Dynamic nuclear polarization (DNP) of molecules in a solution at room temperature has the potential to revolutionize nuclear magnetic resonance spectroscopy and imaging. The prevalent methods for achieving DNP in solutions are typically most effective in the regime of small interaction correlation times between the electron and nuclear spins, limiting the size of accessible molecules. To solve this limitation, we design a mechanism for DNP in the liquid phase that is applicable for large interaction correlation times. Importantly, while this mechanism makes use of a resonance condition similar to solid-state DNP, the polarization transfer is robust to a relatively large detuning from the resonance due to molecular motion. We combine this scheme with optically polarized nitrogen-vacancy (NV) center spins in nanodiamonds to design a setup that employs optical pumping and is therefore not limited by room temperature electron thermal polarization. We illustrate numerically the effectiveness of the model in a flow cell containing nanodiamonds immobilized in a hydrogel, polarizing flowing water molecules 4700-fold above thermal polarization in a magnetic field of 0.35 T, in volumes detectable by current NMR scanners.

Crystal and electronic structures of $\mathrm{BiFe}{\mathrm{O}}_3$ thin films ($\sim 10$ and ∼300 nm) grown on $\mathrm{SrTi}{\mathrm{O}}_3$ substrate have been investigated in terms of $\mathrm{BiFe}{\mathrm{O}}_3$ film thickness dependence using the advanced transmission electron microscopy (TEM) technique. Electron diffraction patterns of both $\mathrm{BiFe}{\mathrm{O}}_3$ thin films acquired along ${\left[011\right]}_{\mathrm{SrTi}{\mathrm{O}}_3}$ cross sections clearly exhibited the existence of extra Bragg's reflections which are absent in that from $\mathrm{SrTi}{\mathrm{O}}_3$. Structure factor calculations unambiguously revealed that the electron diffraction pattern corresponds to the [211] net pattern of rhombohedral $\mathrm{BiFe}{\mathrm{O}}_3$. High-resolution TEM images combined with multislice simulation also demonstrated that the crystalline structure of both $\mathrm{BiFe}{\mathrm{O}}_3$ films is rhombohedral. Electron energy loss spectroscopy results for both $\mathrm{BiFe}{\mathrm{O}}_3$ thin films showed spectra with the characteristics of bulk $\mathrm{BiFe}{\mathrm{O}}_3$, i.e., rhombohedral. The lattice mismatch of <2.5% between $\mathrm{BiFe}{\mathrm{O}}_3$ and $\mathrm{SrTi}{\mathrm{O}}_3$ found in a particular epitaxial relationship is considered to be the reason that $\mathrm{BiFe}{\mathrm{O}}_3$ can grow by maintaining its bulk crystalline, i.e., rhombohedral, structure.

We report site disorder driven ferromagnetic ordering of nearly $8\%$ Ce atoms in single crystalline $\mathrm{Ce}{}_3\mathrm{Os}{}_4{\mathrm{Ge}}_{13}$ below 0.5 K. $\mathrm{Ce}{}_3\mathrm{Os}{}_4{\mathrm{Ge}}_{13}$ crystallizes in a cubic structure with space group $\mathit{Pm}\overline{\mathit{3}}\mathit{n}$. The structural analysis shows the presence of site disorder in the system, where the $2\left(a\right)$ Ge site is partially occupied by the Ce atoms. Due to the small Ce $4f$-ligand hybridization, these Ce atoms are in the ${\mathrm{Ce}}^{3+}$ (magnetic) state while the rest of the Ce atoms in the unit cell are in the ${\mathrm{Ce}}^{4-\delta }$ (nonmagnetic) state. The heat capacity shows a peak below 0.5 K corresponding to the ferromagnetic ordering of the ${\mathrm{Ce}}^{3+}$ moments. The ferromagnetic ordering below 0.5 K is also seen in the ac-susceptibility data. At low temperatures ($1.8\mathrm{K}\le T\le 6$ K), the magnetization shows $\ln \left(T\right)$ dependence, whereas the resistivity shows power-law $\left(T^n\right)$ temperature dependence indicating the non-Fermi-liquid behavior of the quasiparticles.

Recently there has been substantial interest in realizations of skyrmions, in particular in quasi-two-dimensional (2D) systems due to increased stability resulting from reduced dimensionality. A stable skyrmion, representing the smallest realizable magnetic texture, could be an ideal element for ultradense magnetic memories. Here we use the most general form of the quasi-2D free energy with Dzyaloshinskii-Moriya interactions constructed from general symmetry considerations reflecting the underlying system. We predict that the skyrmion phase is robust and it is present even when the system lacks the in-plane rotational symmetry. In fact, the lowered symmetry leads to increased stability of vortex-antivortex lattices with fourfold symmetry and in-plane spirals, in some instances even in the absence of an external magnetic field. Our results relate different hexagonal and square cell phases to the symmetries of materials used for realizations of skyrmions. This will give clear directions for experimental realizations of hexagonal and square cell phases, and will allow engineering of skyrmions with unusual properties. We also predict striking differences in gyrodynamics induced by spin currents for isolated skyrmions and for crystals where spin currents can be induced by charge carriers or by thermal magnons. We find that under certain conditions, isolated skyrmions can move along the current without a side motion which can have implications for realizations of magnetic memories.

In the Landau free energy, which is a powerful tool for describing the physical properties and phase transitions in condensed-matter physics, it has been generally believed that time-reversal symmetry allows only even-power polynomials in the magnetic moment when magnetic transition happens. Although no experimental evidence supports it, this symmetry constraint is very strict in theory. On the other hand, MnO, CoO, and NiO have been extensively studied for several decades since these materials are used to test advance experimental and theoretical methods. However, their precise spin directions and phase-transition mechanism have remained as a long-standing problem until now. To resolve these issues, we used the cutting-edge time-of-flight neutron powder diffractometer (SuperHRPD) at the Japan Proton Accelerator Research Complex (J-PARC) to study the spin-direction-dependent magnetoelastic coupling in MnO, CoO, and NiO. We also constructed a group-subgroup structure relation from $Fm-3m$ to $C2/m$ with exchange striction and a type of magnetostriction (dipolar and roto magnetostriction). These unified viewpoints and the high-resolution neutron powder diffractometer enable us to construct an order-parameter vector diagram. The order-parameter vector diagram reveals distinguished order-parameter coupling and phase-transition characters by different ${\mathrm{\Gamma }}_1^{+}\left(A_g\right)$ and ${\mathrm{\Gamma }}_2^{+}\left(B_g\right)$ spin direction. Moreover, the experiment results show clearly that the ${\mathrm{CoO}}_6$ octahedral distortion and the Co magnetic moment couple through a magnetoelastic coupling ${\epsilon }_{\text{oct}}\sim M^3$, which is forbidden by time-reversal symmetry but allowed by the more general magnetic symmetry—the so-called rotation-time-reversal symmetry in the double antisymmetry space group. Rotation-time-reversal symmetry allows the coupling of odd-power polynomials in the spin vector and odd-power ones in the roto vector when both spin- and roto-axial vectors belong to the same irreducible representation in Landau free energy. Here, we show experimental evidence that the magnetic order parameter and static structural order parameter belong to the same irreducible representation in the materials. Experimental discovery of general magnetic symmetry opens a way to find the physics forbidden by time-reversal symmetry in condensed matter physics.

The natural excitations of an interacting one-dimensional system at low energy are the hydrodynamic modes of a Luttinger liquid, protected by the Lorentz invariance of the linear dispersion. We show that beyond low energies, where the quadratic dispersion reduces the symmetry to Galilean, the main character of the many-body excitations changes into a hierarchy: calculations of dynamic correlation functions for fermions (without spin) show that the spectral weights of the excitations are proportional to powers of ${\mathcal{R}}^2/L^2$, where $\mathcal{R}$ is a length-scale related to interactions and $L$ is the system length. Thus only small numbers of excitations carry the principal spectral power in representative regions on the energy-momentum planes. We have analyzed the spectral function in detail and have shown that the first-level (strongest) excitations form a mode with parabolic dispersion, like that of a renormalized single particle. The second-level excitations produce a singular power-law line shape to the first-level mode and multiple power laws at the spectral edge. We have illustrated a crossover to a Luttinger liquid at low energy by calculating the local density of states through all energy scales: from linear to nonlinear, and to above the chemical potential energies. In order to test this model, we have carried out experiments to measure the momentum-resolved tunneling of electrons (fermions with spin) from/to a wire formed within a GaAs heterostructure. We observe a well-resolved spin-charge separation at low energy with appreciable interaction strength and only a parabolic dispersion of the first-level mode at higher energies. We find a structure resembling the second-level excitations, which dies away rapidly at high momentum in line with the theoretical predictions here.

An Anderson model for a magnetic impurity in a two-dimensional electron gas with bulk Rashba spin-orbit interaction is solved using the numerical renormalization group under two different experimental scenarios. For a fixed Fermi energy, the Kondo temperature $T_K$ varies weakly with Rashba coupling ${\lambda }_R$, as reported previously. If instead the band filling is low and held constant, increasing ${\lambda }_R$ can drive the system into a helical regime with exponential enhancement of $T_K$. Under either scenario, thermodynamic properties at low temperatures $T$ exhibit the same dependencies on $T/T_K$ as are found for ${\lambda }_R=0$. Unlike the conventional Kondo effect, however, the impurity exhibits static spin correlations with conduction electrons of nonzero orbital angular momentum about the impurity site. We also consider a magnetic field that Zeeman splits the conduction band but not the impurity level, an effective picture that arises under a proposed route to access the helical regime in a driven system. The impurity contribution to the system's ground-state angular momentum is found to be a universal function of the ratio of the Zeeman energy to a temperature scale that is not $T_K$ (as would be the case in a magnetic field that couples directly to the impurity spin), but rather is proportional to $T_K$ divided by the impurity hybridization width. This universal scaling is explained via a perturbative treatment of field-induced changes in the electronic density of states.

We study electrostatic screening in two classes of metamaterials. The first class consists of a cubic array of metal cubes, which is known to give rise to a positive local dielectric function. We show that such a local positive dielectric function also correctly describes its electrostatic behavior. The second class consists of a variety of wire media that have a strong nonlocal response in their dielectric function. We show that in these wire media, the electrostatic potential generated by a point charge decays exponentially as a function of distance from the charge, and such an exponential screening is intrinsically related to the nonlocal behavior of the dielectric function. We also show, surprisingly, that the electrostatic behavior in some of these wire media is isotropic in spite of the strong anisotropy in the dielectric tensor. Our work here provides an understanding of how to geometrically control electrostatic screening and Coulomb interaction in metamaterials.

We present a theory of the electron structure and the Zeeman effect for the helical edge states emerging in two-dimensional topological insulators based on HgTe/HgCdTe quantum wells with strong natural interface inversion asymmetry. The interface inversion asymmetry, reflecting the real atomistic structure of the quantum well, drastically modifies both bulk and edge states. For the in-plane magnetic field, this asymmetry leads to a strong anisotropy of the edge-state effective $g$ factor, which becomes dependent on the edge orientation. The interface inversion asymmetry also couples the counterpropagating edge states in the out-of-plane magnetic field leading to the opening of the gap in the edge-state spectrum by arbitrary small fields.

We study the spectral function of the two-dimensional Hubbard model using cluster perturbation theory, and a density matrix renormalization group as a cluster solver. We reconstruct the two-dimensional dispersion at and away from half-filling using $2\times L$ ladders, with $L$ up to 80 sites, yielding results with unprecedented resolution in excellent agreement with quantum Monte Carlo. The main features of the spectrum can be described with a mean-field dispersion, with kinks and pseudogap traced back to scattering between spin and charge degrees of freedom.

We study the Hall conductance of a Chern insulator after a global quench of the Hamiltonian. The Hall conductance in the long time limit is obtained by applying the linear response theory to the diagonal ensemble. It is expressed as the integral of the Berry curvature weighted by the occupation number over the Brillouin zone. We identify a topologically driven nonequilibrium phase transition, which is indicated by the nonanalyticity of the Hall conductance as a function of the energy gap $m_f$ in the post-quench Hamiltonian ${\widehat{H}}_f$. The topological invariant for the quenched state is the winding number of the Green's function $W$, which equals the Chern number for the ground state of ${\widehat{H}}_f$. In the limit $m_f\to 0$, the derivative of the Hall conductance with respect to $m_f$ is proportional to $ln|m_f|$, with the constant of proportionality being the ratio of the change of $W$ at $m_f=0$ to the energy gap in the initial state. This nonanalytic behavior is universal in two-band Chern insulators such as the Dirac model, the Haldane model, or the Kitaev honeycomb model in the fermionic basis.

The accuracy of $\mathrm{DFT}+U$ calculations, applied to the study of electronic structure and energetics of strongly correlated materials, heavily depends on $U$ parameters, chosen for adequate treatment of $d$ and $f$ states. Computational evaluation of $U$ parameters, which does not require fitting to experimental measurements or results of computationally expensive schemes, is highly desirable for the study of novel materials and even more so for materials not yet synthesized to date. Within this work, we show that the linear response method could provide $U$ parameters which can yield redox potentials of battery cathode materials in much better agreement with experiment than conventional density functional theory (DFT). In our approach, we evaluate $U$ values self-consistently, ensuring agreement between $U$ calculated using linear response with the value used for $\mathrm{DFT}+U$ calculations. We find that such self-consistency is necessary for determination of adequate values of $U$. We also studied the impact of using various PAW (projector augmented wave) potentials for transition-metal ions, that differ by the number of electrons treated as valence. We find that redox potentials are reasonably well reproduced for all cases, although a slightly higher degree of accuracy corresponds to PAW potentials with semicore electrons treated as valence. Importantly, we find that converged values of $U$ are substantially different for various PAW potentials of transition-metal ions of the same material. Overall, we find that self-consistent $\mathrm{DFT}+U$/linear response calculations provide quite accurate values of redox potentials for materials with purely ionic bonding (e.g., ${\mathrm{LiFePO}}_4$, ${\mathrm{LiCoPO}}_4$, ${\mathrm{LiCoO}}_2$, ${\mathrm{LiMnPO}}_4$, ${\mathrm{NaFePO}}_4$), whereas for materials with covalent $pd$ hybridization (e.g., ${\mathrm{LiNiO}}_2$) or conducting materials (e.g., ${\mathrm{LiTiS}}_2$) the agreement with experimental redox voltage is lower. This emphasizes the need for application of more advanced techniques (e.g., $\mathrm{DFT}+U+V$ method) for accurate study of partially covalent and metallic materials, which contain transition-metal ions.

Materials with a negative dielectric permittivity (e.g., metals) display high reflectance and can be shaped into nanoscale optical resonators exhibiting extreme mode confinement, a central theme of nanophotonics. However, the ability to actively tune these effects remains elusive. By photoexciting free carriers in $4H-\mathrm{SiC}$, we induce dramatic changes in reflectance near the “reststrahlen band” where the permittivity is negative due to charge oscillations of the polar optical phonons in the midinfrared. We infer carrier-induced changes in the permittivity required for useful tunability $(\sim 40\mathrm{c}{\mathrm{m}}^{-1})$ in nanoscale resonators, providing a direct avenue towards the realization of actively tunable nanophotonic devices in the midinfrared to terahertz spectral range.

Surface plasmon spectrum of a metallic hyperbola can be found analytically with the separation of variables in elliptic coordinates. The spectrum consists of two branches: a symmetric, low-frequency branch, $\omega <{\omega }_0/\sqrt{2}$, and an antisymmetric high-frequency branch, $\omega >{\omega }_0/\sqrt{2}$, where ${\omega }_0$ is the bulk plasmon frequency. The frequency width of the plasmon band increases with decreasing the angle between the asymptotes of the hyperbola. For the simplest multiconnected geometry of two hyperbolas separated by an air spacer the plasmon spectrum contains two low-frequency branches and two high-frequency branches. Most remarkably, the lower of the two low-frequency branches exists at $\omega \to 0$; i.e., unlike a single hyperbola, it is “thresholdless.” We study how the complex structure of the plasmon spectrum affects the energy transfer between two emitters located on the surface of the same hyperbola and on the surfaces of different hyperbolas.

This paper studies short-range order (SRO) in the semiconductor alloy ${\left(\mathrm{GaN}\right)}_{1-x}{\left(\mathrm{ZnO}\right)}_x$. Monte Carlo simulations performed on a density functional theory (DFT)-based cluster expansion model show that the heterovalent alloys exhibit strong SRO because of the energetic preference for the valence-matched nearest-neighbor Ga-N and Zn-O pairs. To represent the SRO-related structural correlations, we introduce the concept of special quasiordered structure (SQoS). Subsequent DFT calculations reveal the dramatic influence of SRO on the atomic, electronic, and vibrational properties of the ${\left(\mathrm{GaN}\right)}_{1-x}{\left(\mathrm{ZnO}\right)}_x$ alloy. Due to the enhanced statistical presence of the energetically unfavored Zn-N bonds with the strong $\mathrm{Zn}3d-\mathrm{N}2p$ repulsion, the disordered alloys exhibit much larger lattice bowing and band-gap reduction than those of the short-range ordered alloys. Lattice vibrational entropy tilts the alloy toward less SRO.

The analysis of low-temperature specific heat of rare-earth (RE)-containing quasicrystals and periodic approximants and consequent interpretation of their electronic properties in the $T\to 0$ limit is frequently hampered by the Schottky effect, where crystalline electric fields lift the degeneracy of the RE-ion Hund's rule ground state and introduce additional contribution to the specific heat. In this paper we study the low-temperature specific heat of a thulium-containing $i$-Zn-Ag-Sc-Tm icosahedral quasicrystal and its 1/1 Zn-Sc-Tm approximant, both being classified as “Schottky” systems. We have derived the crystal-field Hamiltonian for pentagonal symmetry of the crystalline electric field, pertinent to the class of Tsai-type icosahedral quasicrystals and their approximants, where the RE ions are located on fivefold axes of the icosahedral atomic cluster. Using the leading term of this Hamiltonian, we have calculated analytically the Schottky specific heat in the presence of an external magnetic field and made comparison to the experimental specific heat of the investigated quasicrystal and approximant. When the low-temperature specific heat $C$ is analyzed in a $C/T$ versus $T^2$ scale (as it is customarily done for metallic specimens), the Schottky specific heat yields an upturn in the $T\to 0$ limit that cannot be easily distinguished from a similar upturn produced by the electron-electron interactions in exchange-enhanced systems and strongly correlated systems. Our results show that extraction of the electronic properties of RE-containing quasicrystals from their low-temperature specific heat may be uncertain in the presence of the Schottky effect.

We present a comparative far-infrared reflection spectroscopy study of phonons, phase transitions, spin-phonon, and electron-phonon interactions in isostructural multiferroic iron borates of gadolinium and terbium. The behavior of phonon modes registered in a wide temperature range is consistent with a weak first-order structural phase transition [$T_s=143$ for $\mathrm{GdF}{\mathrm{e}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ and 200 K for $\mathrm{TbF}{\mathrm{e}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$] from a high-symmetry high-temperature $R32$ structure into a low-symmetry low-temperature $P{3}_{1}21$ one. The temperature dependences of frequencies, oscillator strengths, and damping constants of some low-frequency modes reveal an appreciable lattice anharmonicity. Peculiarities in the phonon mode behavior in both compounds at the temperature of an antiferromagnetic ordering [$T_N=32\mathrm{K}$ for $\mathrm{GdF}{\mathrm{e}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ and 40 K for $\mathrm{TbF}{\mathrm{e}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$] evidence the spin-phonon interaction. In the energy range of phonons, $\mathrm{GdF}{\mathrm{e}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ has no electronic levels, but $\mathrm{TbF}{\mathrm{e}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ possesses several. We observe an onset of new bands in the excitation spectrum of $\mathrm{TbF}{\mathrm{e}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ due to a resonance interaction between a lattice phonon and $4f$ electronic crystal-field (CF) excitations of $\mathrm{T}{\mathrm{b}}^{3+}$. This interaction causes delocalization of the CF excitations, their Davydov splitting, and formation of coupled electron-phonon modes.

The martensite-austenite phase transition in a series of $\mathrm{N}{\mathrm{i}}_2\mathrm{M}{\mathrm{n}}_{1-x}\mathrm{C}{\mathrm{r}}_x\mathrm{Ga}$ Heusler alloys has been investigated by x-ray diffraction, dc magnetization, and electrical resistivity measurements. With increasing Cr concentration, the martensitic phase transformation shifts to higher temperature while the ferromagnetic transition shifts to lower temperature. For $x<0.5$, the martensitic transformation occurs in a ferromagnetic state, while for $x>0.5$, the transition occurs in a paramagnetic state. The Cr doping results in a reconstruction of the electronic structure, particularly, near the Fermi level, which is indicated in the resistivity data where a systematic jumplike anomaly is observed in the vicinity of the martensite-austenite phase transformation. With increasing Cr concentration, the magnitude of the jump in resistivity changes dramatically from less than $1\%$ to nearly $18\%$ The results are discussed considering the fundamental interactions in Heusler alloys.

The anomalous Nernst coefficient, anomalous Hall angle, and Seebeck coefficient have been measured in the same series of ${\left[\mathrm{Pt}/\mathrm{Co}\right]}_n$ superlattices in which spin-orbit coupling is dominated by interfaces. With an increase in the number of interfaces from 1 to 15, the anomalous Nernst coefficient and anomalous Hall angle simultaneously increase, by $350\%$ and $430\%$, respectively. Furthermore, they even scale linearly with each other, while the Seebeck coefficient and magnetization of the superlattices vary a little. Based on the linear scaling relation, a physical scenario behind the anomalous Nernst effect, as well as a formula relating the Nernst coefficient, the Hall angle, and the Seebeck coefficient, is proposed. The work not only demonstrates an effective way to enhance the anomalous Nernst effect in ferromagnetic conductors but also can bring about deeper insight into the anomalous Nernst effect.

Very often it is an implied paradigm of molecular magnetism that magnetic molecules in a crystal interact so weakly that measurements of dc magnetic observables reflect ensemble properties of single molecules. But the number of cases where the assumption of virtually noninteracting molecules does not hold grows steadily. A deviation from the noninteracting case can especially clearly be seen in clusters with antiferromagnetic couplings, where steps of the low-temperature magnetization curve are smeared out with increasing intermolecular interaction. In this investigation we demonstrate with examples in one, two, and three space dimensions how intermolecular interactions influence typical magnetic observables such as magnetization, susceptibility, and specific heat.