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Spin degree of freedom generally plays an important role in unconventional superconductivity. In many of the iron-based compounds, superconductivity is found in close proximity to long-range antiferromagnetic order, whereas monolayer FeSe grown on SrTiO3, with enhanced superconductivity, exhibits no magnetic or nematic ordering. Here we grow monolayer and multilayer FeSe on antiferromagnetic EuTiO3(001) layers, in an effort to introduce a spin polarization in proximity to the superconductivity of FeSe. By X-ray magnetic dichroism, we observe an antiferromagnet–ferromagnet switching on Eu and Ti sites in EuTiO3 driven by the applied magnetic field, with no concomitant spin polarization on the Fe site of FeSe. Transport measurements show enhanced superconductivity of monolayer FeSe on EuTiO3 with a transition temperature of ~30 K. The band structure revealed by photoemission spectroscopy is analogous to that of FeSe/SrTiO3. Our work creates a platform for the interplay of spin and unconventional superconductivity in the two-dimensional limit.

We study the coupling of the fully symmetric vibration mode of arsenic atoms to magnetism in a $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Au}}_{x})}_{2}{\mathrm{As}}_{2}$ system by polarization-resolved Raman spectroscopy and neutron diffraction. In this system, there are two phase transitions: a tetragonal-to-orthorhombic structural phase transition at temperature ${T}_{S}$ and a magnetic phase transition into collinear spin-density wave (SDW) state at temperature ${T}_{N}$ ($\ensuremath{\le}{T}_{S}$). ${T}_{S}$ and ${T}_{N}$ almost coincide in the pristine compound, whereas they differ by as much as 8 K for compounds with dilute gold substitution for iron. Raman coupling to the ${A}_{g}$(As) phonon is forbidden for the $XY$ scattering geometry in the tetragonal phase above ${T}_{S}$, whereas it becomes allowed in the orthorhombic phase below ${T}_{S}$: The emerging mode's intensity indicates the lattice orthorhombicity. We find that upon cooling below ${T}_{S}$, first, weak ${A}_{g}$(As) phonon mode intensity appears in the $XY$ scattering geometry spectra; however, the mode's intensity is significantly enhanced in the magnetic phase below ${T}_{N}$. The ${A}_{g}$(As) phonon also shows an asymmetric line shape below ${T}_{N}$ and an anomalous linewidth broadening upon Au doping. We describe the anomalous behavior of the ${A}_{g}$(As) mode in the $XY$ scattering geometry using a Fano model involving the ${A}_{g}$(As) phonon interacting with the ${B}_{2g}({D}_{4h}$)-symmetry-like electron-hole continuum. We conclude that the temperature dependence of light coupling amplitude to the ${A}_{g}$(As) phonon follows the evolution of the magnetic order parameter $M(T)$. We propose that the intensity enhancement of the ${A}_{g}$(As) phonon in the $XY$ scattering geometry below ${T}_{N}$ is due to electronic anisotropy induced by the collinear SDW order parameter.

The protected electron states at the boundaries or on the surfaces of topological insulators (TIs) have been the subject of intense theoretical and experimental investigations. Such states are enforced by very strong spin-orbit interaction in solids composed of heavy elements. Here, we study the composite particles -- chiral excitons -- formed by the Coulomb attraction between electrons and holes residing on the surface of an archetypical three-dimensional topological insulator (TI), Bi$_2$Se$_3$. Photoluminescence (PL) emission arising due to recombination of excitons in conventional semiconductors is usually unpolarized because of scattering by phonons and other degrees of freedom during exciton thermalization. On the contrary, we observe almost perfectly polarization-preserving PL emission from chiral excitons. We demonstrate that the chiral excitons can be optically oriented with circularly polarized light in a broad range of excitation energies, even when the latter deviate from the (apparent) optical band gap by hundreds of meVs, and that the orientation remains preserved even at room temperature. Based on the dependences of the PL spectra on the energy and polarization of incident photons, we propose that chiral excitons are made from massive holes and massless (Dirac) electrons, both with chiral spin textures enforced by strong spin-orbit coupling. A theoretical model based on such proposal describes quantitatively the experimental observations. The optical orientation of composite particles, the chiral excitons, emerges as a general result of strong spin-orbit coupling in a 2D electron system. Our findings can potentially expand applications of TIs in photonics and optoelectronics., 22 pages, 11 figures

We performed an optical spectroscopy study of electronic and magnetic excitations for a rare-earth system with a single electron quasi-localized in the f-shell on an ion at high-symmetry crystallographic site in application to CeB$_{6}$ heavy-fermion metal. We carried out group-theoretical classification of the electronic crystal field (CF) transitions and assessed their coupling to light cross-sections for polarization resolved Raman scattering processes. We discuss applicability of symmetrized Raman susceptibility to studies of exotic charge and spin high multiplet ordering phases in f-electron systems. We study temperature effects on intra- and inter-multiplet CF transitions and also on the coupling between the CF excitations with the lattice vibrations. We acquired temperature dependence of the low-frequency polarization resolved Raman response and obtained the static Raman susceptibility for all Raman-allowed symmetry channels: A$_{1g}$, E$_{g}$, T$_{1g}$, and T$_{2g}$ of the cubic O$_{h}$ point group. We demonstrate that for CeB$_{6}$ system only T$_{1g}$-symmetry static Raman susceptibility shows an anomalous temperature dependence which is consistent with the magnetic susceptibility data measured by other techniques. This anomalous behavior in the T$_{1g}$-channel signifies the presence of long wavelength magnetic fluctuations, while the lack of susceptibility enhancement for all the remaining symmetry channels indicates that long wavelength charge quadrupole fluctuations at low-temperature are weak.

Topological magnets comprising 2D magnetic layers with Curie temperatures (TC ) exceeding room temperature are key for dissipationless quantum transport devices. However, the identification of a material with 2D ferromagnetic planes that exhibits an out-of-plane-magnetization remains a challenge. This study reports a ferromagnetic, topological, nodal-line, and semimetal MnAlGe composed of square-net Mn layers that are separated by nonmagnetic Al-Ge spacers. The 2D ferromagnetic Mn layers exhibit an out-of-plane magnetization below TC ≈ 503 K. Density functional calculations demonstrate that 2D arrays of Mn atoms control the electrical, magnetic, and therefore topological properties in MnAlGe. The unique 2D distribution of the Berry curvature resembles the 2D Fermi surface of the bands that form the topological nodal line near the Fermi energy. A large anomalous Hall conductivity of ≈700 S cm-1 is obtained at 2 K and related to this nodal-line-induced 2D Berry curvature distribution. The high transition temperature, large anisotropic out-of-plane magnetism, and natural heterostructure-type atomic arrangements consisting of magnetic Mn and nonmagnetic Al/Ge elements render nodal-line MnAlGe one of the few, unique, and layered topological ferromagnets that have ever been observed.

The transition metal dichalcogenide TiSe$_2$ has received significant research attention over the past four decades. Different studies have presented ways to suppress the 200~K charge density wave transition, vary low temperature resistivity by several orders of magnitude, and stabilize magnetism or superconductivity. Here we give the results of a new synthesis technique whereby samples were grown in a high pressure environment with up to 180~bar of argon gas. Above 100~K, properties are nearly unchanged from previous reports, but a hysteretic resistance region that begins around 80~K, accompanied by insulating low temperature behavior, is distinct from anything previously observed. An accompanying decrease in carrier concentration is seen in Hall effect measurements, and photoemission data show a removal of an electron pocket from the Fermi surface in an insulating sample. We conclude that high inert gas pressure synthesis accesses an underlying nonmetallic ground state in a material long speculated to be an excitonic insulator., Comment: 11 pages, 7 figures

My thesis research focuses on using low temperature polarization resolved Raman spectroscopy to identify optically excited collective modes in strongly correlated electron systems and three-dimensional (3D) topological insulators. In particular, we are interested in collective modes presenting in the pseudovector symmetry channel, such as the A2g representation of D4h group, which is antisymmetric with respect to in-plane reections. Previously, such excitations are primarily seen in magnetic materials, where the time reversal symmetry is broken. Here, we present two examples where A2g collective modes arises from time reversal symmetry preserved ground states. In the first example, we study a heavy fermion metal URu2-xFexSi2, which holds long standing interest in the strongly correlated electron community due to an emergent long range order it exhibits at low temperature and x < 0.1, known as the hidden order" (HO). By studying the temperature dependent Raman response function in all allowed symmetry channels for various iron concentrations x, we found evidences of broken symmetries and a possible order parameter. In the low doped HO phase, we observed a sharp in gap mode with A2g symmetry and small leakage into the A1g symmetry. Our results show strong indication of the local reflection symmetries broken at the uranium site in the HO phase [Kung et al., Science 347, 6228 (2015)]. As we increase x, the energy of an A2g collective mode decreases to almost zero at the phase boundary, and recovers in a well known AFM phase. This is a direct evidence that both HO and AFM phases are connected by an A2g type order parameter, that arranges the uranium-5f orbitals into orders breaking either chirality or time reversal symmetry [Kung et al., Phys. Rev. Lett. 117, 227601 (2016)]. In the second example, we study a 3D topological insulator Bi2Se3, which has a rhombohedral lattice with the D3d point group symmetry. At the crystal surface characterized by the C6v point group, strong spin orbit coupling and time reversal symmetry conspire to form topologically protect Dirac states with chiral spin texture. While the existence and spin texture of the surface states are mostly understood for Bi2Se3 through photoemission and scanning tunneling spectroscopies, very few examples of collective excitations from the Dirac surface states have so far been identified, and therefore interactions between the Dirac particles are yet unclear. In our study, we tune the excitation energy into resonance between two surface Dirac cones about 1.8 eV apart to enhance the signal contributed by surface electrons. We observed a sharp collective mode with energy of about 150meV appearing below the edge of a gapped continuum, in the pseudovector-like A2 symmetry of the C6v group. By comparing the data with calculations, we identify this peak as the transverse collective spin mode of surface Dirac fermions: a collective spin flip excitation from the lower to upper Dirac cone [Kung et al., Phys. Rev. Lett. 119, 136802 (2017)]. This is a new and direct optical measurement of the dynamical response from the Dirac fermions in topological materials. The spin mode survives even at room temperature with slightly decreased intensity, which is likely due to the more available decay channels through interacting with surface phonons [Kung et al., Phys. Rev. B 95, 245406 (2017)]. The robustness of the chiral spin mode suggests potential applications in room temperature magnonics and optoelectronic devices.

In a superconducting nanostructure, phase fluctuations are prominent and give rise to finite resistance below superconducting transition temperature ${T}_{c}$. By using a monolayer polymer/nanosphere hybrid we developed previously, we fabricated a large array of interconnected niobium (Nb) honeycomb lattices with the thinnest interconnected linewidth $d$ ranging from 36 nm to 89 nm. The honeycomb cells form a highly ordered triangular lattice with more than ${10}^{8}$ unit cells extending over few ${\mathrm{mm}}^{2}$ area, which enables the detailed transport study at nanometer scales. We found ${T}_{c}$ gradually drops with decreasing $d$ due to the phase-slip effect, while the critical field at lower temperature tends to follow that of a continuous Nb thin film. One likely scenario is to consider a model system of numerous superconducting islands interconnected by short phase-slip junctions, where the phase coherence is dictated by the phase slippage in the nanoconstriction. This was strongly supported by the excellent fitting to the thermally activated phase-slip model and also the unusual phenomena of transition width narrowing in high fields.

We present a polarization resolved Raman scattering study of surface vibration modes in the topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ single crystal and thick films. Besides the four Raman active bulk phonons, we observed four additional modes with much weaker intensity and slightly lower energy than the bulk counterparts. Using symmetry analysis, we assigned these additional modes to out-of-plane surface phonons. Comparing with first-principle calculations, we conclude that the appearance of these modes is due to $c$-axis lattice distortion and van der Waals gap expansion near the crystal surface. Two of the surface modes at 60 and 173 ${\mathrm{cm}}^{\ensuremath{-}1}$ are associated with Raman active ${A}_{1g}$ bulk phonon modes, the other two at 136 and 158 ${\mathrm{cm}}^{\ensuremath{-}1}$ are associated with infrared active bulk phonons with ${A}_{2u}$ symmetry. The latter become Raman allowed due to reduction of crystalline symmetry from ${D}_{3d}$ in the bulk to ${C}_{3v}$ on the crystal surface. In particular, the 158 ${\mathrm{cm}}^{\ensuremath{-}1}$ surface phonon mode shows a Fano line shape under resonant excitation, suggesting interference in the presence of electron-phonon coupling of the surface excitations.

Using polarization-resolved resonant Raman spectroscopy, we explore collective spin excitations of the chiral surface states in a three dimensional topological insulator, Bi$_2$Se$_3$. We observe a sharp peak at 150 meV in the pseudovector $A_2$ symmetry channel of the Raman spectra. By comparing the data with calculations, we identify this peak as the transverse collective spin mode of surface Dirac fermions. This mode, unlike a Dirac plasmon or a surface plasmon in the charge sector of excitations, is analogous to a spin wave in a partially polarized Fermi liquid, with spin-orbit coupling playing the role of an effective magnetic field., Comment: 9 pages, 7 figures, accepted for publication in Phys. Rev. Lett

In URu$_2$Si$_2$ two types of staggered phases involving long range ordering of the uranium-5f electrons compete at low temperature when a critical parameter $x$ is tuned, where $x$ can be chemical substituent concentration, pressure or magnetic field. When cooled at below the critical value $x_c$, the non-magnetic `hidden order' (HO) phase with broken local chiral symmetry emerges, whereas above $x_c$, unconventional antiferromagnetic (AF) phase with broken local time-reversal symmetry appears. The two phases show strikingly similar electronic properties. `Janus faces' nature of the HO and AF phases has been theorized before, but the experimental signatures of the interplay between them are still lacking. Here, we use polarized Raman scattering to study the dynamical fluctuations between the two competing ground states as a function of $x$. Albeit the distinct discrete symmetries are broken above and below $x_c$, we detect a resonance continuously evolving with parameter $x$, providing evidence for a unified order parameter across the URu$_2$Si$_2$ phase diagram.

In this study, we use angle resolved photoemission spectroscopy, Raman spectroscopy, low energy electron diffraction, and x-ray scattering to reveal an unusual electronically mediated charge density wave (CDW) in K0.9Mo6O17. Not only does K0.9Mo6O17 lack signatures of electron-phonon coupling, but it also hosts an extraordinary surface CDW, with TS_CDW = 220 K nearly twice that of the bulk CDW, TB_CDW = 115 K. While the bulk CDW has a BCS-like gap of 12 meV, the surface gap is 10 times larger and well in the strong coupling regime. Strong coupling behavior combined with the absence of signatures of strong electron-phonon coupling indicates that the CDW is likely mediated by electronic interactions enhanced by low dimensionality.

Uncovering the symmetry of a hidden order Cooling matter generally makes it more ordered and may induce dramatic transitions: Think of water becoming ice. With increased order comes loss of symmetry; water in its liquid form will look the same however you rotate it, whereas ice will not. Kung et al. studied the symmetry properties of a mysteriously ordered phase of the material URu 2 Si 2 that appears at 17.5 K. They shone laser light on the crystal and studied the shifts in the frequency of the light. The electron orbitals of the uranium had a handedness to them that alternated between the atomic layers. Science , this issue p. 1339

We studied the defects of Bi2Se3 generated from Bridgman growth of stoichiometric and nonstoichiometric self-fluxes. Growth habit, lattice size, and transport properties are strongly affected by the types of defects generated. Major defect types of the BiSe antisite and partial Bi2-layer intercalation are identified through combined studies of direct atomic-scale imaging with scanning transmission electron microscopy in conjunction with energy-dispersive x-ray spectroscopy, x-ray diffraction, and Hall effect measurements. We propose a consistent explanation to the origin of defect type, growth morphology, and transport property.

Hole-doped single crystals of misfit-layered cobaltate Bi2−xPbxSr2−zCo2Oy (x = 0–0.61, y = 8.28–8.62, and z = 0.01–0.22) have been successfully grown using the optical floating-zone method. Heavier hole doping has been achieved through both Pb substitution in the Bi site and the more effective Sr vacancy formation. The Co4+ : Co3+ ratio can be raised significantly from its original ∼1 : 1 to 4.5 : 1, as confirmed by iodometric titration. A spin-glass transition temperature of Tg ∼ 70 K is confirmed by ac susceptibility measurement when the Co4+ : Co3+ ratio becomes higher than 2 : 1, presumably owing to the significantly increased probability of triangular geometrical frustration among antiferromagnetically coupled localized Co4+ spins.

We have developed an approach to prepare large array nanostructured thin films using polyethylene oxide assisted self-assembly of polystyrene nanospheres as a template. By tuning the antidot diameter in the cobalt thin films, we found a crossover behavior in the magnetization reversal process. In addition, the effective moment per cobalt atom turns out to drop with growing antidot diameter which effectively increases the surface to bulk volume fraction. Our results suggest a tendency of reduced saturation magnetization in a ferromagnet/normal metal interface, where the mixing of itinerant electrons with ferromagnetic d electrons can give rise to observable effect.