The magnetization configuration in two stacked micron-size ferromagnetic disks can assume different equilibrium states depending on the interfacial coupling between the disks. Here I examine the magnetization dynamics in response to an out-of-plane field pulse for different equilibrium states. For antiferromagnetic coupling, the response spectrum generally consists of a lower-frequency part and a higher-frequency part. The former is related to the response of the core region, which has a significant in-plane response coupled to the out-of-plane one; the latter is related to spin waves generated at the edges of the disk. For a meron structure the response in the two disks to an out-of-plane pulse is also asymmetric.

The $S=1/2$ antiferromagnetic Heisenberg spin chain compound ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$ doped with $1\%$ and $2\%$ of Ni impurities has been studied by means of ${}^{63}\mathrm{Cu}$ nuclear magnetic resonance. A strong decrease of the spin-lattice relaxation rate $T_1^{-1}$ at low temperatures points toward a spin gap, while a stretching exponent $\lambda <1$ and a frequency dependence of $T_1^{-1}$ indicate that this spin gap varies spatially and should rather be characterized as a spin pseudogap. The magnitude of the spin pseudogap scales with doping level. Our results therefore evidence the finite-size character of this phenomenon. Moreover, an unusual narrowing of the low-temperature NMR lines reveals the suppression of the impurity-induced staggered paramagnetic response with increasing doping level.

The properties of doped and natural impurities in lithium niobate single crystals are studied using the whispering gallery mode method at low temperatures as a function of magnetic field. The study reveals considerable coupling of microwave photon modes to the ${\mathrm{Fe}}^{3+}$ spin ensemble in iron-doped and nondoped crystals. The $S=5/2$ structure of the ${\mathrm{Fe}}^{3+}$ impurities demonstrates zero field splittings of $11.21$ and $20.96\mathrm{GHz}$, a significant asymmetry of the Zeeman lines, and additional lines with anomalous $g$ factors of $1.37$ and $3.95$. Also, interactions between different transitions of the ${\mathrm{Fe}}^{3+}$ ion are observed. An additional ion impurity ensemble with a splitting of about $1.7\mathrm{GHz}$ is shown to couple to the dominating ${\mathrm{Fe}}^{3+}$ spins, and the effect on the $Q$ factors of the microwave photon modes due to the ${\mathrm{Fe}}^{3+}$ ion ensemble is also demonstrated. Measurements down to less than one photon level are made, with a loss tangent of order ${10}^{-5}$ determined.

The quasiharmonic (QH) approximation uses harmonic vibrational frequencies ${\omega }_{Q,H}\left(V\right)$ computed at volumes $V$ near $V_0$ where the Born-Oppenheimer (BO) energy $E_{\mathrm{el}}\left(V\right)$ is minimum. When this is used in the harmonic free energy, QH approximation gives a good zeroth order theory of thermal expansion and first-order theory of bulk modulus, where $n^{\mathrm{th}}$-order means smaller than the leading term by ${\epsilon }^n$, where $\epsilon =\hslash {\omega }_{\mathrm{vib}}/E_{\mathrm{el}}$ or $k_{B}T/E_{\mathrm{el}}$, and $E_{\mathrm{el}}$ is an electronic energy scale, typically 2 to 10 eV. Experiment often shows evidence for next-order corrections. When such corrections are needed, anharmonic interactions must be included. The most accessible measure of anharmonicity is the quasiparticle (QP) energy ${\omega }_Q(V,T)$ seen experimentally by vibrational spectroscopy. However, this cannot just be inserted into the harmonic free energy $F_H$. In this paper, a free energy is found that corrects the double-counting of anharmonic interactions that is made when $F$ is approximated by $F_H({\omega }_Q(V,T))$. The term “QP thermodynamics” is used for this way of treating anharmonicity. It enables $(n+1)$-order corrections if QH theory is accurate to order $n$. This procedure is used to give corrections to the specific heat and volume thermal expansion. The QH formulas for isothermal $\left(B_T\right)$ and adiabatic $\left(B_S\right)$ bulk moduli are clarified, and the route to higher-order corrections is indicated.

We study the dynamics of skyrmions in Dzyaloshinskii-Moriya materials with easy-axis anisotropy. An important link between topology and dynamics is established through the construction of unambiguous conservation laws obtained earlier in connection with magnetic bubbles and vortices. In particular, we study the motion of a topological skyrmion with skyrmion number $Q=1$ and a nontopological skyrmionium with $Q=0$ under the influence of an applied field gradient. The $Q=1$ skyrmion undergoes Hall motion perpendicular to the direction of the field gradient with a drift velocity proportional to the gradient. In contrast, the nontopological $Q=0$ skyrmionium is accelerated in the direction of the field gradient, thus exhibiting ordinary Newtonian motion. When the applied field is switched off the $Q=1$ skyrmion is spontaneously pinned around a fixed guiding center, whereas the $Q=0$ skyrmionium moves with constant velocity $v$. We give a systematic calculation of a skyrmionium traveling with any constant velocity $v$ that is smaller than a critical velocity $v_c$.

The longitudinal spin Seebeck effect (LSSE) in $\mathrm{Pt}/{\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}\left(\mathrm{YIG}\right)$ junction systems has been investigated at various magnetic fields and temperatures. We found that the LSSE voltage in a Pt/YIG-slab system is suppressed by applying high magnetic fields and this suppression is critically enhanced at low temperatures. The field-induced suppression of the LSSE in the Pt/YIG-slab system is too large at around room temperature to be explained simply by considering the effect of the Zeeman gap in magnon excitation. This result requires us to introduce a magnon-frequency-dependent mechanism into the scenario of LSSE; low-frequency magnons dominantly contribute to the LSSE. The magnetic field dependence of the LSSE voltage was observed to change by changing the thickness of YIG, suggesting that the thermospin conversion by the low-frequency magnons is suppressed in thin YIG films due to the long characteristic lengths of such magnons.

We show that a physical realization of the phase diagram proposed by Minnhagen et al. [Phys. Rev. B 78, 184432 (2008)] for the so-called generalized fully frustrated XY model on a square lattice can be achieved in arrays of superconductor-ferromagnet-superconductor junctions near the transition of the junctions to the $\pi$ state. Moreover, the phase diagram with such a topology has to be reproduced twice, on both sides of the $0\text{−}\pi$ transition.

${\mathrm{Mo}}_3{\mathrm{Sb}}_7$ single crystals lightly doped with Cr, Ru, or Te are studied in order to explore the interplay between superconductivity, magnetism, and the cubic-tetragonal structural transition. The structural transition at 53 K is extremely sensitive to Ru or Te substitution which introduces additional electrons, but robust against Cr substitution. No sign of a structural transition was observed in superconducting ${\mathrm{Mo}}_{2.91}{\mathrm{Ru}}_{0.09}{\mathrm{Sb}}_7$ and ${\mathrm{Mo}}_3{\mathrm{Sb}}_{6.975}{\mathrm{Te}}_{0.025}$. In contrast, 3 at.$\%$ Cr doping only slightly suppresses the structural transition to 48 K while leaving no trace of superconductivity above 1.8 K. Analysis of magnetic properties suggests that the interdimer interaction in ${\mathrm{Mo}}_3{\mathrm{Sb}}_7$ is near a critical value and essential for the structural transition. All dopants suppress the superconductivity of ${\mathrm{Mo}}_3{\mathrm{Sb}}_7$. The tetragonal structure is not necessary for superconductivity.

The notion of a dynamical quantum phase transition (DQPT) was recently introduced [Heyl et al., Phys. Rev. Lett. 110, 135704 (2013)] as the nonanalytic behavior of the Loschmidt echo at critical times in the thermodynamic limit. In this work the quench dynamics in the ground state sector of the two-dimensional Kitaev honeycomb model is studied regarding the occurrence of DQPTs. For general two-dimensional systems of BCS type it is demonstrated how the zeros of the Loschmidt echo coalesce to areas in the thermodynamic limit, implying that DQPTs occur as discontinuities in the second derivative. In the Kitaev honeycomb model DQPTs appear after quenches across a phase boundary or within the massless phase. In the 1d limit of the Kitaev honeycomb model it becomes clear that the discontinuity in the higher derivative is intimately related to the higher dimensionality of the nondegenerate model. Moreover, there is a strong connection between the stationary value of the rate function of the Loschmidt echo after long times and the occurrence of DQPTs in this model.

A three-dimensional (3D) Dirac semimetal is a novel state of quantum matter which has recently attracted much attention as an apparent 3D version of graphene. In this paper, we report results on the electronic structure of the 3D Dirac semimetal ${\mathrm{Na}}_3\mathrm{Bi}$ at a surface that reveals its nontrivial ground state. Our studies reveal that the two 3D Dirac cones go through a topological change in the constant energy contour as a function of the binding energy, featuring a Lifshitz point, which is missing in a strict 3D analog of graphene. Our results identify an example of a band saddle-point singularity in 3D Dirac materials. This is in contrast to its two-dimensional analogs such as graphene and the Dirac surface states of a topological insulator. The observation of multiple Dirac nodes in ${\mathrm{Na}}_3\mathrm{Bi}$ connecting via a Lifshitz point along its crystalline rotational axis away from the Kramers point serves as a decisive signature for the symmetry-protected nature of the Dirac semimetal's topological bulk ground state.

The possibility of realizing bosonic fractional quantum Hall effect in ultracold atomic systems suggests a new route to producing and manipulating anyons, by introducing auxiliary bosons of a different species that capture quasiholes and thus inherit their nontrivial braiding properties. States with localized quasiholes at any desired locations can be obtained by annihilating the auxiliary bosons at those locations. We explore how this method can be used to generate non-Abelian quasiholes of the Moore-Read Pfaffian state for bosons at filling factor $\nu =1$. We show that a Hamiltonian with an appropriate three-body interaction can produce two-quasihole states in two distinct fusion channels of the topological “qubit.” Characteristics of these states that are related to the non-Abelian nature can be probed and verified by a measurement of the effective relative angular momentum of the auxiliary bosons, which is directly related to their pair distribution function. Moore-Read states of more than two quasiholes can also be produced in a similar fashion. We investigate some issues related to the experimental feasibility of this approach, in particular, how large the systems should be for a realization of this physics and to what extent this physics carries over to systems with the more standard two-body contact interaction.

We analyzed the valence band (VB) of the 90 K high-temperature superconductor ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ by photoelectron spectroscopy under standing-wave excitation employing hard x rays. Precisely positioning the standing-wave intensity in the unit cell allows selectively probing the VB yield from the CuO chains and ${\mathrm{CuO}}_2$ planes, respectively. Both contribute strongly over the whole VB but the spectral weight of the planes is significantly higher than the chains within about 2 eV from the Fermi level. In the x-ray regime, the major contribution to the VB emission is coming from Cu $3d$.

We have measured x-ray absorption spectra at the $\mathrm{Sm}{L}_3$ edge to investigate the Sm-ion valence of $\left(\mathrm{S}{\mathrm{m}}_x\mathrm{L}{\mathrm{a}}_{1-x}\right)\mathrm{O}{\mathrm{s}}_4\mathrm{S}{\mathrm{b}}_{12}$, in which field-insensitive heavy-fermion behavior appears at low temperatures for $x=1$. It has been found that the Sm-ion valance shifts to $2+$ with La ion substitution; from $v=+2.78(x=1)$ to $v=+2.73(x=0.2)$ at 10 K. For all $x$ investigated, its temperature dependence shows a $logT$ behavior, indicating that the valence change is caused by “an unconventional Kondo effect” associated with $\mathrm{Sm}4f$-electron charge degrees of freedom. Almost $x$ independence of “the associated Kondo temperature” $({\tilde{T}}_{\mathrm{K}}=56\pm 10\mathrm{K})$ indicates that the Kondo effect has a local nature, attributable to the cage structure of the filled skutterudite.

Using cluster perturbation theory, we explain the origin of the strongly dispersive feature found at high binding energy in the spectral function of the Hubbard model. By comparing the Hubbard and $t-J-3s$ model spectra, we show that this dispersion does not originate from either coupling to spin fluctuations $\left(\propto J\right)$ or the free hopping $\left(\propto t\right)$. Instead, it should be attributed to a long-range, correlated hopping $\propto t^2/U$, which allows an effectively free motion of the hole within the same antiferromagnetic sublattice. This origin explains both the formation of the high-energy anomaly in the single-particle spectrum and the sensitivity of the high-binding-energy dispersion to the next-nearest-neighbor hopping $t^{\prime }$.

We generalize Nozières' Fermi-liquid theory for the low-energy behavior of the Kondo model to that of the single-impurity Anderson model. In addition to the electrons' phase shift at the Fermi energy, the low-energy Fermi-liquid theory is characterized by four Fermi-liquid parameters: the two given by Nozières that enter to first order in the excitation energy, and two additional ones that enter to second order and are needed away from particle-hole symmetry. We express all four parameters in terms of zero-temperature physical observables, namely the local charge and spin susceptibilities and their derivatives with respect to the local level position. We determine these in terms of the bare parameters of the Anderson model using Bethe ansatz and numerical renormalization group (NRG) calculations. Our low-energy Fermi-liquid theory applies throughout the crossover from the strong-coupling Kondo regime via the mixed-valence regime to the empty-orbital regime. From the Fermi-liquid theory, we determine the conductance through a quantum dot symmetrically coupled to two leads in the regime of small magnetic field, low temperature, and small bias voltage, and compute the coefficients of the $\sim B^2, \sim T^2$, and $\sim V^2$ terms exactly in terms of the Fermi-liquid parameters. The coefficients of $T^2, V^2$, and $B^2$ are found to change sign during the Kondo to empty-orbital crossover. The crossover becomes universal in the limit that the local interaction is much larger than the level width. For completeness, we also compute the shot noise and discuss the resulting Fano factor.

Vanadium sesquioxide $\left({\mathrm{V}}_2{\mathrm{O}}_3\right)$ is an antiferromagnetic insulator below $T_{\mathrm{N}}\approx 155\text{K}$. The magnetic order does not consist of only antiferromagnetic nearest-neighbor bonds, possibly excluding the interplane vanadium pairs, as one would infer from the bipartite character of the hexagonal basal plane in the high-temperature corundum structure. In fact, a magnetic structure with one ferromagnetic bond and two antiferromagnetic ones in the honeycomb plane is known experimentally to be realized, accompanied by a monoclinic distortion that makes the ferromagnetic bond inequivalent from the other two. We show here that the magnetic ordering, the accompanying monoclinic structural distortion, the magnetic anisotropy, and also the recently discovered high-pressure nonmagnetic monoclinic phase, can all be accurately described by conventional electronic structure calculations within GGA and GGA+$U$. Remarkably, our calculations yield that the corundum phase would be unstable to a monoclinic distortion even without magnetic ordering, thus suggesting that magnetism and lattice distortion are independent phenomena, though they reinforce each other. By means of GGA+$U$, we find a metal-to-insulator transition at a critical $U_c$. Both metal at $U\le U_c$ and insulator above $U_c$ have the same magnetic order as that actually observed below $T_{\mathrm{N}}$, but different monoclinic distortions. Reassuringly, the distortion on the insulating side agrees with the experimental one. Our results are in line with DMFT calculations for the paramagnetic phase [A. I. Poteryaev et al., Phys. Rev. B 76, 085127 (2007)], which predict that the insulating character is driven by a correlation-enhanced crystal-field splitting between $e_g^{\pi }$ and $a_{1g}$ orbitals that pushes the latter above the chemical potential. We find that the $a_{1g}$ orbital, although almost empty in the insulating phase, is actually responsible for the unusual magnetic order as it leads to magnetic frustration whose effect is similar to a next-nearest-neighbor exchange in a Heisenberg model on a honeycomb lattice.

We conduct a detailed investigation of the polaron self-interaction (pSI) error in standard approximations to the exchange-correlation (XC) functional within density-functional theory (DFT). The pSI leads to delocalization error in the polaron wave function and energy, as calculated from the Kohn-Sham (KS) potential in the native charge state of the polaron. This constitutes the origin of the systematic failure of DFT to describe the polaron formation in band insulators. It is shown that the delocalization error in these systems is, however, largely absent in the KS potential of the closed-shell neutral charge state. This leads to a modification of the DFT total-energy functional that corrects the pSI in the XC functional. The resulting pSIC-DFT method constitutes an accurate parameter-free ab initio methodology for calculating polaron properties in insulators at a computational cost that is orders of magnitude smaller than hybrid XC functionals. Unlike approaches that rely on parametrized localized potentials such as DFT+$U$, the pSIC-DFT method properly captures both site and bond-centered polaron configurations. This is demonstrated by studying formation and migration of self-trapped holes in alkali halides (bond-centered) as well as self-trapped electrons in an elpasolite compound (site-centered). The pSIC-DFT approach consistently reproduces the results obtained by hybrid XC functionals parametrized by DFT+$G_0{W}_0$ calculations. Finally, we generalize the pSIC approach to hybrid functionals, and show that in stark contrast to conventional hybrid calculations of polaron energies, the pSIC-hybrid method is insensitive to the parametrization of the hybrid XC functional. On this basis, we further rationalize the success of the pSIC-DFT approach.

We consider exciton polaritons in a semiconductor microcavity with a saturable absorber in the growth direction of the heterostructure. This feature promotes additional nonlinear losses of the system with the emergence of bistability of the condensate particles number on the nonresonant (electrical or optical) excitation intensity. Furthermore, we demonstrate a new type of bright spatial dissipative exciton-polariton soliton which emerges in the equilibrium between the regions with different particle density. We develop protocols of soliton creation and destruction. The switch to a solitonlike behavior occurs if the cavity is exposed by a short strong laser pulse with certain energy and duration. We estimate the characteristic times of soliton switch on and off and the time of return to the initial cycle. In particular, we demonstrate surprising narrowing of the spatial profile of the soliton and its vanishing at certain temperature due to interaction of the system with the thermal bath of acoustic phonons. We also address the role of polariton-polariton interaction (Kerr-like nonlinearity) on formation of dissipative solitons and show that the soliton may exist both in its presence and its absence.

Making use of droplet epitaxy, we systematically controlled the height of self-assembled GaAs quantum dots by more than one order of magnitude. The photoluminescence spectra of single quantum dots revealed the strong dependence of the spectral linewidth on the dot height. Tall dots with a height of $\sim 30\mathrm{nm}$ showed broad spectral peaks with an average width as large as $\sim 5 \mathrm{meV}$, but shallow dots with a height of $\sim 2 \mathrm{nm}$ showed resolution-limited spectral lines $(\le 120\mu \mathrm{eV})$. The measured height dependence of the linewidths is in good agreement with Stark coefficients calculated for the experimental shape variation. We attribute the microscopic source of fluctuating electric fields to the random motion of surface charges at the vacuum-semiconductor interface. Our results offer guidelines for creating frequency-locked photon sources, which will serve as key devices for long-distance quantum key distribution.

MXenes are a set of two-dimensional transition metal carbides and nitrides that offer many potential applications in energy storage and electronic devices. As an important parameter to design new electronic devices, we investigate the work functions of bare MXenes and their functionalized ones with F, OH, and O chemical groups using first-principles calculations. From our calculations, it turns out that the OH-terminated MXenes attain ultralow work functions between 1.6 and 2.8 eV. Moreover, depending on the type of the transition metal, the F or O functionalization affects increasing or decreasing the work functions. We show that the changes in the work functions upon functionalizations are linearly correlated with the changes in the surface dipole moments. It is shown that the work functions of the F- or O-terminated MXenes are controlled by two factors: the induced dipole moments by the charge transfers between F/O and the substrate, and the changes in the total surface dipole moments caused by surface relaxation upon the functionalization. However, in the cases of the OH-terminated MXenes, in addition to these two factors, the intrinsic dipole moments of the OH groups play an important role in determining the total dipole moments and consequently justify their ultralow work functions.