Using a mixed-ligand synthetic scheme, we create a family of quasi-two-dimensional antiferromagnets, namely, $\left[\mathrm{Cu}\left({\mathrm{HF}}_2\right){\left(\mathrm{pyz}\right)}_2\right]{\mathrm{ClO}}_4$ [pyz = pyrazine], $\left[\mathrm{Cu}{L}_2{\left(\mathrm{pyz}\right)}_2\right]{\left({\mathrm{ClO}}_4\right)}_2 [L$ = pyO = pyridine-N-oxide and 4-phpy-O = 4-phenylpyridine-N-oxide. These materials are shown to possess equivalent two-dimensional ${\left[\text{Cu}{\left(\mathrm{pyz}\right)}_2\right]}^{2+}$ nearly square layers, but exhibit interlayer spacings that vary from 6.5713 to 16.777 Å, as dictated by the axial ligands. We present the structural and magnetic properties of this family as determined via x-ray diffraction, electron-spin resonance, pulsed- and quasistatic-field magnetometry and muon-spin rotation, and compare them to those of the prototypical two-dimensional magnetic polymer $\text{Cu}{\left(\mathrm{pyz}\right)}_2{\left({\mathrm{ClO}}_4\right)}_2$. We find that, within the limits of the experimental error, the two-dimensional, intralayer exchange coupling in our family of materials remains largely unaffected by the axial ligand substitution, while the observed magnetic ordering temperature (1.91 K for the material with the ${\mathrm{HF}}_2$ axial ligand, 1.70 K for the pyO and 1.63 K for the 4-phpy-O) decreases slowly with increasing layer separation. Despite the structural motifs common to this family and $\text{Cu}{\left(\mathrm{pyz}\right)}_2{\left({\mathrm{ClO}}_4\right)}_2$, the latter has significantly stronger two-dimensional exchange interactions and hence a higher ordering temperature. We discuss these results, as well as the mechanisms that might drive the long-range order in these materials, in terms of departures from the ideal $S=1/2$ two-dimensional square-lattice Heisenberg antiferromagnet. In particular, we find that both spin-exchange anisotropy in the intralayer interaction and interlayer couplings (exchange, dipolar, or both) are needed to account for the observed ordering temperatures, with the intralayer anisotropy becoming more important as the layers are pulled further apart.

Over the past decade, donor spin qubits in isotopically enriched ${}^{28}\mathrm{Si}$ have been intensely studied due to their exceptionally long coherence times. More recently, bismuth donor electron spins have become popular because Bi has a large nuclear spin which gives rise to clock transitions (first-order insensitive to magnetic field noise). At every clock transition there are two nearly degenerate transitions between four distinct states which can be used as a pair of qubits. Here it is experimentally demonstrated that these transitions are excited by microwaves of opposite helicity such that they can be selectively driven by varying microwave polarization. This work uses a combination of a superconducting coplanar waveguide (CPW) microresonator and a dielectric resonator to flexibly generate arbitrary elliptical polarizations while retaining the high sensitivity of the CPW.

We report the preparation of single crystals of tetragonal iron sulfide (FeS) which exhibits a nearly ideal tetrahedral geometry with S–Fe–S bond angles of ${\mathrm{110.2(2)}}^{\circ }$ and ${\mathrm{108.1(2)}}^{\circ }$. Grown via hydrothermal de-intercalation of $\mathrm{K}{}_x\mathrm{Fe}{}_{2-y}\mathrm{S}{}_2$ crystals under basic and reducing conditions, the silver, platelike crystals of FeS remain stable up to $200{}^{\circ }\mathrm{C}$ under air and $250{}^{\circ }\mathrm{C}$ under inert conditions, even though the mineral “mackinawite” (FeS) is known to be metastable. FeS single crystals exhibit a superconducting state below $T_c=4$ K as determined by electrical resistivity, magnetic susceptibility, and heat capacity measurements, confirming the presence of a bulk superconducting state. Normal state measurements yield an electronic specific heat of 5 mJ/mol ${\mathrm{K}}^2$, and paramagnetic, metallic behavior with a low residual resistivity of 250 $\mu \mathrm{\Omega }\mathrm{cm}$. Magnetoresistance measurements performed as a function of magnetic field angle tilted toward both transverse and longitudinal orientations with respect to the applied current reveal remarkable two-dimensional behavior. This is paralleled in the superconducting state, which exhibits the largest known upper critical field $H_{c2}$ anisotropy of all iron-based superconductors, with $H_{c2}^{\left|\right|ab}\left(0\right)/H_{c2}^{\left|\right|c}\left(0\right)=\left(2.75T\right)/\left(0.275T\right)=10$. Comparisons to theoretical models for two-dimensional and anisotropic three-dimensional superconductors, however, suggest that FeS is the latter case with a large effective mass anisotropy. We place FeS in context to other closely related iron-based superconductors and discuss the role of structural parameters such as anion height on superconductivity.

We present the magnetic field dependencies of transport properties for $R\mathrm{PtBi}$ ($R=\mathrm{Gd}$, Dy, Tm, and Lu) half-Heusler compounds. Temperature- and field-dependent resistivity measurements of high-quality $R\mathrm{PtBi}$ single crystals reveal an unusually large, nonsaturating magnetoresistance (MR) up to 300 K under a moderate magnetic field of $H=140$ kOe. At 300 K, the large MR effect decreases as the rare earth is traversed from Gd to Lu and the magnetic field dependence of MR shows a deviation from the conventional $H^2$ behavior. The Hall coefficient $\left(R_H\right)$ for $R=\mathrm{Gd}$ indicates a sign change around 120 K, whereas $R_H$ curves for $R=\mathrm{Dy}$, Tm, and Lu remain positive for all measured temperatures. At 300 K, the Hall resistivity reveals a deviation from the linear field dependence for all compounds. Thermoelectric power measurements on this family show strong temperature and magnetic field dependencies which are consistent with resistivity measurements. A highly enhanced thermoelectric power under applied magnetic field is observed as high as $\sim 100 \mu \mathrm{V}/\mathrm{K}$ at 140 kOe. Analysis of the transport data in this series reveals that the rare-earth-based half-Heusler compounds provide opportunities to tune MR effect through lanthanide contraction and to elucidate the mechanism of nontrivial MR.

The optical properties of transition metal dichalcogenide monolayers such as the two-dimensional semiconductors ${\mathrm{MoS}}_2$ and ${\mathrm{WSe}}_2$ are dominated by excitons, Coulomb bound electron-hole pairs. The light emission yield depends on whether the electron-hole transitions are optically allowed (bright) or forbidden (dark). By solving the Bethe-Salpeter equation on top of $GW$ wave functions in density functional theory calculations, we determine the sign and amplitude of the splitting between bright and dark exciton states. We evaluate the influence of the spin-orbit coupling on the optical spectra and clearly demonstrate the strong impact of the intra-valley Coulomb exchange term on the dark-bright exciton fine structure splitting.

Repeated injection of spin-polarized carriers in a quantum dot (QD) leads to the polarization of nuclear spins, a process known as dynamic nuclear spin polarization (DNP). Here, we report the observation of p-shell carrier assisted DNP in single QDs at zero external magnetic field. The nuclear field—measured by using the Overhauser shift of the singly charged exciton state of the QDs—continues to increase, even after the carrier population in the s-shell saturates. This is also accompanied by an abrupt increase in nuclear spin buildup time as p-shell emission overtakes that of the $s$ shell. We attribute the observation to p-shell electrons strongly altering the nuclear spin dynamics in the QD, supported by numerical simulation results based on a rate equation model of coupling between electron and nuclear spin system. Dynamic nuclear spin polarization with p-shell carriers could open up avenues for further control to increase the degree of nuclear spin polarization in QDs.

We derive a dielectric function tensor model approach to render the optical response of monoclinic and triclinic symmetry materials with multiple uncoupled infrared and far-infrared active modes. We apply our model approach to monoclinic $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ single-crystal samples. Surfaces cut under different angles from a bulk crystal, (010) and $\left(\overline{2}01\right)$, are investigated by generalized spectroscopic ellipsometry within infrared and far-infrared spectral regions. We determine the frequency dependence of 4 independent $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ Cartesian dielectric function tensor elements by matching large sets of experimental data using a point-by-point data inversion approach. From matching our monoclinic model to the obtained 4 dielectric function tensor components, we determine all infrared and far-infrared active transverse optic phonon modes with $A_u$ and $B_u$ symmetry, and their eigenvectors within the monoclinic lattice. We find excellent agreement between our model results and results of density functional theory calculations. We derive and discuss the frequencies of longitudinal optical phonons in $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$. We derive and report density and anisotropic mobility parameters of the free charge carriers within the tin-doped crystals. We discuss the occurrence of longitudinal phonon plasmon coupled modes in $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ and provide their frequencies and eigenvectors. We also discuss and present monoclinic dielectric constants for static electric fields and frequencies above the reststrahlen range, and we provide a generalization of the Lyddane-Sachs-Teller relation for monoclinic lattices with infrared and far-infrared active modes. We find that the generalized Lyddane-Sachs-Teller relation is fulfilled excellently for $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$.

We investigate the effect of the filling factor on transport anisotropies, known as stripes, in high Landau levels of a two-dimensional electron gas. We find that at certain in-plane magnetic fields, the stripes orientation is sensitive to the filling factor within a given Landau level. This sensitivity gives rise to the emergence of stripes away from half-filling while orthogonally oriented, native stripes reside at half-filling. This switching of the anisotropy axes within a single Landau level can be attributed to a strong dependence of the native symmetry-breaking potential on the filling factor.

While the odd-parity electronic excited states of the prototypical acceptor boron in silicon have received detailed experimental and theoretical study, the properties of the even-parity excited states are less well known. Indeed, a recent study has strongly questioned the existing assignment of the first even-parity excited state above the $1{\mathrm{\Gamma }}_8^{+}$ ground state, and further questioned whether the assigned state is even in the silicon band gap. Here we report the results of electronic Raman scattering spectroscopy, acceptor bound exciton two-hole spectroscopy, and thermally induced infrared absorption spectroscopy, all of which support the original assignment of the lowest-lying even-parity excited state of boron in silicon to the $1{\mathrm{\Gamma }}_7^{+}$ state associated with the split-off valence band.

A method is described for calculating the energetics of planar defects in alloys based on the special-quasirandom-structure (SQS) approach. We examine the accuracy of the approach employing atomistic calculations based on a classical embedded-atom-method (EAM) interatomic potential for hexagonal close packed (hcp) alloys, for which benchmark results can be obtained by direct configurational averaging. The results of these calculations demonstrate that the SQS-based approach can be employed to derive the concentration dependence of the energies of twin boundaries, unstable stacking faults, and surfaces to within an accuracy of approximately 10%. The SQS considered in this study contain up to 72 atoms and hence are small enough to be considered in first-principles density-functional-theory (DFT) based calculations. The application of the SQS-based approach in direct DFT-based calculations is demonstrated in a study of the concentration dependence of interfacial energies for $\left\{11\overline{2}1\right\}$ twins in hcp Ti-Al alloys.

Optical vortices possess unique topology which can benefit a broad variety of applications, ranging from particle trapping and microscopy to communications and light-matter interactions. Notwithstanding their proven applicability, being subjected to the diffraction limit typically inhibits the facilitation of vortex beams into nanoscale applications. Here, we present optical nanovortices of 60 nm size employing silicon-based waveguides that support guided modes with short wavelengths. We introduce nanovortices with on-demand topological charge and show azimuthal phase variations almost ten times faster than free-space oscillations of the illumination wavelength (671 nm) using high-resolution phase-resolved near-field measurements. Moreover, we apply superoscillations of two short-wavelength guided modes to further decrease the radial size of the vortex, achieving nanosized dimensions in both the azimuthal and radial directions.

We report on the unusual and counterintuitive behavior of spin lifetime of excitons in coupled semiconductor quantum wells (CQWs) in the presence of in-plane magnetic field. Instead of conventional acceleration of spin relaxation due to the Larmor precession of electron and hole spins, we observe a strong increase of the spin relaxation time at low magnetic fields followed by saturation and decrease at higher fields. We argue that this nonmonotonic spin relaxation dynamics is a fingerprint of the magnetic quantum confined Stark effect. In the presence of electric field along the CQW growth axis, an applied magnetic field efficiently suppresses the exciton spin coherence, due to inhomogeneous broadening of the $g$-factor distribution.

The newly discovered 112-type ferropnictide superconductors contain chains of As atoms that break the tetragonal symmetry between the $a$ and $b$ axes. This feature eliminates the need for uniaxial strain that is usually required to stabilize large single domains in the electronic nematic state that exists in the vicinity of magnetic order in the iron-based superconductors. We report detailed structural symmetry measurements of 112-type ${\mathrm{Ca}}_{0.73}{\mathrm{La}}_{0.27}{\mathrm{FeAs}}_2$ using rotational anisotropy optical second-harmonic generation. This technique is complementary to diffraction experiments and enables a precise determination of the point-group symmetry of a crystal. By combining our measurements with density functional theory calculations, we uncover a strong optical second-harmonic response of bulk electric dipole origin from the Fe and Ca $3d$-derived states that enables us to assign $C_2$ as the crystallographic point group. This makes the 112-type materials high-temperature superconductors without a center of inversion, allowing for the possible mixing of singlet and triplet Cooper pairs in the superconducting state. We also perform pump-probe transient reflectivity experiments that reveal a 4.6-THz phonon mode associated with the out-of-plane motion of As atoms in the FeAs layers. We do not observe any suppression of the optical second-harmonic response or shift in the phonon frequency upon cooling through the reported monoclinic-to-triclinic transition at 58 K. This allows us to identify $C_1$ as the low-temperature crystallographic point group but suggests that structural changes induced by long-range magnetic order are subtle and do not significantly affect electronic states near the Fermi level.

Optical properties of orthorhombic $\mathrm{DyFe}{\mathrm{O}}_3$ single crystals have been studied in the far-infrared spectral range of $11\text{–}120\mathrm{c}{\mathrm{m}}^{-1}$ at temperatures between 1.5 and 50 K and in magnetic fields up to 7 T. The temperature and magnetic-field dependencies of the antiferromagnetic (AFM) resonance spectra have been measured below and above the AFM ordering temperature of $\mathrm{D}{\mathrm{y}}^{3+}$ moments with $T_N^{\mathrm{Dy}}=4.2\mathrm{K}$. Hardening of the quasiferromagnetic mode frequency of the AFM resonance due to ordering of $\mathrm{D}{\mathrm{y}}^{3+}$ moments is observed. Below $T_N^{\mathrm{Dy}}$ two electric dipole-active magnetic excitations, or electromagnons, appear in the spectra at $\sim 20$ and $\sim 50{\mathrm{cm}}^{-1}$. The electromagnons vanish with the application of a strong magnetic field along the $a$ or $b$ axis which changes the magnetic structures of $\mathrm{F}{\mathrm{e}}^{3+}$ and $\mathrm{D}{\mathrm{y}}^{3+}$ moments into the ones compatible with the spatial inversion symmetry. We show that the electromagnon at $\sim 20\mathrm{c}{\mathrm{m}}^{-1}$ provides a significant contribution to the static electric permittivity ε. The energies of the electromagnons as well as static electric permittivity manifest a strong hysteresis upon cycling of an external magnetic field at $T<T_N^{\mathrm{Dy}}$.

Perovskite ${\mathrm{SrRuO}}_3$, a prototypical conductive ferromagnetic oxide, exhibits a kink in its band dispersion signaling the unusual electron dynamics therein. However, the origin of this kink remains elusive. By taking advantage of the combo of reactive molecular beam epitaxy and in situ angle-resolved photoemission spectroscopy, we systematically studied the evolution of the low-energy electronic structure of ${\mathrm{SrRuO}}_3$ films with thickness thinning down to a nearly two-dimensional limit in a well-controlled way. The kink structure persists even in the four-unit-cell-thick film. Moreover, through quantitative self-energy analysis, we observed the negligible thickness dependence of the binding energy of the kink, which is in sharp contrast to the downward trend of the Curie temperature with reducing the film thickness. Together with previously reported transport and Raman studies, this finding suggests that the kink of perovskite ${\mathrm{SrRuO}}_3$ should originate from the electron-phonon coupling rather than magnetic collective modes, and the in-plane phonons may play a dominant role. Considering such a kink structure of ${\mathrm{SrRuO}}_3$ is similar to those of many other correlated oxides, we suggest the possible ubiquity of the coupling of electrons to oxygen-related phonons in correlated oxides.

The excitation of coherent optical phonons in solids provides a pathway for ultrafast modulation of light on a sub-ps time scale. Here, we report on efficient 3.6 THz modulation of light reflected from hybrid metal/semiconductor plasmonic crystals caused by lattice vibrations in a few-nm-thick layer of elemental tellurium. We observe that surface plasmon polaritons contribute significantly to the photoinduced formation of the tellurium layer at the interface between a telluride-based II-VI semiconductor, such as (Cd,Mg)Te or (Cd,Mn)Te, and a one-dimensional gold grating. The change in interface composition is monitored via the excitation and detection of coherent optical tellurium phonons of $A_1$ symmetry by femtosecond laser pulses in a pump-probe experiment. The patterning of a plasmonic grating onto the semiconductor enhances the transient signal which originates from the interface region. This allows one to monitor the layer formation and observe the shift of the phonon frequency caused by confinement of the lattice vibrations in the nm-thick segregated layer. Efficient excitation and detection of coherent optical phonons by means of surface plasmon polaritons are evidenced by the dependence of the signal strength on polarization of the pump and probe pulses and its spectral distribution.

We investigate the physical properties of PdTeI single crystal containing quasi-one-dimensional (1D) PdTe chains. Surprisingly, we find that the carrier concentration decreases gradually before the long-range charge density wave (CDW) ordering state occurring at $T_1\sim 110$ K, reflecting the existence of strong CDW fluctuation with possible pseudogap state at $T\gg T_1$ because of dynamic charge separation of Pd ions $\left({\mathrm{Pd}}^{3+}\to {\mathrm{Pd}}^{2+}+{\mathrm{Pd}}^{4+}\right)$. Moreover, the sliding CDW state appears below $T_2\sim 6$ K. Combining such low $T_2$ with the feature of multiple quasi-1D bands, PdTeI exhibits exotic crossover behavior from negative to huge positive magnetoresistance under magnetic field and field-induced localization. Thus, PdTeI provides a novel platform for studying the CDW fluctuation and the interplay between magnetic field and CDW state.

Surface spin waves in thin permalloy films are studied by means of propagative-spin-wave spectroscopy. We observe a systematic difference of up to several tens of MHz when comparing the frequencies of counterpropagating waves. This frequency nonreciprocity effect is modeled using an analytical dipole-exchange theory that considers the mutual influence of nonreciprocal spin wave modal profiles and differences in magnetic anisotropies at the two film surfaces. At moderate film thickness (20 nm and below), the frequency nonreciprocity scales linearly with the wave vector and quadratically with the thickness, whereas a more complex nonmonotonic behavior is observed at larger thickness. Our paper suggests that surface wave frequency nonreciprocity can be used as an accurate spectroscopic probe of magnetic asymmetries in thin ferromagnetic films.

We describe the quantum dynamics of a magnetic rigid rotor in the mesoscopic scale where the Einstein-De Haas effect is predominant. In particular, we consider a single-domain magnetic nanoparticle with uniaxial anisotropy in a magnetic trap. Starting from the basic Hamiltonian of the system under the macrospin approximation, we derive a bosonized Hamiltonian describing the center-of-mass motion, the total angular momentum, and the macrospin degrees of freedom of the particle treated as a rigid body. This bosonized Hamiltonian can be approximated by a simple quadratic Hamiltonian that captures the rich physics of a nanomagnet tightly confined in position, nearly not spinning, and with its macrospin antialigned to the magnetic field. The theoretical tools derived and used here can be applied to other quantum mechanical rigid rotors.