We present a scanning tunneling microscopy/spectroscopy study of compressed Cl adlayers on Cu(111) under ultrahigh-vacuum conditions. We describe a rational scheme to assign Cl adatoms to different surface sites. The dominant electronic state visible in scanning tunneling spectroscopy (STS) corresponds to an antibonding interaction between the Cl adlayer and the copper surface. This state was observed to be $200\mathrm{meV}$ higher in energy at hcp sites compared to fcc sites, and it is attributed to the greater charge transfer to Cl adatoms at hcp sites. Although there was no STS signature associated with bridging sites, the presence of bridging Cl adatoms along the periphery of fcc domains caused a shift in the energy of the interface state in the latter. These results shed important light on the ordering and interaction between Cl adatoms on Cu(111) and their influence of the local electronic structure of the surface.

We calculate the pure dephasing time of three-electron exchange-only qubits due to interaction with the nuclear hyperfine field. Within the $S=S_z=1/2$ spin subspace, we derive formulas for the dephasing time as a function of the position within the stability diagram consisting of the $(1,1,1)$ charge region and the neighboring charge sectors coupled by tunneling. The nuclear field and the tunneling are taken into account in a second order approximation. The analytical solutions accurately reproduce the numerical evaluation of the full problem, and in comparison with existing experimental data, we find that the dephasing times are longer but on the same time scale as for single spins.

The paraelectric-ferroelectric transition in $\mathrm{CdTi}{\mathrm{O}}_3$ has been monitored using high-resolution neutron diffraction data. This necessitated preparing a sample enriched in ${}^{114}\mathrm{Cd}$. A subtle, but significant, anisotropy in the thermal expansion of the lattice parameters for $\mathrm{CdTi}{\mathrm{O}}_3$ associated with the transition to the polar structure was observed. First-principles calculations are presented to understand energies, phonon dispersion, and structures of possible phases with different symmetries.

Size effects on vibrational modes in complex crystals remain largely unexplored, despite their importance in a variety of electronic and energy conversion technologies. Enabled by advances in a four-probe thermal transport measurement method, we report the observation of glass-like thermal conductivity in $\sim 20$-nm-thick single crystalline ribbons of higher manganese silicide, a complex, anisotropic crystal with a $\sim 10$-nm-scale lattice constant along the incommensurate $c$ axis. The boundary-scattering effect is strong for many vibrational modes because of a strong anisotropy in their group velocities or diffusive nature, while confinement effects are pronounced for acoustic modes with long wavelengths along the $c$ axis. Furthermore, the transport of the nonpropagating, diffusive modes is suppressed in the nanostructures by the increased incommensurability between the two substructures as a result of the unusual composition of the nanostructure samples. These unique effects point to diverse approaches to suppressing the lattice thermal conductivity in complex materials.

Antiferromagnetic hexagonal MnTe is a promising material for spintronic devices relying on the control of antiferromagnetic domain orientations. Here we report on neutron diffraction, magnetotransport, and magnetometry experiments on semiconducting epitaxial MnTe thin films together with density functional theory (DFT) calculations of the magnetic anisotropies. The easy axes of the magnetic moments within the hexagonal basal plane are determined to be along $〈1\overline{1}00〉$ directions. The spin-flop transition and concomitant repopulation of domains in strong magnetic fields is observed. Using epitaxially induced strain the onset of the spin-flop transition changes from $\sim 2$ to $\sim 0.5$ T for films grown on InP and ${\mathrm{SrF}}_2$ substrates, respectively.

Quantum critical (QC) phenomena can be accessed by studying quantum magnets under an applied magnetic field ($B$). The QC points are located at the end points of magnetization plateaus and separate gapped and gapless phases. In one dimension, the low-energy excitations of the gapless phase form a Luttinger liquid (LL), and crossover lines bound insulating (plateau) and LL regimes, as well as the QC regime. Alternating ferrimagnetic chains have a spontaneous magnetization at $T=0$ and gapped excitations at zero field. Besides the plateau at the fully polarized (FP) magnetization, due to the gap there is another magnetization plateau at the ferrimagnetic (FRI) magnetization. We develop spin-wave theories to study the thermal properties of these chains under an applied magnetic field: one from the FRI classical state and another from the FP state, comparing their results with quantum Monte Carlo data. We deepen the theory from the FP state, obtaining the crossover lines in the $T$ vs $B$ low-$T$ phase diagram. In particular, from local extreme points in the susceptibility and magnetization curves, we identify the crossover between an LL regime formed by excitations from the FRI state to another built from excitations of the FP state. These two LL regimes are bounded by an asymmetric domelike crossover line, as observed in the phase diagram of other quantum magnets under an applied magnetic field.

Applying thermodynamic integration within an ab initio-based free-energy approach is a state-of-the-art method to calculate melting points of materials. However, the high computational cost and the reliance on a good reference system for calculating the liquid free energy have so far hindered a general application. To overcome these challenges, we propose the two-optimized references thermodynamic integration using Langevin dynamics (TOR-TILD) method in this work by extending the two-stage upsampled thermodynamic integration using Langevin dynamics (TU-TILD) method, which has been originally developed to obtain anharmonic free energies of solids, to the calculation of liquid free energies. The core idea of TOR-TILD is to fit two empirical potentials to the energies from density functional theory based molecular dynamics runs for the solid and the liquid phase and to use these potentials as reference systems for thermodynamic integration. Because the empirical potentials closely reproduce the ab initio system in the relevant part of the phase space the convergence of the thermodynamic integration is very rapid. Therefore, the proposed approach improves significantly the computational efficiency while preserving the required accuracy. As a test case, we apply TOR-TILD to fcc Cu computing not only the melting point but various other melting properties, such as the entropy and enthalpy of fusion and the volume change upon melting. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional and the local-density approximation (LDA) are used. Using both functionals gives a reliable ab initio confidence interval for the melting point, the enthalpy of fusion, and entropy of fusion.

Antiferromagnetic domain distribution is analyzed by a scanning superconducting quantum interference device microscopy for a pyrochlore $\mathrm{T}{\mathrm{b}}_2\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ thin film with an all-in–all-out (AIAO) spin arrangement. The local magnetic field on the surface is found to originate predominantly from the $\mathrm{T}{\mathrm{b}}^{3+}(J=6)$ magnetic moment. While $\mathrm{I}{\mathrm{r}}^{4+}$ magnetic moment ($J_{\mathrm{eff}}=1/2$) is too small to detect even below the Néel temperature (120 K), $\mathrm{T}{\mathrm{b}}^{3+}$ AIAO domains grow below 40 K following the background $\mathrm{I}{\mathrm{r}}^{4+}$ domains rather than the external magnetic field, clarifying the decisive role of Ir-Tb exchange interaction in magnetic ordering.

We report x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements as well as multiplet calculations for Dy, Ho, Er, and Tm atoms adsorbed on Pt(111), Cu(111), Ag(100), and Ag(111). In the gas phase, all four elements are divalent and we label their $4f$ occupancy as $4f^n$. Upon surface adsorption, and depending on the substrate, the atoms either remain in that state or become trivalent with $4f^{n-1}$ configuration. The trivalent state is realized when the sum of the atomic correction energies ($4f$→$5d$ promotion energy $E_{fd}+$ intershell coupling energy $\delta E_c$) is low and the surface binding energy is large. The latter correlates with a high substrate density of states at the Fermi level. The magnetocrystalline anisotropy of trivalent RE atoms is larger than the one of divalent RE atoms. We ascribe this to the significantly smaller covalent radius of the trivalent state compared to the divalent one for a given RE element. For a given valency of the RE atom, the anisotropy is determined by the overlap between the $spd$ states of the RE and the $d$ states of the surface. For all investigated systems, the magnetization curves recorded at 2.5 K show absence of hysteresis indicating that magnetic relaxation is faster than about 10 s.

The magnitude of the powder spin susceptibility of an optimally doped superconductor ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$ (Hg1201) in the normal state is found to be nearly the same as that of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ near the optimally doped level. The Stoner enhancement factor of Hg1201 is larger than that of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$. The magnitude correlation of the Stoner enhancement factor is inconsistent with the effect of the recent theoretical Coulomb repulsion between $3d$ electrons and that of the superexchange interaction of a charge transfer type.

Quantum orbital-selective Mott (OSM) transitions are investigated within dynamical mean-field theory based on a two-orbital Hubbard model with different bandwidth at half filling. We find two distinct OSM phases both showing coexistence of itinerant electrons and localized spins, dependent on whether the Hund's coupling is full or of Ising type. The critical values and the nature of the OSM transitions are efficiently determined by entanglement entropy. We reveal that vanishing of the Kondo energy scale evidenced by absence of local spin fluctuations at low frequency in local dynamical spin susceptibility is responsible for the appearance of non-Fermi-liquid OSM phase in Ising Hund's coupling case. We argue that this scenario can also be applied to account for emergent quantum non-Fermi liquid in the one-band Hubbard model when short-range antiferromagnetic order is considered.

Recent magnetoconductance measurements performed on magnetic topological insulator candidates have revealed butterfly-shaped hysteresis. This hysteresis has been attributed to the formation of gapless chiral domain-wall bound states during a magnetic-field sweep. We treat this phenomenon theoretically, providing a link between microscopic magnetization dynamics and butterfly hysteresis in magnetoconductance. Further, we illustrate how a spatially resolved conductance measurement can probe the most striking feature of the domain-wall bound states: their chirality. This work establishes a regime where a definitive link between butterfly hysteresis in longitudinal magneto-conductance and domain-wall bound states can be made. This analysis provides an important tool for the identification of magnetic topological insulators.

Quantum oscillations, conventionally thought to be a metallic property, have recently been shown to arise in certain kinds of insulators, with properties very different from those in metals. All departures from the canonical behavior found so far appear in the amplitude and the phase but not in the frequency. Here I show that such robustness in the behavior of the frequency is only valid for a particle-hole symmetric insulator; in a strongly particle-hole asymmetric insulator, de Haas–van Alphen oscillations (oscillations in magnetization and susceptibility) and Shubnikov–de Haas oscillations (oscillations in the density of states) exhibit different frequencies, with the frequency of the latter changing with temperature. I demonstrate these effects with numerical calculations on a lattice model, and provide a theory to account for the unusual behavior. The relevance of the theory in the context of recent experiments on ${\mathrm{SmB}}_6$ is discussed.

We report on an electron-induced modification of alkali halides in the ultrathin film regime. The reaction kinetics and products of the modifications are investigated in the case of NaCl films grown on Ag(001). Their structural and chemical modification upon irradiation with electrons of energy 52–60 eV and 3 keV is studied using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), respectively. The irradiation effects on the film geometry and thickness (ranging from between two and five atomic layers) are examined using scanning tunneling microscopy (STM). We observe that Cl depletion follows different reaction kinetics, as compared to previous studies on NaCl thick films and bulk crystals. Na atoms produced from NaCl dissociation diffuse to bare areas of the Ag(001) surface, where they form Na-Ag superstructures that are known for the Na/Ag(001) system. The modification of the film is shown to proceed through two processes, which are interpreted as a fast disordering of the film with removal of NaCl from the island edges and a slow decrease of the structural order in the NaCl with formation of holes due to Cl depletion. The kinetics of the Na-Ag superstructure growth is explained by the limited diffusion on the irradiated surface, due to aggregation of disordered NaCl molecules at the substrate step edges.

The control of a ferromagnet's magnetization via only electric currents requires the efficient generation of current-driven spin torques. In magnetic structures based on topological insulators (TIs) current-induced spin-orbit torques can be generated. Here we show that the addition of graphene, or bilayer graphene, to a TI-based magnetic structure greatly enhances the current-induced spin-density accumulation and significantly reduces the amount of power dissipated. We find that this enhancement can be as high as a factor of 100, giving rise to a giant Edelstein effect. Such a large enhancement is due to the high mobility of graphene (bilayer graphene) and to the fact that the graphene (bilayer graphene) sheet very effectively screens charge impurities, the dominant source of disorder in topological insulators. Our results show that the integration of graphene in spintronics devices can greatly enhance their performance and functionalities.

Vertically (off-plane) ferroelectric ordering in ultrathin films has been pursued for decades. We predict the existence of intrinsic vertical polarization orderings in ultrathin metal chalcogen-diphosphates (MCDs). Taking ${\mathrm{CuInP}}_2{\mathrm{Se}}_6$ as an example, the first-principles calculation and electrostatic-energy model show that, under the open-circuit boundary condition, the ground state of bulk ${\mathrm{CuInP}}_2{\mathrm{Se}}_6$ is ferroelectric (FE) while that of monolayer is antiferroelectric (AFE), and the critical thickness for this FE/AFE transition is around six layers. Interestingly, under the closed-circuit boundary condition, the FE state can hold even for monolayer. Particularly, because of the small energy difference but the large barrier between FE and AFE orderings, the FE state can be stabilized in a free-standing monolayer, giving rise to intrinsic, off-plane two-dimensional ferroelectrics. Applying Monte Carlo simulations, we further calculate the ferroelectric Curie temperature ($T_c$) and electric hysteresis.

Atomic nanolines are one-dimensional systems realized by assembling many atoms on a substrate into long arrays. The electronic properties of the nanolines depend on those of the substrate. Here, we demonstrate that to fully understand the electronic properties of Bi nanolines on clean Si(001) several different contributions must be accounted for. Scanning tunneling microscopy reveals a variety of different patterns along the nanolines as the imaging bias is varied. We observe an electronic phase shift of the Bi dimers, associated with imaging atomic $p$ orbitals, and an electronic coupling between the Bi nanoline and neighboring Si dimers, which influences the appearance of both. Understanding the interplay between the Bi nanolines and Si substrate could open a novel route to modifying the electronic properties of the nanolines.

Using an enhanced optically heterodyned optical Kerr effect method and a theoretical description of the interactions between an optical beam, a single layer of graphene, and its substrate, we provide experimental answers to questions raised by theoretical models of graphene third-order nonlinear optical response. In particular, we measure separately the time response of the two main tensor components of the nonlinear susceptibility, we validate the assumption that the out-of-plane tensor components are small, and we quantify the optical impact of the substrate on the measured coefficients. Our method can be applied to other two-dimensional materials, as it relies mainly on the small ratio between the thickness and the wavelength.

We report low-temperature transport measurements in strained $\mathrm{InAs}/\mathrm{G}{\mathrm{a}}_{0.68}\mathrm{I}{\mathrm{n}}_{0.32}\mathrm{Sb}$ quantum wells, which supports time-reversal symmetry-protected helical edge states. The temperature and bias voltage dependence of the helical edge conductance for devices of various sizes are consistent with the theoretical expectation of a weakly interacting helical edge state. Moreover, we found that the magnetoresistance of the helical edge states is related to the edge interaction effect and the disorder strength.