Magnetic excitations of single atoms on surfaces have been widely studied experimentally in the past decade. Lately, systems with unprecedented magnetic stability started to emerge. Here, we present a general theoretical investigation of the stability of rare-earth magnetic atoms exposed to crystal or ligand fields of various symmetry and to exchange scattering with an electron bath. By analyzing the properties of the atomic wave function, we show that certain combinations of symmetry and total angular momentum are inherently stable against first or even higher-order interactions with electrons. Further, we investigate the effect of an external magnetic field on the magnetic stability.

Motivated by the recent developments on cluster Mott insulating materials such as the cluster magnet ${\mathrm{LiZn}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$, we consider the strong plaquette charge ordered regime of the extended Hubbard model on a breathing kagome lattice and reveal the properties of the cluster Mottness. The plaquette charge order arises from the intersite charge interaction and the collective motion of three localized electrons on the hexagon plaquettes. This model leads naturally to a reduction of the local moments by 2/3, as observed in ${\mathrm{LiZn}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$. Furthermore, at low temperatures, each hexagon plaquette contains an extra orbital-like degree of freedom in addition to the remaining spin 1/2. We explore the consequence of this emergent orbital degree of freedom. We point out the interaction between the local moments is naturally described by a Kugel-Khomskii spin-orbital model. We develop a parton approach and suggest a spin-liquid ground state with spinon Fermi surfaces for this model. We further predict an emergent orbital order when the system is under a strong magnetic field. Various experimental consequences for ${\mathrm{LiZn}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ are discussed, including an argument that the charge ordering must be short ranged if the charge per Mo is slightly off stoichiometric.

The discovery of monolayer graphene has initiated two fertile fields in condensed matter physics: Dirac semimetals and atomically thin layered materials. When these trends meet again in transition metal compounds, which possess spin and orbital degrees of freedom and strong electron correlations, more exotic phenomena are expected to emerge in the cross section of topological states of matter and Mott physics. Here, we show by using ab initio calculations that a monolayer form of transition metal trichalcogenides (TMTs), which has a honeycomb network of $4d$ and $5d$ transition metal cations, may exhibit multiple Dirac cones in the electronic structure of the half-filled $e_g$ orbitals. The Dirac cones are gapped by the spin-orbit coupling under the trigonal lattice distortion and, hence, can be tuned by tensile strain. Furthermore, we show that electron correlations and carrier doping turn the multiple Dirac semimetal into a topological ferromagnet with high Chern number. Our findings indicate that the honeycomb-monolayer TMTs provide a good playground for correlated Dirac electrons and topologically nontrivial magnetism.

Using density functional theory calculations including an on-site Coulomb term, we explore electronic and possibly topologically nontrivial phases in $3d$ transition-metal oxide honeycomb layers confined in the corundum structure ($\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$) along the [0001] direction. In most cases the ground state is a trivial antiferromagnetic Mott insulator, often with distinct orbital or spin states compared to the bulk phases. With imposed symmetry of the two sublattices the ferromagnetic phases of ($X_2{\mathrm{O}}_3$)${}_1$/(${\mathrm{Al}}_2{\mathrm{O}}_3$)${}_5$(0001) with $X=$ Ti, Mn, Co, and Ni exhibit a characteristic set of four bands, two that are relatively flat and two with a Dirac crossing at $K$, associated with the single-electron occupation of $e_g^{\prime }$ (Ti) or $e_g$ (Mn, Co, Ni) orbitals. Our results indicate that the Dirac point can be tuned to the Fermi level using strain. Applying spin-orbit coupling (SOC) leads to a substantial anomalous Hall conductivity with values up to 0.94 $e^2/h$. Moreover, at $a_{{\mathrm{Al}}_2{\mathrm{O}}_3}=4.81$ Å we identify a particularly strong effect of SOC with an out-of-plane easy axis for (${\mathrm{Ti}}_2{\mathrm{O}}_3$)${}_1$/(${\mathrm{Al}}_2{\mathrm{O}}_3$)${}_5$(0001) which stabilizes the system dynamically. Due to the unusually high orbital moment of $-0.88{\mu }_{\mathrm{B}}$ that nearly compensates the spin moment of $1.01{\mu }_{\mathrm{B}}$, this system emerges as a candidate for the realization of the topological Haldane model of spinless fermions. Parallels to the perovskite analogs ($\mathrm{La}X{\mathrm{O}}_3$)${}_2$/(${\mathrm{LaAlO}}_3$)${}_4$(111) are discussed.

We address the long-term controversy on the fundamental question of the role of the surface state on the Kondo effect with Co adatoms on a Ag(111) surface. The width of the Kondo resonance oscillates with the same period of half Fermi wavelength of the surface state. But the amplitude increases for a Co adatom placed next to another Co adatom, at the vicinity of a step edge, and quantum confined within nanocorrals. A greater than three times enhancement of the resonance width can be achieved when quantum confinement is introduced. The experimental results are described quantitatively utilizing an analytical model where the contributions of the bulk and surface states are weighted by their exchange values with the magnetic impurity. These findings clarify the role of the surface state on the Kondo effect and pave a pathway to tailor the Kondo effect via quantum confinement of the surface state.

We propose a realization of mechanically tunable Ruderman-Kittel-Kasuya-Yosida interaction in a double quantum dot nanoelectromechanical device. The coupling between spins of two quantum dots suspended above a metallic plate is mediated by conduction electrons. We show that the spin-mechanical interaction can be driven by a slow modulation of charge density in the metallic plate. We propose to use Stückelberg oscillations as a sensitive tool for detection of the spin and charge states of the coupled quantum dots. Theory of mechanical back action induced by a dynamical spin-spin interaction is discussed.

Landau level broadening mechanisms in electrically neutral and quasineutral graphene were investigated through micro-magneto-Raman experiments in three different samples, namely, a natural single-layer graphene flake and a back-gated single-layer device, both deposited over ${\mathrm{Si}/\mathrm{SiO}}_2$ substrates, and a multilayer epitaxial graphene employed as a reference sample. Interband Landau level transition widths were estimated through a quantitative analysis of the magnetophonon resonances associated with optically active Landau level transitions crossing the energy of the $E_{2g}$ Raman-active phonon. Contrary to multilayer graphene, the single-layer graphene samples show a strong damping of the low-field resonances, consistent with an additional broadening contribution of the Landau level energies arising from a random strain field. This extra contribution is properly quantified in terms of a pseudomagnetic field distribution $\mathrm{\Delta }B=1.0-1.7$ T in our single-layer samples.

Uranium diantimonide (${\mathrm{USb}}_2$) reflects enigmatic properties posing a substantial challenge for a microscopic modeling. Among others, it develops a nonstandard antiferromagnetic order of a ($\uparrow \downarrow \downarrow \uparrow$)-type along the [001] crystallographic direction, and under pressure it undergoes a transition to the ferromagnetic phase. Here we propose a minimal low-energy model of ${\mathrm{USb}}_2$ which, as we demonstrate at the mean-field level, accommodates a physical mechanism for the mentioned observations. Relying on the obtained results we also comment on the features of magnetism observed in other U-based compounds: ${\mathrm{UAs}}_2,{\mathrm{UBi}}_2$, UAsSe, ${\mathrm{URh}}_x{\mathrm{Ir}}_{1-x}\mathrm{Ge}$, and ${\mathrm{UGe}}_2$.

Nonequilibrium electrochemistry raises new challenges for atomistic simulation: we need to perform molecular dynamics for the nuclear degrees of freedom with an explicit description of the electrons, which in turn must be free to enter and leave the computational cell. Here we present a limiting form for electron open boundaries that we expect to apply when the magnitude of the electric current is determined by the drift and diffusion of ions in a solution and which is sufficiently computationally efficient to be used with molecular dynamics. We present tight-binding simulations of a parallel-plate capacitor with nothing, a dimer, or an atomic wire situated in the space between the plates. These simulations demonstrate that this scheme can be used to perform molecular dynamics simulations when there is an applied bias between two metal plates with, at most, weak electronic coupling between them. This simple system captures some of the essential features of an electrochemical cell, suggesting this approach might be suitable for simulations of electrochemical cells out of equilibrium.

The emergence of superconductivity upon progressively suppressing the long-range, charge-density-wave (CDW) order characterizes the phase diagram of several materials of interest in the on-going solid-state physics research. Se-doped ${\mathrm{ZrTe}}_3$ compounds provide the most recent, suitable arena in order to investigate the interplay of otherwise competing orders in layeredlike two-dimensional systems. We present an optical study of the CDW state in ${\mathrm{ZrTe}}_{3-x}{\mathrm{Se}}_x$ at selected Se dopings, based on the measurement of the reflectivity from the far-infrared up to the ultraviolet, as a function of temperature. We particularly focus our attention to the redistribution of the spectral weight, which images the impact of the CDW state within the optical conductivity across the phase diagram of the title compounds. The electrodynamic response is consistent with a scenario based on a long-range CDW condensate at low Se doping. Upon increasing the Se content, this then gives way to local, short-range order CDW segments. Our spectral weight analysis reveals the presence of a pseudogap phase, as fingerprint of the CDW precursor effects and thus shaping the charge dynamics of the title compounds in their normal state, preceding the onset of superconductivity.

We study the dynamics of photoexcited electrons and holes in single negatively charged CdSe/ZnSe quantum dots with two-color femtosecond pump-probe spectroscopy. An initial characterization of the energy level structure is performed at low temperatures and magnetic fields of up to 5 T. Emission and absorption resonances are assigned to specific transitions between few-fermion states by a theoretical model based on a configuration interaction approach. To analyze the dynamics of individual charge carriers, we initialize the quantum system into excited trion states with defined energy and spin. Subsequently, the time-dependent occupation of the trion ground state is monitored by spectrally resolved differential transmission measurements. We observe subpicosecond dynamics for a hole excited to the $D$ shell. The energy dependence of this $D$-to-$S$ shell intraband transition is investigated in quantum dots of varying size. Excitation of an electron-hole pair in the respective $p$ shells leads to the formation of singlet and triplet spin configurations. Relaxation of the $p$-shell singlet is observed to occur on a time scale of a few picoseconds. Pumping of $p$-shell triplet transitions opens up two pathways with distinctly different scattering times. These processes are shown to be governed by the mixing of singlet and triplet states due to exchange interactions enabling simultaneous electron and hole spin flips. To isolate the relaxation channels, we align the spin of the residual electron by a magnetic field and employ laser pulses of defined helicity. This step provides ultrafast preparation of a fully inverted trion ground state of the quantum dot with near unity probability, enabling deterministic addition of a single photon to the probe pulse. Therefore our experiments represent a significant step towards using single quantum emitters with well-controled inversion to manipulate the photon statistics of ultrafast light pulses.

Creation of high-quality $p-n$ junctions in graphene monolayer is vital in studying many exotic phenomena of massless Dirac fermions. However, even with the fast progress of graphene technology for more than ten years, it remains conspicuously difficult to generate nanoscale and atomically sharp $p-n$ junctions in graphene. Here, we realized nanoscale $p-n$ junctions with atomically sharp boundaries in graphene monolayer by using monolayer vacancy island of Cu surface. The generated sharp $p-n$ junctions with the height as high as 660 meV isolate the graphene above the Cu monolayer vacancy island as nanoscale graphene quantum dots (GQDs) in a continuous graphene sheet. Massless Dirac fermions are confined by the $p-n$ junctions for a finite time to form quasibound states in the GQDs. By using scanning tunneling microscopy, we observe resonances of quasibound states in the GQDs with various sizes and directly visualize effects of geometries of the GQDs on the quantum interference patterns of the quasibound states, which allow us to test the quantum electron optics based on graphene in atomic scale.

We investigate the statistical properties of interfering directed paths in disordered media. At long distance, the average sign of the sum over paths may tend to zero (sign disordered) or remain finite (sign ordered) depending on dimensionality and the concentration of negative scattering sites $x$. We show that in two dimensions the sign-ordered phase is unstable even for arbitrarily small $x$ by identifying rare destabilizing events. In three dimensions, we present strong evidence that there is a sign phase transition at a finite $x_c>0$. These results have consequences for several different physical systems. In two-dimensional insulators at low temperature, the variable-range-hopping magnetoresistance is always negative, while in three dimensions, it changes sign at the point of the sign phase transition. We also show that in the sign-disordered regime a small magnetic field may enhance superconductivity in a random system of $D$-wave superconducting grains embedded in a metallic matrix. Finally, the existence of the sign phase transition in three dimensions implies new features in the spin-glass phase diagram at high temperature.

Traditionally, strain localization in metallic glasses is related to the thickness of the shear defect, which is confined to the nanometer scale. Using site-specific x-ray photon correlation spectroscopy, we reveal significantly accelerated relaxation dynamics around a shear band in a metallic glass at a length scale that is orders of magnitude larger than the defect itself. The relaxation time in the shear-band vicinity is up to ten times smaller compared to the as-cast matrix, and the relaxation dynamics occurs in a characteristic three-stage aging response that manifests itself in the temperature-dependent shape parameter known from classical stretched exponential relaxation dynamics of disordered materials. We demonstrate that the time-dependent correlation functions describing the aging at different temperatures can be captured and collapsed using simple scaling functions. These insights highlight how a ubiquitous nanoscale strain-localization mechanism in metallic glasses leads to a fundamental change of the relaxation dynamics at the mesoscale.

For half-doped manganese oxides that have a perovskite structure, $R{E}_{1-\mathrm{x}}A{E}_{\mathrm{x}}\mathrm{Mn}{\mathrm{O}}_3$ ($x=0.5$) (RE and AE are rare-earth and alkaline-earth elements, respectively), the phase competition (stability) between the antiferromagnetic charge- or orbital-ordered insulator (CO/OO AFI), ferromagnetic metal (FM), layered (A-type) antiferromagnetic phase [AF(A)], and spin-glass-like insulator (SGI), have been studied using single crystals prepared by the floating zone method. The CO/OO AFI, FM, AF(A), and SGI are displayed on the plane of the disorder (the variance of the RE and AE cations) versus the effective one-electron bandwidth (the averaged ionic radius of the RE and AE). In the plane of the disorder versus the effective one-electron bandwidth, similar to the phase diagram of $R{E}_{1-\mathrm{x}}A{E}_{\mathrm{x}}\mathrm{Mn}{\mathrm{O}}_3$ ($x=0.45$), the CO/OO AFI, FM, and SGI dominate at the lower-left, right, and upper regions, respectively. However, the CO/OO AFI for $x=0.5$ is more stable than that for $x=0.45$, and it expands to the plane points that correspond to the $R{E}_{0.5}\mathrm{S}{\mathrm{r}}_{0.5}\mathrm{Mn}{\mathrm{O}}_3$ ($RE=\mathrm{Nd}$ and Sm) specimens as the hole concentration is commensurate with the ordering of $\mathrm{M}{\mathrm{n}}^{3+}$/$\mathrm{M}{\mathrm{n}}^{4+}$ with a ratio of 1/1. The $y$-dependent electronic phases for $R{E}_{0.5}{\left(\mathrm{S}{\mathrm{r}}_{1-\mathrm{y}}\mathrm{B}{\mathrm{a}}_{\mathrm{y}}\right)}_{0.5}\mathrm{Mn}{\mathrm{O}}_3$ ($0\le y\le 0.5$) ($RE=\mathrm{Sm}$, $\mathrm{N}{\mathrm{d}}_{0.5}\mathrm{S}{\mathrm{m}}_{0.5}$, Nd, and Pr) show that the AF(A) intervenes between the CO/OO AFI and FM. Besides the region around ${\left(\mathrm{L}{\mathrm{a}}_{1-\mathrm{y}}\mathrm{P}{\mathrm{r}}_{\mathrm{y}}\right)}_{0.5}\mathrm{S}{\mathrm{r}}_{0.5}\mathrm{Mn}{\mathrm{O}}_3$ ($0\le y\le 1$) that has a smaller disorder, the AF(A) also exists at the regions around $R{E}_{0.5}{\left(\mathrm{S}{\mathrm{r}}_{1-\mathrm{y}}\mathrm{B}{\mathrm{a}}_{\mathrm{y}}\right)}_{0.5}\mathrm{Mn}{\mathrm{O}}_3$ ($0<y<0.3$) ($RE=\mathrm{Sm}$ and $\mathrm{N}{\mathrm{d}}_{0.5}\mathrm{S}{\mathrm{m}}_{0.5}$) that have a relatively larger disorder. This indicates that the AF(A) is rather robust against the increased disorder, even though an ordering of the ($x^2-y^2$) orbital occurs. This study has comprehensively investigated the effects of the disorder on the AF(A) as well as on the competition between the CO/OO AFI, FM, and AF(A) that is unique to $x=0.5$. The comparison of phase diagrams between $x=0.45$ and 0.5 brings further insights into the understanding of the rich electronic phases of manganites.

We study a superconductor–normal-state–superconductor Josephson junction along the edge of a quantum spin Hall insulator with a superconducting $\pi$ phase across the junction. We solve self-consistently for the superconducting order parameter and find both real junctions, where the order parameter is fully real throughout the system, and junctions where the order parameter has a complex phase winding. Real junctions host two Majorana zero modes (MZMs), while phase-winding junctions have no subgap states close to zero energy. At zero temperature we find that the phase-winding solution always has the lowest free energy, which we establish as being due to a strong proximity effect into the N region. With increasing temperature this proximity effect is dramatically decreased and we find a phase transition into a real junction with two MZMs.

In a closed single-particle quantum system, spatial disorder induces Anderson localization of eigenstates and halts wave propagation. The phenomenon is vulnerable to interaction with environment and decoherence that is believed to restore normal diffusion. We demonstrate that for a class of experimentally feasible non-Hermitian dissipators, which admit signatures of localization in asymptotic states, quantum particle opts between diffusive and ballistic regimes, depending on the phase parameter of dissipators, with sticking about localization centers. In a diffusive regime, statistics of quantum jumps is non-Poissonian and has a power-law interval, a footprint of intermittent locking in Anderson modes. Ballistic propagation reflects dispersion of an ordered lattice and introduces the second timescale for jumps, resulting in non-nonmonotonous probability distribution. Hermitian dephasing dissipation makes localization features vanish, and Poissonian jump statistics along with normal diffusion are recovered.

We demonstrate magnetic-field sensing using an ensemble of nitrogen-vacancy centers by recording the variation in the pump-light absorption due to the spin-polarization dependence of the total ground-state population. Using a 532 nm pump laser, we measure the absorption of native nitrogen-vacancy centers in a chemical-vapor-deposited diamond placed in a resonant optical cavity. For a laser pump power of 0.4 W and a cavity finesse of 45, we obtain a noise floor of $\sim 100 \mathrm{nT}/\sqrt{\mathrm{Hz}}$ spanning a bandwidth up to 125 Hz. We project a photon shot-noise-limited sensitivity of $\sim 1 \mathrm{pT}/\sqrt{\mathrm{Hz}}$ by optimizing the nitrogen-vacancy concentration and the detection method.

Materials that form the spinel structure are known to exhibit geometric frustration, which can lead to magnetic frustration as well. Through magnetization and neutron diffraction measurements, we find that $\mathrm{FeM}{\mathrm{n}}_2{\mathrm{O}}_4$ undergoes one structural and two magnetic transitions. The structural transition occurs at $T_s\sim 595\mathrm{K}$ from cubic at high temperatures to tetragonal at low temperatures. Two magnetic transitions are ferrimagnetic at $T_{\mathrm{FI}-1}\sim 373\mathrm{K}$ and $T_{\mathrm{FI}-2}\sim 50\mathrm{K}$, respectively. Further investigation of the specific heat, thermal conductivity, and Seebeck coefficient confirms both magnetic transitions. Of particular interest is that there is a significant magnetic contribution to the low-temperature specific heat and thermal conductivity, providing a unique system to study heat transport by magnetic excitations.