Recent advances in ultracold atoms in optical lattices and developments in surface science have allowed for the creation of artificial lattices as well as the control of many-body interactions. Such systems provide new settings to investigate interaction-driven instabilities and nontrivial topology. In this paper, we explore the interplay between molecular electric dipoles on a two-dimensional triangular lattice with fermions hopping on the dual decorated honeycomb lattice which hosts Dirac and flat band states. We show that short-range dipole-dipole interaction can lead to ordering into various stripe and vortex crystal ground states. We study these ordered states and their thermal transitions as a function of the interaction range using simulated annealing and Monte Carlo methods. For the special case of zero-wave-vector ferrodipolar order, incorporating dipole-electron interactions and integrating out the electrons leads to a six-state clock model for the dipole ordering. Finally, we discuss the impact of the various dipole orders on the electronic band structure and the local tunneling density of states. Our work may be relevant to studies of “molecular graphene”—CO molecules arranged on the Cu(111) surface—which have been explored using scanning tunneling spectroscopy, as well as ultracold molecule-fermion mixtures in optical lattices.

Thin films of topological insulators (TI) attract large attention because of expected topological effects from the intersurface hybridization of Dirac points. However, these effects may be depleted by unexpectedly large energy smearing $\mathrm{\Gamma }$ of surface Dirac points by the random potential of abundant Coulomb impurities. We show that in a typical TI film with large dielectric constant $\sim 50$ sandwiched between two low dielectric-constant layers, the Rytova-Chaplik-Entin-Keldysh modification of the Coulomb potential of a charge impurity allows a larger number of the film impurities to contribute to $\mathrm{\Gamma }$. As a result, $\mathrm{\Gamma }$ is large and independent of the TI film thickness $d$ for $d>5$ nm. In thinner films $\mathrm{\Gamma }$ grows with decreasing $d$ due to reduction of screening by the hybridization gap. We study the surface conductivity away from the neutrality point and at the neutrality point. In the latter case, we find the maximum TI film thickness at which the hybridization gap is still able to make a TI film insulating and allow observation of the quantum spin Hall effect, $d_{max}\sim 7$ nm.

We study direct and indirect magnetoexcitons in Rydberg states in monolayers and double-layer heterostructures of Xenes (silicene, germanene, and stanene) in external parallel electric and magnetic fields applied perpendicular to the monolayer and heterostructure. We calculate binding energies of magnetoexcitons for the Rydberg states, $1s$, $2s$, $3s$, and $4s$ by numerical integration of the Schrödinger equation using the Rytova-Keldysh potential for direct and both the Rytova-Keldysh and Coulomb potentials for indirect magnetoexcitons, respectively. The latter allows understanding a role of screening in Xenes. In the external perpendicular electric field, the buckled structure of the Xene monolayers leads to the appearance of potential difference between sublattices allowing us to tune electron and hole masses and, therefore, the binding energies and diamagnetic coefficients (DMCs) of magnetoexcitons. We report the energy contribution from electric and magnetic fields to the binding energies and DMCs. The tunability of the energy contribution of direct and indirect magnetoexcitons by electric and magnetic fields is demonstrated. It is also shown that DMCs of direct excitons can be tuned by the electric field, and the DMCs of indirect magnetoexcitons can be tuned by the electric field and manipulated by the number of hBN layers, therefore allowing the possibility of electronic devices design that can be controlled by external electric and magnetic fields and the number of hBN layers. The calculations of the binding energies and DMCs of magnetoexcitons in Xenes monolayers and heterostructures can be compared with the experimental results when they are available.

The pyrochlore magnet ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ shows a lack of magnetic order to low temperatures and is considered to be a quantum spin liquid candidate. We perform time-domain THz spectroscopy on high-quality ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ crystals and study the low-energy excitations as a function of [111] magnetic field with high energy resolution. The low-energy crystal-field excitations change their energies anomalously under magnetic field. Despite several sharp field-dependent changes, we show that the material's spectrum can be described not by phase transitions but by field-dependent hybridization between the low-energy crystal-field levels. We highlight the strong coupling between spin and lattice degrees of freedom in ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ as evidenced by the magnetic-field tunable crystal-field environment. Calculations based on single ion physics with field-induced symmetry reduction of the crystal-field environment can reproduce our data.

We introduce an effective two-dimer tight-binding model for the family of Shastry-Sutherland models with geometrically tunable triplon excitations. The Rashba pseudospin-orbit coupling induced by the tilted external magnetic field leads to elementary excitations having nontrivial topological properties with $\pi$-Berry flux. The interplay between the in-plane and out-of-plane magnetic field thus allows us to effectively engineer the band structure in this bosonic system. In particular, the in-plane magnetic field gives rise to a Berry curvature hotspot near the bottom of the triplon band and at the same time significantly increases the critical magnetic field for the topological triplon band. We calculate explicitly the experimental signature of the thermal Hall effect (THE) of triplons in ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$ and show pronounced and tunabled transport signals within the accessible parameter range, particularly with a change of sign of the thermal Hall conductance. The tilted magnetic field is also useful in reducing the bandwidth of the lowest triplon band. We show it can thus be a flexible theoretical and experimental platform for the correlated bosonic topological system.

In this work, cross relaxation between nitrogen-vacancy (NV) centers and substitutional nitrogen in a diamond crystal is investigated. It is demonstrated that optically detected magnetic resonance signals (ODMR) can be used to probe cross relaxation. ODMR is detected at axial magnetic field values around 51.2 mT in a diamond sample with a relatively high (200 ppm) nitrogen concentration. We observe transitions that involve magnetic sublevels that are split by the hyperfine interaction. Microwaves in the frequency ranges from 1.3 GHz to 1.6 GHz ($m_S=0⟶m_S=-1$ NV transitions) and from 4.1 to 4.6 GHz ($m_S=0⟶m_S=+1$ NV transitions) were used. To understand the cross-relaxation process in more detail and, as a result, reproduce measured signals more accurately, a model is developed that describes the microwave-initiated transitions between hyperfine levels of the NV center at anticrossing that are strongly mixed. Additionally, we simulate the extent to which the microwave radiation driving ODMR in the NV center also induces transitions in the substitutional nitrogen via cross relaxation. The improved understanding of the NV processes in the presence of magnetic field will be useful for designing NV-diamond-based devices for a wide range of applications from implementation of q bits to hyperpolarization of large molecules to various quantum technological applications such as field sensors.

Understanding of the interaction of antiferromagnetic solitons including domain walls and skyrmions with boundaries of chiral antiferromagnetic slabs is important for the design of prospective antiferromagnetic spintronic devices. Here, we derive the transition from spin lattice to micromagnetic nonlinear $\sigma$ model with the corresponding boundary conditions for a chiral cubic G-type antiferromagnet and analyze the impact of the slab boundaries and antisymmetric exchange (Dzyaloshinskii-Moriya interaction) on the vector order parameter. We apply this model to evaluate modifications of antiferromagnetic domain walls and skyrmions upon interaction with boundaries for different strengths of the antisymmetric exchange. Due to the presence of the antisymmetric exchange, both types of antiferromagnetic solitons become broader when approaching the boundary and transform to a mixed Bloch-Néel structure. Both textures feel the boundary at the distance of about five magnetic lengths. In this respect, our model provides design rules for antiferromagnetic racetracks, which can support bulklike properties of solitons.

The properties of Kitaev materials are attracting ever increasing attention owing to their exotic properties. In realistic two-dimensional materials, the Kitaev interaction is often accompanied by the Dzyaloshinskii-Moriya interaction, which poses a challenge for distinguishing their magnitudes separately. In this paper, we demonstrate that it can be done by accessing magnonic transport properties. By studying honeycomb ferromagnets exhibiting Dzyaloshinskii-Moriya and Kitaev interactions simultaneously, we reveal nontrivial magnonic topological properties accompanied by intricate magnonic transport characteristics as given by thermal Hall and magnon Nernst effects. We also investigate the effect of a magnetic field, showing that it does not only break the symmetry of the system but also brings drastic modifications to magnonic topological transport properties, which serve as hallmarks of the relative strength of anisotropic exchange interactions. Based on our findings, we suggest strategies to estimate the importance of Kitaev interactions in real materials.

We study theoretically spin pumping in bilayers consisting of superconductors (SC) and antiferromagnetic insulators (AFI). We consider both compensated and uncompensated interfaces and include both the regular scattering channel and the Umklapp scattering channel. We find that at temperatures close to the critical temperatures and precession frequencies much lower than the gap, the spin current is enhanced in superconductors as compared to normal metals. Otherwise, the spin current is suppressed. The relevant precession frequencies, where the spin current in SC/AFI is enhanced compared to normal metals/AFI, is much lower than the typical resonance frequencies of antiferromagnets, which makes the detection of this effect experimentally challenging. A possible solution lies in the shifting of the resonance frequency by a static magnetic field.

The crystal structure, magnetic, electronic, and thermal properties of $\mathrm{Sr}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x\right)}_2{\mathrm{As}}_2$ crystals grown by transition metal arsenide flux have been systematically investigated. A miscibility gap with $x$ ranging from 0.4362(4) to 0.9612(9) is found in the $\mathrm{Sr}\left({\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x\right){\mathrm{As}}_2$ system—for $x<0.4362\left(4\right)$, the phase remains in the parent tetragonal structure [space group $I4/mmm$ (No. 139)], whereas for $\mathrm{x}>0.9612\left(9\right)$, the phase exhibits a trigonal structure [space group $P\text{−}3\mathrm{m}1$ (No. 164)]. Based on our observation, the spin density wave order for $x<0.0973\left(1\right)$ is suppressed, followed by an abnormal and broadened increase in the ordering temperature from $x=0.0973\left(1\right)$ to $x=0.2055\left(2\right)$. No real-space phase separation of Mn and Fe was detected for $x<0.2055\left(2\right)$, indicating that the broadened increase in ordering temperature is attributable to a possibly intrinsic Griffiths phase. No superconducting signal was observed down to 2 K in the whole composition range with $0<x<1$. A phase diagram with multicritical points of Mn-doped $\mathrm{Sr}{\mathrm{Fe}}_2{\mathrm{As}}_2$ system is established accordingly.

Transient heat transfer in superfluid ${}^4\mathrm{He}$ (He II) is a complex process that involves the interplay of the unique counterflow heat-transfer mode, the emission of second-sound waves, and the creation of quantized vortices. Many past researches focused on homogeneous heat transfer of He II in a uniform channel driven by a planar heater. In this paper, we report our systematic study of He II transient heat transfer in nonhomogeneous geometries that are pertinent to emergent applications. By solving the He II two-fluid equations of motion coupled with Vinen's equation for vortex-line density, we examine and compare the characteristics of transient heat transfer from planar, cylindrical, and spherical heaters in He II. Our results show that as the heater turns on, an outgoing second-sound pulse emerges, within which the vortex-line density grows rapidly. These vortices attenuate the second sound and result in a heated He II layer in front of the heater, i.e., the thermal layer. In the planar case where the vortices are created throughout the space, the second-sound pulse is continuously attenuated, leading to a thick thermal layer that diffusely spreads following the heat pulse. On the contrary, in the cylindrical and the spherical heater cases, vortices are created mainly in a thin thermal layer near the heater surface. As the heat pulse ends, a rarefaction tail develops following the second-sound pulse, in which the temperature drops. This rarefaction tail can promptly suppress the thermal layer and take away the deposited thermal energy. The effects of the heater size, heat flux, pulse duration, and temperature on the thermal-layer dynamics are discussed. We also show how the peak heat flux for the onset of boiling in He II can be studied in our model.

Rare-earth triangular lattice materials have been proposed as a good platform for the investigation of frustrated magnetic ground states. ${\mathrm{KErSe}}_2$, with the delafossite structure, contains perfect two-dimensional ${\mathrm{Er}}^{3+}$ triangular layers separated by potassium ions, realizing this ideal configuration and inviting study. Here we investigate the magnetism of ${\mathrm{KErSe}}_2$ at millikelvin temperatures by heat capacity and neutron powder diffraction. Heat capacity results reveal a magnetic transition at 0.2 K in zero applied field. This long-range order is suppressed by an applied magnetic field of 0.5 T below 0.08 K. Neutron powder diffraction suggests that the zero-field magnetic structure orders with $k=(\frac{1}{2},0,\frac{1}{2})$ in a stripe spin structure. Unexpectedly, Er is found to have a reduced moment of 3.06(1)${\mu }_B$/Er in the ordered state, and diffuse magnetic scattering, which originates at higher temperatures, is found to persist in the ordered state, potentially indicating magnetic fluctuations. Neutron diffraction collected under an applied field shows a metamagnetic transition at $\sim 0.5$ T to ferromagnetic order with $k=(0,0,0)$ and two possible structures, which are likely dependent on the applied field direction. The zero-field stripe spin structure can be explained by the anisotropic interactions or the first-, second-, and third-neighbor couplings in the antiferromagnetic triangular lattice.

In the XY regime of the XXZ Heisenberg model phase diagram, we demonstrate that the origin of magnetically ordered phases is influenced by the presence of solvable points with exact quantum coloring ground states featuring a quantum-classical correspondence. Using exact diagonalization and density matrix renormalization group calculations, for both the square and the triangular lattice magnets, we show that the ordered physics of the solvable points in the extreme XY regime, at $\frac{J_z}{J_{\perp }}=-1$ and $\frac{J_z}{J_{\perp }}=-\frac{1}{2}$, respectively, with $J_{\perp }>0$, adiabatically extends to the more isotropic regime $\frac{J_z}{J_{\perp }}\sim 1$. We highlight the projective structure of the coloring ground states to compute the correlators in fixed magnetization sectors which enables an understanding of the features in the static spin structure factors and correlation ratios. These findings are contrasted with an anisotropic generalization of the celebrated one-dimensional Majumdar-Ghosh model, which is also found to be (ground-state) solvable. For this model, both exact dimer and three-coloring ground states exist at $\frac{J_z}{J_{\perp }}=-\frac{1}{2}$ but only the two dimer ground states survive for any $\frac{J_z}{J_{\perp }}>-\frac{1}{2}$.

Based on the nonequilibrium Green's function formalism, the spinor Boltzmann equation beyond gradient approximation is derived in a ferromagnetic metal with a single domain wall (DW). We further obtain the charge continuity equation and the spin diffusion equation by integrating over the momentum. By using the spin diffusion equation, we get a generalized spin transfer torque (STT), in which the usual STT is extended to the case beyond the gradient approximation and with inhomogeneous current. We also calculate numerically the physical observables such as charge density $n\left(x\right)$, spin accumulation $\mathit{m}\left(x\right)$, current density $j\left(x\right)$, spin current density ${\mathit{j}}_s\left(x\right)$, etc., by the use of the spinor distribution function. Along with the Landau-Lifshitz-Gilbert-Slonczewski equation that contains the above generalized STT, we can study the motion domain wall, and the critical electric field at an initial velocity of the DW is obtained in terms of the linear stability analysis method.

We study spin-orbit torques and transport properties of Dirac electrons on the surface of ferromagnetic topological insulators. A focus is on the effects of deviation from the ideal Dirac model with linear dispersion, which takes an additional scalar form, quadratic in wave vector, and breaks particle-hole symmetry. This term removes some peculiar features of the linear Dirac model, and, combined with in-plane magnetization, gives rise to in-plane anisotropies. We study in detail the chemical potential dependence of longitudinal/transverse conductivities and reactive/dissipative spin-orbit torques, including the out-of-plane spin polarization. It is found that the in-plane anisotropy is generally stronger for $p$-type carriers than $n$-type carriers (for positive curvature of the quadratic term). It is also found that the in-plane anisotropy of the dissipative spin-orbit torque changes sign across the Dirac point. Because of the similarity of the model, we also study the magnetic Rashba model focusing on its Dirac-like parameter region. In this model, with an in-plane magnetization, a curious discontinuity is found at the Dirac point in the current-induced out-of-plane spin polarization.

Chevrel ternary superconductors show an intriguing coexistence of molecular aspects, large electron-phonon and electron-electron correlations, which to some extent still impedes their quantitative understanding. We present a first principles study on the prototypical Chevrel compound ${\mathrm{PbMo}}_6{\mathrm{S}}_8$, including electronic, structural and vibrational properties at zero and high pressure. We confirm the presence of an extremely strong electron-phonon coupling, linked to the proximity to a $\mathrm{R}\overline{3}-\mathrm{P}\overline{1}$ structural phase transition, which weakens as the system, upon applied pressures, is driven away from the phase boundary. A detailed description of the superconducting state is obtained by means of fully ab initio superconducting density-functional theory (SCDFT). SCDFT accounts for the role of phase instability, electron-phonon coupling with different intra- and intermolecular phonon modes, and without any empirical parameter, and accurately reproduces the experimental critical temperature and gap. This study provides the conclusive confirmation that Chevrel phases are phonon driven superconductors mitigated, however, by an uncommonly strong Coulomb repulsion. The latter is generated by the combined effect of repulsive Mo states at the Fermi energy and a band gap in close proximity to the Fermi level. This is crucial to rationalize why Chevrel phases, in spite of their extreme electron-phonon coupling, have critical temperatures below 15 K. In addition, we predict the evolution of the superconducting critical temperature as a function of the external pressure, showing an excellent agreement with available experimental data.

We study the effects of electron-electron interactions and hole doping on the electronic structure of Cu-doped NaFeAs using the density functional theory plus dynamical mean-field theory ($\text{DFT}+\text{DMFT}$) method. In particular, we employ an effective multiorbital Hubbard model with a realistic band structure of NaFeAs in which Cu-doping was modeled within a rigid band approximation and compute the evolution of the spectral properties, orbital-selective electronic mass renormalizations, and magnetic properties of NaFeAs on doping with Cu. In addition, we perform fully charge self-consistent $\text{DFT}+\text{DMFT}$ calculations for the long-range antiferromagnetically ordered Na(Fe,Cu)As with Cu $x=0.5$ with a real-space ordering of Fe and Cu ions. Our results reveal a crucial importance of strong electron-electron correlations and local potential difference between the Cu and Fe ions for understanding the k-resolved spectra of Na(Fe,Cu)As. On Cu-doping, we observe a strong orbital-selective localization of the Fe $3d$ states accompanied by a large renormalization of the Fe $xy$ and $xz/yz$ orbitals. Na(Fe,Cu)As exhibits bad-metal behavior associated with a coherence-to-incoherence crossover of the Fe $3d$ electronic states and local moments formation near a Mott metal-insulator transition (MIT). For heavily doped NaFeAs with Cu $x\sim 0.5$ we obtain a Mott insulator with a band gap of $\sim 0.3$ eV which is characterized by divergence of the quasiparticle effective mass of the Fe $xy$ states. In contrast to this, the quasiparticle weights of the Fe $xz/yz$ and $e$ states remain finite at the MIT. The MIT occurs via an orbital-selective Mott phase to appear at Cu $x\simeq 0.375$ with the Fe $xy$ states being Mott localized. We propose the possible importance of Fe/Cu disorder to explain the magnetic properties of Cu-doped NaFeAs.

The partial (up to 7%) substitution of Cd for Zn in the Yb-based heavy-fermion material $\mathrm{Yb}{\mathrm{Fe}}_2{\mathrm{Zn}}_{20}$ is known to induce a slight ($\sim 20\%$) reduction of the Sommerfeld specific-heat coefficient $\gamma$ and a huge (up to two orders of magnitude) reduction of the $T^2$ resistivity coefficient $A$, corresponding to a drastic and unexpected reduction of the Kadowaki-Woods ratio $A/{\gamma }^2$. Here, Yb $L_3$-edge x-ray absorption spectroscopy shows that the Yb valence state is close to $3+$ for all $x$, whereas x-ray diffraction reveals that Cd replaces the Zn ions only at the $16c$ site of the $Fd\overline{3}m$ cubic structure, leaving the $48f$ and $96g$ sites with full Zn occupation. Ab initio electronic structure calculations in pure and Cd-doped materials, carried out without considering correlations, show multiple conduction bands with only minor modifications of the band dispersions near the Fermi level and therefore do not explain the resistivity drop introduced by Cd substitution. We propose that the site-selective Cd substitution introduces light conduction bands with a substantial contribution of $\mathrm{Cd}(16c$) $5p$ levels that have weak coupling to the ${\mathrm{Yb}}^{3+}4f$ moments. These light fermions coexist with heavy fermions that originate from other conduction bands with larger participation of $\mathrm{Zn}\left(96g\right) 4p$ levels that remain strongly coupled with the ${\mathrm{Yb}}^{3+}$ local moments.

First-principles calculations are performed to study the electronic structures and topological phases of magnetic layered materials ${\mathrm{MnBi}}_2{\mathrm{Te}}_4, {\mathrm{MnBi}}_2{\mathrm{Se}}_4$, and ${\mathrm{MnSb}}_2{\mathrm{Te}}_4$ under different film thicknesses, strains, and spin-orbit coupling (SOC) strengths. All these compounds energetically prefer the antiferromagnetic (AFM) state. ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and ${\mathrm{MnSb}}_2{\mathrm{Te}}_4$ bulks are AFM topological insulators (TIs) in the AFM state, while they become Weyl semimetals in the ferromagnetic (FM) state. ${\mathrm{MnBi}}_2{\mathrm{Se}}_4$ is trivially insulating in both the AFM and FM states, but it becomes an AFM TI or a Weyl semimetal with increasing SOC strength or applying compressive strains. Under equilibrium lattice constants, the FM ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ slabs thicker than two septuple layers (SLs), the AFM ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ slabs thicker than three SLs, and the FM ${\mathrm{MnSb}}_2{\mathrm{Te}}_4$ slabs thicker than five SLs are all Chern insulators. In addition, Chern insulators can also be obtained by compressing the in-plane lattice constants of the FM ${\mathrm{MnBi}}_2{\mathrm{Se}}_4$ slabs thicker than four SLs and the FM ${\mathrm{MnSb}}_2{\mathrm{Te}}_4$ slabs of three or four SLs, but cannot be obtained using the same method in the AFM slabs of these two materials. In-plane tensile strains about 1% to 2% turn the Chern insulators into trivial insulators.