We present a theoretical study of Coulomb-exchange interaction for electrons confined in a cylindrical quantum wire and subject to a Rashba-type spin-orbit coupling with radial electric field. The effect of spin splitting on the single-particle band dispersions, the quasiparticle effective mass, and the system's total exchange energy per particle are discussed. Exchange interaction generally suppresses the quasiparticle effective mass in the lowest nanowire sub-band, and a finite spin splitting is found to significantly increase the magnitude of the quasiparticle-mass suppression (by up to 15% in the experimentally relevant parameter regime). In contrast, spin-orbit coupling causes a modest (1%-level) reduction of the magnitude of the exchange energy per particle. Our results shed light on the interplay of spin-orbit coupling and Coulomb interaction in quantum-confined systems, including those that are expected to host exotic quasiparticle excitations.

Films of $\mathrm{Pb}\left(111\right)$ with thickness ranging from 1 to 16 monolayers (ML) were grown on $\mathrm{Ge}\left(111\right)$ and examined by angle-resolved photoemission spectroscopy. The measured thickness dependences of the work function and surface energy from the same set of samples were both well described by damped sinusoidal oscillatory functions with the same period, but the oscillation phases differ by $1/4$ period between the two quantities, as judged by the node positions. This difference agrees with the prediction of a generic quantum-well model. The varied node positions of damped oscillations of the work function and surface energy were examined in terms of the details of Pb subband dispersions, indicating the different roles of the density of states at the Fermi level to both thin-film properties. The surface dipoles of Pb films were found to have no relevant effects on the phase shift of thickness-dependent work functions except for the amplitudes of oscillations. The total boundary phase shifts of the quantum well states, extracted from the energy positions of the quantum-well states for both $\mathrm{Pb}/\mathrm{Ge}\left(111\right)$ and freestanding Pb films, are compared and related to the thickness-dependent work function and surface energy.

We present a method by which the relatively weak electromagnetic force exerted on a surface can be dramatically enhanced. By structuring a metal surface at the nanoscale, we show that the force can be substantially increased over that on the planar metallic surface. The basis for this effect is found to be cavity-enhanced fields and the excitation of surface waves, and results are related to theory. In practice, this force enhancement could be expanded to other materials in various frequency regimes. This increased electromagnetic force should facilitate an expansion of applications related to optomechanics.

We theoretically study the quantized topological Hall effect (QTHE) in skyrmion crystal (SkX) without external magnetic field. The emergent magnetic field in SkX could be gigantic, as much as $4000\mathrm{T}$, when its lattice constant is $1\mathrm{nm}$. The band structure is not flat but has a finite gap in the low electron-density regime. We also study the conditions to realize the QTHE for the skyrmion size, carrier density, disorder strength, and temperature. Comparing the SkX and the system under the corresponding uniform magnetic field, the former is more fragile against the temperature compared with the latter since the gap is reduced by a factor of 1/5, while they are almost equally robust against the disorder. Therefore, it is expected that the QTHE of the SkX system is realized even with strong disorder at room temperature when the electron density is of the order of 1 per skyrmion.

Recently, the chiral-induced spin selectivity in molecular systems has attracted extensive interest among the scientific communities. Here, we investigate the effect of the gate voltage on spin-selective electron transport through the $\alpha$-helical peptide/protein molecule contacted by two nonmagnetic electrodes. Based on an effective model Hamiltonian and the Landauer-Büttiker formula, we calculate the conductance and the spin polarization under an external electric field, which is perpendicular to the helix axis of the $\alpha$-helical peptide/protein molecule. Our results indicate that both the magnitude and the direction of the gate field have a significant effect on the conductance and the spin polarization. The spin filtration efficiency can be improved by properly tuning the gate voltage, especially in the case of the strong dephasing regime. And the spin polarization increases monotonically with the molecular length without the gate voltage, which is consistent with the recent experiment, and presents oscillating behavior in the presence of the gate voltage. In addition, the spin selectivity is robust against the dephasing, the on-site energy disorder, and the space angle disorder under the gate voltage. Our results could motivate further experimental and theoretical works on the chiral-based spin selectivity in molecular systems.

We have performed an ab initio investigation of transition metals $(\mathrm{TMs}=\mathrm{Mo},\mathrm{Ru},\mathrm{Co},\text{and}\mathrm{Pt})$ embedded in twisted bilayer graphene (tBG) layers. Our total energy results reveal that, triggered by the misalignment between the graphene layers, Mo and Ru atoms may form a quasiperiodic (triangular) array of intercalated atoms. In contrast, the formation of those structures is not expected for the other TMs, the Co and Pt atoms. The net magnetic moment ($m$) of Mo and Ru atoms may be quenched upon intercalation, depending on the stacking region (AA or AB). For instance, we find a magnetic moment of $0.3{\mu }_{\mathrm{B}}\left(1.8{\mu }_{\mathrm{B}}\right)$ for Ru atoms intercalated between the AA (AB) regions of the stacked twisted layers. Through simulated scanning tunneling microscopy (STM) images, we verify that the presence of intercalated TMs can be identified by the formation of bright (hexagonal) spots lying on the graphene surface.

We study the thermal relaxation dynamics of ${\mathrm{VO}}_2$ films after the ultrafast photoinduced metal-insulator transition for two ${\mathrm{VO}}_2$ film samples grown on ${\mathrm{Al}}_2{\mathrm{O}}_3$ and ${\mathrm{TiO}}_2$ substrates. We find two orders of magnitude difference in the recovery time (a few nanoseconds for the ${\mathrm{VO}}_2/{\mathrm{Al}}_2{\mathrm{O}}_3$ sample versus hundreds of nanoseconds for the ${\mathrm{VO}}_2/{\mathrm{TiO}}_2$ sample). We present a theoretical model to take into account the effect of inhomogeneities in the films on the relaxation dynamics. We obtain quantitative results that show how the microstructure of the ${\mathrm{VO}}_2$ film and the thermal conductivity of the interface between the ${\mathrm{VO}}_2$ film and the substrate affect long time-scale recovery dynamics. We also obtain a simple analytic relationship between the recovery time-scale and the film's parameters.

Epitaxial growth of graphene on transition metal substrates is an important route for obtaining large scale graphene. However, the interaction between graphene and the substrate often leads to multiple orientations, distorted graphene band structure, large doping, and strong electron-phonon coupling. Here we report the growth of monolayer graphene with high crystalline quality on Pt(111) substrate by using a very low concentration of an internal carbon source with high annealing temperature. The controlled growth leads to electronically decoupled graphene: it is nearly charge neutral and has extremely weak electron-phonon coupling (coupling strength $\lambda \approx 0.056$) as revealed by angle-resolved photoemission spectroscopic measurements. The thermodynamics and kinetics of the carbon diffusion process are investigated by density functional theory calculations. Such graphene with negligible graphene-substrate interaction provides an important platform for fundamental research as well as device applications when combined with a nondestructive sample transfer technique.

We analyze the critical properties and the entanglement scaling at the quantum critical points of the spin-half XY model on a two-dimensional square-lattice bilayer and necklace lattice, based on quantum Monte Carlo simulations on finite tori and for different subregion shapes. For both models, the finite-size scaling of the transverse staggered spin structure factor is found in accord with a quantum critical point described by the two-component, three-dimensional ${\varphi }^4$ theory. The second Rényi entanglement entropy in the absence of corners along the subsystem boundary exhibits area-law scaling in both models, with an area-law prefactor of $0.0674\left(7\right)$ $\left[0.0664\right(4\left)\right]$ for the bilayer (necklace) model, respectively. Furthermore, the presence of ${90}^{\circ }$ corners leads to an additive logarithmic term in both models. We estimate a contribution of $-0.010\left(2\right)$ $[-0.009(2\left)\right]$ due to each ${90}^{\circ }$ corner to the logarithmic correction for the bilayer (necklace) model, and compare our findings to recent numerical linked-cluster calculations and series expansion results on related models.

We use electrostatic lithography to fabricate a graphene p-n-p junction and exploit the coherence of weakly confined Dirac quasiparticles to image the underlying scattering potential using low-temperature scanning gate microscopy. The tip-induced perturbation to the junction potential modifies the condition for resonant scattering, enabling us to detect localized Fabry-Pérot subcavities from the focal point of halos in scanning gate images. In addition to halos over the bulk, we also observe ones spatially registered to the physical edge of the graphene. Guided by quantum transport simulations, we attribute these to modified resonant scattering at the edges within elongated cavities that form due to focusing of the electrostatic field.

We investigate the structural and magnetic origins of the unusual ultrafast second-harmonic-generation (SHG) response of femtosecond-laser-excited nickel oxide (NiO) previously attributed to oscillatory reorientation dynamics of the magnetic structure induced by $d-d$ excitations. Using time resolved x-ray diffraction from the ($\frac{3}{2}\frac{3}{2}\frac{3}{2}$) magnetic planes, we show that changes in the magnitude of the magnetic structure factor following ultrafast optical excitation are limited to $\mathrm{\Delta }\langle F_m\rangle /\langle F_m\rangle =1.5\%$ in the first 30 ps. An extended investigation of the ultrafast SHG response reveals a strong dependence on wavelength as well as characteristic echoes, both of which give evidence for an acoustic origin of the dynamics. We therefore propose an alternative mechanism for the SHG response based on perturbations of the nonlinear susceptibility via optically induced strain in a spatially confined medium. In this model, the two observed oscillation periods can be understood as the times required for an acoustic strain wave to traverse one coherence length of the SHG process in either the collinear or anticollinear geometries.

Despite possessing a local spin-2 moment on the iron site and a Curie-Weiss temperature of 45 K, the A-site spinel ${\mathrm{FeSc}}_2{\mathrm{S}}_4$ does not magnetically order down to 50 mK. Previous theoretical work advanced an explanation for this observation in the form of the “$J_2\text{−}\lambda$” model, which places ${\mathrm{FeSc}}_2{\mathrm{S}}_4$ close to a quantum critical point on the disordered side of a quantum phase transition between a Néel ordered phase and a “spin-orbital liquid” in which spins and orbitals are entangled, quenching the magnetization. We present new theoretical studies of the optical properties of the $J_2\text{−}\lambda$ model, including a computation of the dispersion relation for the quasiparticle excitations and the form of the collective response to electric field. We argue that the latter directly probes a low energy excitation continuum characteristic of quantum criticality, and that our results reinforce the consistency of this model with experiment.

The anomalous Nernst effect (ANE) has been investigated in alternately stacked multilayer films comprising paramagnetic and ferromagnetic metals. We found that the ANE is enhanced by increasing the number of the paramagnet/ferromagnet interfaces and keeping the total thickness of the films constant, and that the enhancement appears even in the absence of magnetic proximity effects; similar behavior was observed not only in Pt/Fe multilayers but also in Au/Fe and Cu/Fe multilayers free from proximity ferromagnetism. This universal enhancement of the ANE in metallic multilayers suggests the presence of unconventional interface-induced thermoelectric conversion in the Fe films attached to the paramagnets.

${\mathrm{SrEr}}_2{\mathrm{O}}_4$ is a geometrically frustrated magnet which demonstrates rather unusual properties at low temperatures including a coexistence of long- and short-range magnetic order, characterized by two different propagation vectors. In the present work, the effects of crystal fields (CFs) in this compound containing four magnetically inequivalent erbium sublattices are investigated experimentally and theoretically. We combine the measurements of the CF levels of the ${\mathrm{Er}}^{3+}$ ions made on a powder sample of ${\mathrm{SrEr}}_2{\mathrm{O}}_4$ using neutron spectroscopy with site-selective optical and electron paramagnetic resonance measurements performed on single-crystal samples of the lightly Er-doped nonmagnetic analog, ${\mathrm{SrY}}_2{\mathrm{O}}_4$. Two sets of CF parameters corresponding to the ${\mathrm{Er}}^{3+}$ ions at the crystallographically inequivalent lattice sites are derived which fit all the available experimental data well, including the magnetization and dc susceptibility data for both lightly doped and concentrated samples.

$B2$-ordered ${\mathrm{Fe}}_{100-x}{\mathrm{Rh}}_x$ alloys, which exhibit a first-order magnetic phase transition from an antiferromagnetic state to a ferromagnetic one at around 350 K, have so far been formed by high-temperature heat treatments above $600{}^{\circ }\mathrm{C}$. Here we demonstrate low-temperature ($200{}^{\circ }\mathrm{C}$) growth of $B2$-ordered ${\mathrm{Fe}}_{100-x}{\mathrm{Rh}}_x$ films on body-centered-cubic (bcc) alloys. By using molecular beam epitaxy with an insertion of $D{0}_3$-ordered ${\mathrm{Fe}}_3\mathrm{Si}$ buffer layers, which have good atomic arrangement matching with ${\mathrm{Fe}}_{100-x}{\mathrm{Rh}}_x$ along $〈111〉, B2$-ordered ${\mathrm{Fe}}_{100-x}{\mathrm{Rh}}_x$ films having a sharp magnetic phase transition can be obtained at $200{}^{\circ }\mathrm{C}$ even in as-grown conditions. We find that the disordered structures are mainly derived from the lattice strain in the ${\mathrm{Fe}}_{100-x}{\mathrm{Rh}}_x$ films due to the lattice mismatch between ${\mathrm{Fe}}_{100-x}{\mathrm{Rh}}_x$ and the buffer layers. We discuss the role of heat treatments and other buffer structures on the $B2$-ordering and magnetic properties for ${\mathrm{Fe}}_{100-x}{\mathrm{Rh}}_x$ films.

We investigate the structural, electronic, and magnetic properties of the newly synthesized mineral barlowite ${\mathrm{Cu}}_4{\left(\mathrm{OH}\right)}_6\mathrm{FBr}$ which contains ${\mathrm{Cu}}^{2+}$ ions in a perfect kagome arrangement. In contrast to the spin-liquid candidate herbertsmithite ${{\mathrm{ZnCu}}_3{\left(\mathrm{OH}\right)}_6\mathrm{Cl}}_2$, kagome layers in barlowite are perfectly aligned due to the different bonding environments adopted by ${\mathrm{F}}^{-}$ and ${\mathrm{Br}}^{-}$ compared to ${\mathrm{Cl}}^{-}$. With the synthesis of this material we unveil a design strategy for layered kagome systems with possible exotic magnetic states. Density functional theory calculations and effective model considerations for ${\mathrm{Cu}}_4{\left(\mathrm{OH}\right)}_6\mathrm{FBr}$, which has a ${\mathrm{Cu}}^{2+}$ site coupling the kagome layers, predict a three-dimensional network of exchange couplings, which together with a substantial Dzyaloshinskii-Moriya coupling lead to canted antiferromagnetic ordering of this compound in excellent agreement with magnetic susceptibility measurements on single crystals yielding $T_N=15\mathrm{K}$.

The electron spin of the negatively charged the nitrogen vacancy center $({\mathrm{NV}}^{-}$) in diamond can be optically polarized through intersystem crossing, which enables the defect to be used for quantum computation and metrology. In this work, we studied the electron spin depolarization effect of the NV center induced by charge state conversion, which was proven to be a spin-independent process. The spin-state initialization fidelity was largely affected by the charge state conversion process. As a result, the optical polarization of the electron spin decreased about $14\%(31\%$) with a high-power continuous-wave (pulsed) green laser. Moreover, the undefined fluorescence anomalous saturation effect of the NV center was analyzed and explained in detail based on the spin depolarization. The results demonstrated that a weak laser should be used for initialization of the NV center. In addition, the power and polarization of a laser for NV spin detection should be carefully adjusted to obtain the highest fluorescence signal. Our work also provided information that can increase the understanding of the charge state conversion and spin polarization processes of the NV center for quantum information and sensing.

The chiral langasite ${\mathrm{Ba}}_3{\mathrm{NbFe}}_3{\mathrm{Si}}_2{\mathrm{O}}_{14}$ is a multiferroic compound. While its magnetic order below $T_N=27$ K is now well characterized, its polar order is still controversial. We thus looked at the phonon spectrum and its temperature dependence to unravel possible crystal symmetry breaking. We combine optical measurements (both infrared and Raman spectroscopy) with ab initio calculations and show that the signatures of a polar state are clearly present in the phonon spectrum even at room temperature. An additional symmetry lowering occurs below 120 K as seen from the emergence of softer phonon modes in the terahertz range. These results confirm the multiferroic nature of this langasite and open new routes to understand the origin of the polar state.

Magnetodynamical properties of nanomagnets are affected by the demagnetizing fields created by the same nanoelements. In addition, magnetocrystalline anisotropy produces an effective field that also contributes to the spin dynamics. We show how the dimensions of magnetic elements can be used to balance crystalline and shape anisotropies, and that this can be used to tailor the magnetodynamic properties. We study ferromagnetic ellipses patterned from a 10-nm-thick epitaxial Fe film with dimensions ranging from $50\times 150$ to $150\times 450$ nm. The study combines ferromagnetic resonance (FMR) spectroscopy with analytical calculations and micromagnetic simulations, and proves that the dynamical properties can be effectively controlled by changing the size of the nanomagnets. We also show how edge defects in the samples influence the magnetization dynamics. Dynamical edge modes localized along the sample edges are strongly influenced by edge defects, and this needs to be taken into account in understanding the full FMR spectrum.