.45. The phonon-mediated scattering of electrons near the Weyl point is found to be weak enough to warrant well-defined quasiparticles. Most of the modes actively involved in electron scattering are middle- and high-frequency lattice vibrations, while the participation of low-energy acoustic phonons is significantly suppressed by electron-phonon matrix elements. Of the long-wavelength optical phonons, only the $T_{2g}$ Raman active modes generated by opposite vibrations of Co atoms make a noticeable contribution to the scattering of electrons.

Ultrafast control of the magnetization in ps timescales by fs laser pulses offers an attractive avenue for applications such as fast magnetic devices for logic and memory. However, ultrafast helicity-independent all-optical switching (HI-AOS) of the magnetization has thus far only been observed in Gd-based, ferrimagnetic amorphous ($a$-) rare earth-transition metal ($a$-RE-TM) systems, and a comprehensive understanding of the reversal mechanism remains elusive. Here, we report HI-AOS in ferrimagnetic $a\text{−}{\mathrm{Gd}}_{22-x}{\mathrm{Tb}}_x{\mathrm{Co}}_{78}$ thin films, from $x=0$ to 18, and elucidate the role of Gd in HI-AOS in $a$-RE-TM alloys and multilayers. Increasing Tb content results in increasing perpendicular magnetic anisotropy and coercivity, without modifying magnetization density, and slower remagnetization rates and higher critical fluences for switching but still shows picosecond HI-AOS. Simulations of the atomistic spin dynamics based on the two-temperature model reproduce these results qualitatively and predict that the lower damping on the RE sublattice arising from the small spin-orbit coupling of Gd (with $L=0$) is instrumental for the faster dynamics and lower critical fluences of the Gd-rich alloys. Annealing $a\text{−}{\mathrm{Gd}}_{10}{\mathrm{Tb}}_{12}{\mathrm{Co}}_{78}$ leads to slower dynamics which we argue is due to an increase in damping. These simulations strongly indicate that accounting for element-specific damping is crucial in understanding HI-AOS phenomena. The results suggest that engineering the element-specific damping of materials can open up new classes of materials that exhibit low-energy, ultrafast HI-AOS.

We theoretically investigate the origin of the square-type skyrmion crystal in centrosymmetric itinerant magnets, motivated by the recent experimental findings in ${\mathrm{GdRu}}_2{\mathrm{Si}}_2$ [N. D. Khanh et al., Nat. Nanotech. 15, 444 (2020)]. By simulated annealing for an effective spin model derived from the Kondo lattice model on a square lattice, we find that a square skyrmion crystal composed of a superposition of two spin helices is stabilized in a magnetic field by synergy between the positive biquadratic, bond-dependent anisotropic, and easy-axis anisotropic interactions. This is in stark contrast to triangular skyrmion crystals, which are stabilized by only one of the three, suggesting that the square skyrmion crystal is characteristic of itinerant magnets with magnetic anisotropy. We also show that a variety of noncollinear and noncoplanar spin textures appear, depending on the model parameters as well as the applied magnetic field. The present systematic study will be useful not only for identifying the key ingredients in ${\mathrm{GdRu}}_2{\mathrm{Si}}_2$ but also for exploring further skyrmion-hosting materials in centrosymmetric itinerant magnets.

A kinetic approach to the heat transport by phonons and magnons through a ferromagnetic insulator (FI) layer located between two massive insulators $(I_1$ and $I_2)$ is analytically considered. The effective transverse heat conductivity of such a layered system with an arbitrary thickness of the FI layer is calculated, and the thickness at which the size effect is manifested in the thermal conductivity is found.

Magnetic droplets are dissipative magnetodynamical solitons that can form under current driven nanocontacts in magnetic layers with large perpendicular magnetic anisotropy. Here, we extend the original droplet theory by studying the impact of the dipolar interactions on the dynamics of droplet solitons. By varying the thickness of the free layer of a spin torque nano-oscillator, we systematically tune the internal field of the free layer to investigate the dynamics of droplet solitons. Our numerical results show that increasing the free layer thickness increases the droplet's threshold current, decreases the droplet's frequency and diameter, enlarges the current hysteresis, and also modifies the structure of the droplet. The Oersted field of the current breaks the precessional phase coherency and deteriorates the stability of the droplet in free layers with larger thicknesses. Moreover, our findings show a simple relation to determine the impact of the free layer thickness on the droplet's nucleation boundaries. Our study presents the missing brick on the physics behind magnetic droplet solitons, and further illustrates that magnetic droplets in thinner layers possess more promising characteristics for spintronic applications and enable devices with higher speed of operation.

The $\mathrm{Ba}{\mathrm{Fe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$ compounds with $x=0$ (parent), $x=0.10$ (underdoped), $x=0.31$, 0.33, 0.53 (superconductors with $T_c=27.3$, 27.6, 13.9 K, respectively), and $x=0.70$, 0.77 (overdoped) have been investigated versus temperature using ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy. Special attention was paid to regions of the spin-density wave (SDW) antiferromagnetic order, spin-nematic phase, and superconducting transition. The $\mathrm{Ba}{\mathrm{Fe}}_2{\left({\mathrm{As}}_{0.90}{\mathrm{P}}_{0.10}\right)}_2$ compound exhibits a reduced amplitude of SDW as compared to the parent compound and preserved universality class of two-dimensional magnetic planes with one-dimensional spins. The spin-nematic phase region for $x=0.10$ is characterized by an incoherent magnetic order. $\mathrm{Ba}{\mathrm{Fe}}_2{\left({\mathrm{As}}_{0.69}{\mathrm{P}}_{0.31}\right)}_2$ shows coexistence of a weak magnetic order and superconductivity due to the vicinity of the quantum critical point. The charge-density modulations in the $\mathrm{Ba}{\mathrm{Fe}}_2{\left({\mathrm{As}}_{0.67}{\mathrm{P}}_{0.33}\right)}_2$ and $\mathrm{Ba}{\mathrm{Fe}}_2{\left({\mathrm{As}}_{0.47}{\mathrm{P}}_{0.53}\right)}_2$ superconductors are perturbed near $T_c$. Pronounced hump of the average quadrupole splitting across superconducting transition is observed for the system with $x=0.33$. The phosphorus substitution increases the Debye temperature of the $\mathrm{Ba}{\mathrm{Fe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$ compound. Moreover, experimental electron charge densities at Fe nuclei in this material conclusively show that it should be recognized as a hole-doped system. The measured Mössbauer spectral shift and spectral area are not affected by transition to the superconducting state. This indicates that neither the average electron density at Fe nuclei nor the dynamical properties of the Fe sublattice in $\mathrm{Ba}{\mathrm{Fe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$ are sensitive to the superconducting transition. Theoretical calculations of hyperfine parameters determining the patterns of Mössbauer spectra of $\mathrm{Ba}{\mathrm{Fe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$ with $x=0$, 0.31, 0.5, and 1.0 are performed within the framework of the density-functional theory. These simulations provide an insight into changes of the immediate neighborhood experienced by Fe atoms upon the P for As substitution as well as enable us to explore influence of P doping on the electron density, electric field gradient, and hyperfine field at Fe nuclei in the $\mathrm{Ba}{\mathrm{Fe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$ system.

We study the transport properties of a voltage-biased Josephson junction where the BCS superconducting leads are coupled via the edges of a quantum Hall sample. In this scenario, an out-of-equilibrium Josephson current develops, which is numerically studied within the Floquet-Keldysh Green's function formalism. We particularly focus on the time-averaged current as a function of both the bias voltage and the magnetic flux threading the sample and analyze the resonant multiple Andreev reflection processes that lead to an enhancement of the quasiparticle transmission. We find that a full tomography of the dc current in the voltage-flux plane allows for a complete spectroscopy of the one-way edge modes and could be used as a hallmark of chiral edge mediated transport in these hybrid devices.

Understanding the coexistence of ferromagnetism and insulating behavior in manganites is an unsolved problem. We propose a localized band model involving effective intermediate-range electron-electron (electron-hole) repulsion (attraction) generated by cooperative electron-phonon interaction. A double-exchange mechanism, involving holes virtually hopping to nearest neighbors and back, produces magnetic polarons in an antiferromagnetic environment; when these magnetic polarons coalesce and percolate the system, we get a ferromagnetic insulator. Ferromagnetism gets more pronounced when the concentration of holes (doping) increases or when the ratio of hopping to polaronic-energy dominates over the ratio of superexchange-coupling to hopping.

Optical measurements in doped Mott insulators have discovered the emergence of spectral weights at mid-infrared (MIR) upon chemical doping and photodoping. MIR weights may have a relation to string-type excitation of spins, which is induced by a doped hole generating misarranged spins with respect to their sublattice. There are two types of string effects: one is an $S^z$ string that is reparable by quantum spin flips and the other is a phase string irreparable by the spin flips. We investigate the effect of $S^z$ and phase strings on MIR weights. Calculating the optical conductivity of the single-hole Hubbard model in the strong-coupling regime and the $t\text{−}J$ model on two-leg ladders by using time-dependent Lanczos and density-matrix renormalization group, we find that phase strings make a crucial effect on the emergence of MIR weights as compared with $S^z$ strings. Our findings indicate that a mutual Chern-Simons gauge field acting between spin and charge degrees of freedom, which is the origin of phase strings, is significant for obtaining MIR weights. Conversely, if we remove this gauge field, no phase is picked up by a doped hole. As a result, a spin polaron accompanied by a local spin distortion emerges and a quasiparticle with a cosinelike energy dispersion is formed in single-particle spectral function. Furthermore, we suggest a Floquet engineering to examine the phase-string effect in cold atoms.