The existence of two inequivalent valleys in the band structure of graphene has motivated the search of mechanisms that allow their separation and control for potential device applications. Among the several schemes proposed in the literature, strain-induced out-of-plane deformations (occurring naturally or intentionally designed in graphene samples), ranks among the best candidates to produce separation of valley currents. Because the valley filtering properties in these structures are, however, highly dependent on the type of deformation and setups considered, it is important to identify the relevant factors determining optimal operation and detection of valley currents. In this paper, we present a comprehensive comparison of two typical deformations commonly found in graphene samples: local centrosymmetric bubbles and extended folds/wrinkles. Using the Dirac model for graphene and the second-order Born approximation, we characterize the scattering properties of the bubble deformation, while numerical transmission matrix methods are used for the foldlike deformations. In both cases, we obtain the dependence of valley polarization on the geometrical parameters of deformations and discuss their possible experimental realizations. Our study reveals that extended deformations act as better valley filters in broader energy ranges and present more robust features against variations of geometrical parameters and incident current directions.

Polycrystalline tellurium becomes amorphous after irradiation with strong femtosecond pulses. The amorphization is sensitive to the initial temperature but not very sensitive to the temporal profile of the optical excitation. Above the amorphization threshold, single-shot transient reflectivity traces show clear coherent phonon oscillations within 1 ps. These results suggest that amorphization is due to thermal melting rather than nonthermal melting or switching for pump fluences up to the ablation threshold.

X-ray photoemission electron microscopy (XPEEM) and magneto-optic Kerr effect (MOKE) microscopy have been performed on a metal-insulator multilayer of $[{\mathrm{Co}}_{80}{\mathrm{Fe}}_{20}(t=1.8\mathrm{nm}$)/${\mathrm{Al}}_2{\mathrm{O}}_3{\left(3\mathrm{nm}\right)]}_9$ to image ${360}^{\circ }$ domain walls (DWs) along easy and hard axes, respectively. Their creation and annihilation can be directly visualized under application of a magnetic field. XPEEM experiments and micromagnetic simulations show that ${360}^{\circ }$ DWs occur through the merger of ${180}^{\circ }$ DWs of opposite chiralities along the easy axis. They are stable even under application of large magnetic fields. Formation of ${360}^{\circ }$ DWs observed along the hard axis is attributed to symmetry breaking of the coherent spin rotation. Their formation in metal-insulator multilayers is explained as being due to the presence of an orientational dispersion of anisotropy axes in the film grains that is comparable to an overall uniaxial anisotropy term. Our results are confirmed numerically using micromagnetic simulations.

Using ab initio methods we successfully reproduce an unusual enhancement of magnetization experimentally observed in ferromagnetic ${\mathrm{SrRuO}}_3$ thin films deposited on the spin-triplet superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. We found that this enhancement is due to additional distortions of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ induced by the film. These distortions—rotation of the ${\mathrm{RuO}}_6$ octahedra—lead to a gradual reconstruction of electronic structure of the substrate, such that the Fermi level turns out to be exactly at the peak of the local density of states, corresponding to the Ru ions close to the interface. This results in the magnetic instability. The distortions propagate deep into the substrate and favor the formation of magnetic moments on the Ru ions in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ near the interface. Our estimates show that these distortions may penetrate into ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ substrate up to 40 nm in depth, which agrees with experimental observations.

The spin-disorder resistivity (SDR) of a disordered fcc-(${\mathrm{Ni}}_{1-x},{\mathrm{Fe}}_x$) alloy is determined from first principles. We identify the SDR at and above the critical temperature with the residual resistivity of the corresponding paramagnetic state evaluated in the framework of the disordered local moment (DLM) model. The underlying electronic structure is determined by means of the tight-binding linear muffin-tin orbital method, which employs the coherent potential approximation (CPA) to describe both the DLM state and the chemical disorder in alloys. An extension of the DLM fixed-spin moment method for two independent magnetic moments is used and combined with the paramagnetic lattice gas entropy to determine local moments by minimizing the corresponding free energy. The effect of phonon scattering is included through the mapping of static atomic displacements into a multicomponent random alloy which is then treated in the CPA. Finally, the Kubo-Greenwood-CPA approach is employed to estimate the SDR. We also address the problem of the validity of the Matthiessen rule at the Curie point. Good agreement of calculated and measured SDR is obtained over the whole studied concentration range; the results point to the importance of nonzero Ni magnetic moments in the limit of pure nickel.

In the spinel titanate ${\mathrm{MgTi}}_2{\mathrm{O}}_4$, the tetragonal phase collapses upon substitution of a tiny amount of Mg ions at the Ti site, and the cubic phase with the geometrical frustration is resurrected. The atomic pair distribution function (PDF) and the extended x-ray absorption fine structure (EXAFS) reveal the nanoscale structural fluctuation, in which the Ti atomic displacement has the two-in two-out configuration in the cubic phase. We argue that the geometrical frustration plays an essential role in the collapse of the tetragonal phase and the resultant nanoscale ice-type structural fluctuation.

We present a simultaneous investigation of coherent spin dynamics in both localized and itinerant carriers in Fe/GaAs heterostructures using ultrafast and spin-resolved pump-probe spectroscopy. We find that for excitation densities that push the transient Fermi energy of photocarriers above the mobility edge there exist two distinct precession frequencies in the observed spin dynamics, allowing us to simultaneously monitor both localized and itinerant states. For low magnetic fields (below 3.00 T) the beat frequency between these two excitations evolves linearly, indicating that the nuclear polarization is saturated almost immediately and that the hyperfine coupling to these two states is comparable, despite the 20 times enhancement in nuclear polarization provided by the presence of the Fe layer. At higher magnetic fields (above 3.00 T) the Zeeman energy drives reentrant localization of the photocarriers. Subtracting the constant hyperfine contribution from both sets of data allows us to extract the Lande $g$ factor for each state and estimate their energy relative to the bottom of the conduction band, yielding −2.16 and 17 meV for localized and itinerant states, respectively. This work advances our fundamental understanding of spin-spin interactions between electron and nuclear spin species, as well as between localized and itinerant electronic states, and therefore has implications for future work in both spintronics and quantum information/computation.

We study theoretically the bound state spectrum and $0\text{−}\pi$ transitions in ballistic quasi-one-dimensional superconductor/ferromagnetic insulator/superconductor Josephson junctions. In addition to the Andreev bound states, stemming from the phase coherence, the magnetic barrier gives rise to qualitatively different Yu-Shiba-Rusinov (YSR) bound states with genuine spectral features and spin characteristics. We show that zero-energy YSR states are much more robust against the presence of scalar tunneling than their Andreev counterparts and also fingerprint a quantum phase transition from the junctions' 0 into the $\pi$ phase, connected to a measurable reversal of the Josephson current; this evidence persists also in the presence of Rashba spin-orbit coupling.

We report a technique of a continuously doped surface of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+x}$ through ozone/vacuum annealing and a systematic measurement over the nearly whole superconducting dome on the same sample surface by in situ angle-resolved photoemission spectroscopy. We find that the quasiparticle weight on the antinode is proportional to the doped carrier concentration $x$ within the entire superconducting dome, while the nodal quasiparticle weight changes more mildly. More significantly, we discover that a $d$-wave pairing energy gap extracted from the nodal region scales well with the onset temperature of the Nernst signal. These findings suggest that the emergence of superconducting pairing is concomitant with the onset of free vortices.

A three-step model for high harmonic generation from impurities in solids is developed. The process is found to be similar to high harmonic generation in atomic and molecular gases, with the main difference coming from the nonparabolic nature of the bands. This opens a new avenue for strong-field atomic and molecular physics in the condensed-matter phase. As a first application, our conceptual study demonstrates the feasibility of tomographic measurement of impurity orbitals.

We present angle-dependent high-frequency electron paramagnetic resonance (EPR) studies on a single crystal of a trigonal ${\mathrm{Mn}}_3^{\mathrm{III}}$ cluster with an unusual structure in which the local magnetic easy axes of the constituent $\mathrm{M}{\mathrm{n}}^{\mathrm{III}}$ ions are tilted significantly away from the molecular $C_3$ axis towards the “magic angle” of 54.7°, resulting in an almost complete cancellation of the second-order axial magnetic anisotropy, $D{\widehat{S}}_z^2$, associated with the ferromagnetically coupled total spin $S_{\mathrm{T}}=6$ ground state. This contrasts with the situation in many related ${\mathrm{Mn}}_3^{\mathrm{III}}$ single-molecule magnets (SMMs) that have been studied intensively in the past, for which the local $\mathrm{M}{\mathrm{n}}^{\mathrm{III}}$ anisotropy tensors are reasonably parallel, resulting in substantial barriers to magnetization relaxation $(U_{\mathrm{eff}}\sim 30-35\mathrm{c}{\mathrm{m}}^{-1})$ and magnetization blocking below about 2.5 K. The suppression of the second-order anisotropy [note that the rhombic term, $E\left({\widehat{S}}_x^2-{\widehat{S}}_y^2\right)$, is also zero on symmetry grounds] in the present case results in a situation in which the zero-field splitting (ZFS) of the $S_{\mathrm{T}}=6$ ground state is dominated by fourth- and higher-order interactions. This provides a unique opportunity to study in depth how molecular geometry influences these interactions that are responsible for quantum tunneling of magnetization in high-symmetry SMMs. Angle-dependent EPR measurements provide a full mapping of the molecular magnetoanisotropy. Meanwhile, irreducible tensor operator (ITO) methods are employed in order to obtain analytic expressions that directly relate molecular anisotropy to the microscopic physics, i.e., the ZFS tensors associated with the individual $\mathrm{M}{\mathrm{n}}^{\mathrm{III}}$ ions, their orientations, and the exchange coupling between the three spins. The ITO methodology improves significantly upon previous numerical methods that have been applied to trigonal SMMs. We find that the magic-angle tilting leads to a massive compression of the $S_{\mathrm{T}}=6$ ground-state energy level diagram ($<3.5\mathrm{c}{\mathrm{m}}^{-1}$ separate the lowest- and highest-lying levels in zero field) and strong mixing between spin projection states. Although these characteristics are antagonistic to SMM behavior, they provide important insights into the physics of polynuclear molecular nanomagnets.

We have grown a series of nickel-substituted single crystals of the layered ferromagnet (FM) ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$. The large single-crystalline samples of ${\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_3{\mathrm{GeTe}}_2$ with $x=0–0.84$ were characterized with single-crystal x-ray diffraction, magnetic susceptibility, electrical resistance, and muon spin spectroscopy. We find Fe can be continuously substituted with Ni with only a minor structural variation. In addition, FM order is suppressed from $T_{\mathrm{C}}=212\mathrm{K}$ for $x=0$ down to $T_{\mathrm{C}}=50\mathrm{K}$ for $x=0.3$, which is accompanied by a strong suppression of saturated and effective moments, and Curie-Weiss temperature. Beyond $x=0.3$, the FM order is continuously smeared into a FM cluster-glass phase, with a nearly full magnetic volume fraction. We attribute the observed change in the nature of magnetic order to the intrinsically disordered structure of ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ and subsequent dilution effects from the Ni substitution.

Monolayer ${\mathrm{NbSe}}_2$ is a nodal topological Ising superconductor at magnetic in-plane fields exceeding the Pauli limit, with nodal points strictly on high-symmetry lines in the Brillouin zone. Here, we use a combined numerical and group-theoretical approach in real space to characterize the unconventional superconducting state in monolayer transition metal dichalcogenides. Even with a conventional pairing interaction, the superconducting state is intrinsically parity mixed and robust against onsite disorder. The interplay between the Zeeman magnetic field, strong spin-orbit interaction, and electronic orbital content confer the unique superconducting and topological properties. The discussion also extends to strongly hole-doped ${\mathrm{MoS}}_2$ and its relatives.

The quantum valley-Hall effect (QVHE) manifests in both classical and quantum materials as the emergence of pairs of quasichiral bands along certain interfaces. This bulk-boundary principle is well understood in the limit where the valleys result from a slight splitting of Dirac singularities. However, using a versatile experimental platform based on magnetically coupled spinners, we demonstrate that this regime is not suitable for metamaterial applications due to the delocalization of the interface modes. We also find that a strong splitting of the Dirac singularities washes away the QVHE. We then propose that the enlargement of the bulk gap to be accompanied by a Berry curvature engineering that keeps it localized near the valleys. This is a new regime, entirely outside the umbrella of Dirac physics, which we call the valley-Chern effect (VCE). By establishing an exact relation between VCE and quantum spin-Hall effect, we demonstrate a robust bulk-boundary principle, which could be the foundation of a new wave of applications of topological metamaterials.

Periodic laser pulsing of singly charged semiconductor quantum dots in an external magnetic field leads to a synchronization of the spin dynamics with the optical excitation. The pumped electron spins partially rephase prior to each laser pulse, causing a revival of electron spin polarization with its maximum at the incidence time of a laser pulse. The amplitude of this revival is amplified by the frequency focusing of the surrounding nuclear spins. Two complementary theoretical approaches for simulating up to 20 million laser pulses are developed and employed that are able to bridge between 11 orders of magnitude in time: a fully quantum mechanical description limited to small nuclear bath sizes and a technique based on the classical equations of motion applicable for a large number of nuclear spins. We present experimental data of the nonmonotonic revival amplitude as function of the magnetic field applied perpendicular to the optical axis. The dependence of the revival amplitude on the external field with a profound minimum at $4\mathrm{T}$ is reproduced by both of our theoretical approaches and is ascribed to the nuclear Zeeman effect. Since the nuclear Larmor precession determines the electronic resonance condition, it also defines the number of electron spin revolutions between pump pulses, the orientation of the electron spin at the incidence time of a pump pulse, and the resulting revival amplitude. The magnetic field of $4\mathrm{T}$, for example, corresponds to half a revolution of nuclear spins between two laser pulses.

We show that the spectrum of subbands in an electrostatically defined quantum wire in gapped bilayer graphene (BLG) directly manifests the minivalley structure and reflects Berry curvature via the associated magnetic moment of the states in the low-energy bands of this two-dimensional material. We demonstrate how these appear in degeneracies of the low-energy minibands and their valley splitting, which develops linearly in a weak magnetic field. Consequently, magnetoconductance of a ballistic point contact connecting two nongapped areas of a bilayer through a gapped (top and bottom gated) barrier would reflect such degeneracies by the heights of the first few conductance steps developing upon the increase of the doping of the BLG conduction channel (we consider an adiabatic constriction, where conductance is set by the number of propagating ballistic modes in its narrowest part): $8e^2/h$ steps in a wide channel in BLG with a large gap, $4e^2/h$ steps in narrow channels, all splitting into a staircase of $2e^2/h$ steps upon lifting valley degeneracy by a magnetic field.

We have used scanning tunneling microscopy (STM) to investigate two types of hydrogen defect structures on monolayer graphene supported by hexagonal boron nitride $\left(h\text{−}\mathrm{BN}\right)$ in a gated field-effect transistor configuration. The first H-defect type is created by bombarding graphene with 1-keV ionized hydrogen and is identified as two hydrogen atoms bonded to a graphene vacancy via comparison of experimental data to first-principles calculations. The second type of H defect is identified as dimerized hydrogen and is created by depositing atomic hydrogen having only thermal energy onto a graphene surface. Scanning tunneling spectroscopy (STS) measurements reveal that hydrogen dimers formed in this way open a new elastic channel in the tunneling conductance between an STM tip and graphene.

We report results of systematic measurements of charge transport properties of the 20.5-nm-wide HgTe-based quantum well in perpendicular magnetic field, performed under hydrostatic pressures up to 15.1 kbar. At ambient pressure, transport is well described by the two-band semiclassical model. In contrast, at elevated pressure, we observed nonmonotonic pressure dependence of resistivity at the “charge neutrality point.” For pressures lower than $\approx 9\mathrm{kbar}$, resistivity grows with pressure, in accord with expectations from the band structure calculations and the model incorporating effects of disorder on transport in two-dimensional (2D) semimetals with indirect band overlap. For higher pressures, the resistivity saturates and starts decreasing upon further increase of pressure. Above $\approx 14\mathrm{kbar}$ the resistance value and the magnetoresistance character sharply change, which may indicate significant change of electronic structure due to new electronic phase formation or some structural transitions. The data also reveal strong influence of disorder on transport in 2D electron-hole system with a small band overlap.

Superconductivity is characterized by a nonvanishing superconducting pair amplitude. It has a definite symmetry in spin, momentum, and frequency (time). While the spin and momentum symmetry have been probed experimentally for different classes of superconductivity, the odd-frequency nature of certain superconducting correlations has not been demonstrated yet in a direct way. Here, we propose the thermopower as an unambiguous way to assess odd-frequency superconductivity. This is possible since the thermoelectric coefficient given by Andreev-like processes is only finite in the presence of odd-frequency superconductivity. We illustrate our general findings with a simple example of a superconductor-quantum-dot-ferromagnet hybrid.

We study a two-flavor fermion model where each of the flavors form a Sachdev-Ye-Kitaev (SYK) system with arbitrary $q$-body interactions. The crucial new element is an all-to-all $r$-body interaction between the two flavors. At high temperatures, the model shows a strange metal phase where both flavors are gapless, similar to the usual single-flavor SYK model. Upon reducing temperature, the coupled system undergoes transitions to previously unseen phases in the SYK framework—first, a strange half-metal phase where one flavor remains a strange metal while the other is gapped, and second, an insulating phase of the Mott kind where both flavors are gapped. At a fixed low temperature, we obtain transitions between these phases by tuning the relative fraction of sites for each flavor. We discuss the physics of these phases and the nature of transitions between them. This work provides an example of an instability of the strange metal with potential to provide new routes to study strongly correlated systems through the rich physics contained in SYK-like models.