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${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$ (SCOC) is a model undoped cuprate with $I4/mmm$ crystallographic symmetry, and a simple magnetic space group ${C}_{A}mca$ with associated magnetic point group $mmm{1}^{\ensuremath{'}}$. However, recent second harmonic spectroscopy in the antiferromagnetic phase has challenged this picture, suggesting instead a magnetic point group $4/m{m}^{\ensuremath{'}}{m}^{\ensuremath{'}}$ that coexists with the antiferromagnetism and breaks the two orthogonal mirror planes containing the tetragonal $c$ axis. Here, we analyze the symmetry of SCOC in light of the second harmonic results, and discuss possible ground states that are consistent with the data.

In the presence of electron-phonon coupling, an excitonic insulator harbors two degenerate ground states described by an Ising-type order parameter. Starting from a microscopic Hamiltonian, we derive the equations of motion for the Ising order parameter in the phonon coupled excitonic insulator ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ and show that it can be controllably reversed on ultrashort timescales using appropriate laser pulse sequences. Using a combination of theory and time-resolved optical reflectivity measurements, we report evidence of such order parameter reversal in ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ based on the anomalous behavior of its coherently excited order-parameter-coupled phonons. Our Letter expands the field of ultrafast order parameter control beyond spin and charge ordered materials.

An anomalous optical second-harmonic generation (SHG) signal was previously reported in ${\mathrm{Sr}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ and attributed to a hidden odd-parity bulk magnetic state. Here we investigate the origin of this SHG signal using a combination of bulk magnetic susceptibility, magnetic-field-dependent SHG rotational anisotropy, and overlapping wide-field SHG imaging and atomic force microscopy measurements. We find that the anomalous SHG signal exhibits a twofold rotational symmetry as a function of in-plane magnetic field orientation that is associated with a crystallographic distortion. We also show a change in SHG signal across step edges that tracks the bulk antiferromagnetic stacking pattern. While we do not rule out the existence of hidden order in ${\mathrm{Sr}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$, our results altogether show that the anomalous SHG signal in parent ${\mathrm{Sr}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ originates instead from a surface-magnetization-induced electric-dipole process that is enhanced by strong spin-orbit coupling.

We have used a combination of ultrafast coherent phonon spectroscopy, ultrafast thermometry, and time-dependent Landau theory to study the inversion symmetry breaking phase transition at T_c=200 K in the strongly spin-orbit coupled correlated metal Cd_2Re_2O_7. We establish that the structural distortion at T_c is a secondary effect through the absence of any softening of its associated phonon mode, which supports a purely electronically driven mechanism. However, the phonon lifetime exhibits an anomalously strong temperature dependence that decreases linearly to zero near T_c. We show that this behavior naturally explains the spurious appearance of phonon softening in previous Raman spectroscopy experiments and should be a prevalent feature of correlated electron systems with linearly coupled order parameters.

Neutron single-crystal diffraction and rotational anisotropy optical second harmonic generation data are presented resolving the nature of the structural distortion realized in electron-doped (Sr_(1−x)La_x)_3Ir_2O_7 with x = 0.035 and x = 0.071. Once electrons are introduced into the bilayer spin-orbit assisted Mott insulator Sr_2Ir_2O_7, previous studies have identified the appearance of a low-temperature structural distortion and have suggested the presence of a competing electronic instability in the phase diagram of this material. Our measurements resolve a lowering of the structural symmetry from monoclinic C2/c to monoclinic P2_1/c and the creation of two unique Ir sites within the chemical unit cell as the lattice distorts below a critical temperature T_S. Details regarding the modifications to oxygen octahedral rotations and tilting through the transition are discussed as well as the evolution of the low-temperature distorted lattice as a function of carrier substitution.

A complete structural solution of the bilayer iridate compound Sr_3Ir_2O_7 presently remains outstanding. Previously reported structures for this compound vary and all fail to explain weak structural violations observed in neutron scattering measurements as well as the presence of a net ferromagnetic moment in the basal plane. In this paper, we present single crystal neutron diffraction and rotational anisotropy second harmonic generation measurements unveiling a lower, monoclinic symmetry inherent to Sr_3Ir_2O_7. Combined with density functional theory, our measurements identify the correct structural space group as No. 15 (C2/c) and provide clarity regarding the local symmetry of Ir^(4+) cations within this spin-orbit Mott material.

The newly discovered 112-type ferropnictide superconductors contain chains of As atoms that break the tetragonal symmetry between the ɑ 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 Ca_(0.73)La_(0.27)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.

We report a global structural distortion in Sr2IrO4 using spatially resolved optical second and third harmonic generation rotational anisotropy measurements. A symmetry lowering from an I4_1/acd to I4_1/a space group is observed both above and below the Neel temperature that arises from a staggered tetragonal distortion of the oxygen octahedra. By studying an effective superexchange Hamiltonian that accounts for this lowered symmetry, we find that perfect locking between the octahedral rotation and magnetic moment canting angles can persist even in the presence of large noncubic local distortions. Our results explain the origin of the forbidden Bragg peaks recently observed in neutron diffraction experiments and reconcile the observations of strong tetragonal distortion and perfect magnetoelastic locking in Sr_2IrO_4.

We perform time- and angle-resolved photoemission spectroscopy of a prototypical topological insulator (TI) Bi_2Se_3 to study the ultrafast dynamics of surface and bulk electrons after photoexcitation. By analyzing the evolution of surface states and bulk band spectra, we obtain their electronic temperature and chemical potential relaxation dynamics separately. These dynamics reveal strong phonon-assisted surface-bulk coupling at high lattice temperature and total suppression of inelastic scattering between the surface and the bulk at low lattice temperature. In this low temperature regime, the unique cooling of Dirac fermions in TI by acoustic phonons is manifested through a power law dependence of the surface temperature decay rate on carrier density.

We perform a time-resolved optical study of Sr${}_{2}$IrO${}_{4}$ to understand the influence of magnetic ordering on the low energy electronic structure of a strongly spin-orbit coupled ${J}_{\mathrm{eff}}$ $=$ 1/2 Mott insulator. By studying the recovery dynamics of photoexcited carriers, we find that upon cooling through the N\'eel temperature ${T}_{N}$ the system evolves continuously from a metal-like phase with fast ($\ensuremath{\sim}$50 fs) and excitation density independent relaxation dynamics to a gapped phase characterized by slower ($\ensuremath{\sim}$500 fs) excitation density-dependent bimolecular recombination dynamics, which is a hallmark of a Slater-type metal-to-insulator transition. However our data indicate that the high energy reflectivity associated with optical transitions into the unoccupied ${J}_{\mathrm{eff}}$ $=$ 1/2 band undergoes the sharpest upturn at ${T}_{N}$, which is consistent with a Mott-Hubbard type metal-to-insulator transition involving spectral weight transfer into an upper Hubbard band. These findings show Sr${}_{2}$IrO${}_{4}$ to be a unique system in which Slater- and Mott-Hubbard-type behaviors coexist and naturally explain the absence of anomalies at ${T}_{N}$ in transport and thermodynamic measurements.

A differential coupling of topological surface states to left- versus right-circularly polarized light is the basis of many optospintronics applications of topological insulators. Here we report direct evidence of circular dichroism from the surface states of Bi_2Se_3 using laser-based time-of-flight angle-resolved photoemission spectroscopy. By employing a novel sample rotational analysis, we resolve unusual modulations in the circular dichroism photoemission pattern as a function of both energy and momentum, which perfectly mimic the predicted but hitherto unobserved three-dimensional warped spin texture of the surface states. By developing a microscopic theory of photoemission from topological surface states, we show that this correlation is a natural consequence of spin-orbit coupling. These results suggest that our technique may be a powerful probe of the spin texture of spin-orbit coupled materials in general.

We report on the nonequilibrium quasiparticle dynamics in BaFe_2As_2 on both the hole-doped (Ba_(1−x)K_xFe_2As_2) and electron-doped (BaFe2−yCoyAs2) sides of the phase diagram using ultrafast pump-probe spectroscopy. Below Tc, measurements conducted at low photoinjected quasiparticle densities in the optimally and overdoped Ba1−xKxFe2As2 samples reveal two distinct relaxation processes: a fast component whose decay rate increases linearly with excitation density and a slow component with an excitation density independent decay rate. We argue that these two processes reflect the recombination of quasiparticles in the two hole bands through intraband and interband processes. We also find that the thermal recombination rate of quasiparticles increases quadratically with temperature in these samples. The temperature and excitation density dependence of the decays indicates fully gapped hole bands and nodal or very anisotropic electron bands. At higher excitation densities and lower hole dopings, the dependence of the dynamics on quasiparticle density disappears as the data are more readily understood in terms of a model which accounts for the quasiequilibrium temperature attained by the sample. In the BaFe_(2−y)Co_yAs_2 samples, dependence of the recombination rate on quasiparticle density at low dopings (i.e., y=0.12) is suppressed upon submergence of the inner hole band and quasiparticle relaxation occurs in a slow, density-independent manner.

The development of ferromagnetism in Mn-doped Bi_2Te_3 is characterized through measurements on a series of single crystals with different Mn content. Scanning tunneling microscopy analysis shows that the Mn substitutes on the Bi sites, forming compounds of the type Bi_(2−x)Mn_xTe_3, and that the Mn substitutions are randomly distributed, not clustered. Mn doping first gives rise to local magnetic moments with Curie-like behavior, but by the compositions Bi_(1.96)Mn_(0.04)Te_3 and Bi_(1.91)Mn_(0.09)Te_3, a second-order ferromagnetic transition is observed, with T_C∼9–12 K. The easy axis of magnetization in the ferromagnetic phase is perpendicular to the Bi2Te3 basal plane. Thermoelectric power and Hall effect measurements show that the Mn-doped Bi_2Te_3 crystals are p-type. Angle-resolved photoemission spectroscopy measurements show that the topological surface states that are present in pristine Bi_2Te_3 are also present at 15 K in ferromagnetic Mn-doped Bi2−xMnxTe3 and that the dispersion relations of the surface states are changed in a subtle fashion.

We show that the strongly spin-orbit coupled materials Bi_2Te_3 and Sb_2Te_3 and their derivatives belong to the Z_2 topological-insulator class. Using a combination of first-principles theoretical calculations and photoemission spectroscopy, we directly show that Bi_2Te_3 is a large spin-orbit-induced indirect bulk band gap (δ∼150 meV) semiconductor whose surface is characterized by a single topological spin-Dirac cone. The electronic structure of self-doped Sb_2Te_3 exhibits similar Z_2 topological properties. We demonstrate that the dynamics of spin-Dirac fermions can be controlled through systematic Mn doping, making these materials classes potentially suitable for topological device applications.

We report the first photoemission study of Fe_(1+x) Te—the host compound of the newly discovered iron-chalcogenide superconductors (maximum T_c∼27 K). Our results reveal a pair of nearly electron-hole compensated Fermi pockets, strong Fermi velocity renormalization, and an absence of a spin-density-wave gap. A shadow hole pocket is observed at the “X” point of the Brillouin zone which is consistent with a long-range ordered magnetostructural ground state. No signature of Fermi surface nesting instability associated with Q=(π/2,π/2) is observed. Our results collectively reveal that the Fe_(1+x)Te series is different from the undoped phases of the high Tc pnictides and likely harbor an unusual mechanism for superconductivity and magnetic order.

We present a systematic angle-resolved photoemission spectroscopic study of the high-$Tc$ superconductor class ${(\text{Sr}/\text{Ba})}_{1\ensuremath{-}x}{\text{K}}_{x}{\text{Fe}}_{2}{\text{As}}_{2}$. By utilizing a photon-energy-modulation contrast and scattering geometry we report the Fermi surface and the momentum dependence of the superconducting gap, $\ensuremath{\Delta}(\stackrel{P\vec}{k})$. A prominent quasiparticle dispersion kink reflecting strong scattering processes is observed in a binding-energy range of 25\char21{}55 meV in the superconducting state, and the coherence length or the extent of the Cooper pair wave function is found to be about $20\text{ }\text{\AA{}}$, which is uncharacteristic of a superconducting phase realized by the BCS-phonon-retardation mechanism. The observed $40\ifmmode\pm\else\textpm\fi{}15\text{ }\text{meV}$ kink likely reflects contributions from the frustrated spin excitations in a ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ magnetic background and scattering from the soft phonons. Results taken collectively provide direct clues to the nature of the pairing potential including an internal phase-shift factor in the superconducting order parameter which leads to a Brillouin zone node in a strong-coupling setting.

Momentum-resolved inelastic resonant x-ray scattering is used to map the doping evolution of bulk electronic modes in the doped Mott insulator class Nd_(2−x)Ce_xCuO_4. As the doping induced antiferromagnet/superconductor (AFM/SC) transition is approached, we observe an anisotropic redistribution of the spectral weight of collective excitations over a large energy scale along the Γ→(π,π) direction, whereas the modes exhibit broadening (∼1 eV) with relatively little softening along Γ→(π,0) with respect to the parent Mott state (x=0). Our study reveals a closing of the charge gap in the vicinity of the zone center even though the mode softening and spectral redistribution involve an unusually large energy scale over the full Brillouin zone. The collective behavior of modes in the vicinity of the AFM/SC critical transition is demonstrated.

We report an optical spectroscopy study on the newly discovered superconductor Cu0.07TiSe2. Consistent with the development from a semimetal or semiconductor with a very small indirect energy gap upon doping TiSe2, it is found that the compound has a low carrier density. Most remarkably, the study reveals a substantial shift of the screened plasma edge in reflectance towards high energy with decreasing temperature. This phenomenon, rarely seen in metals, indicates either a sizable increase of the conducting carrier concentration or/and a decrease of the effective mass of carriers with reducing temperature. We attribute the shift primarily to the latter effect.

We report an infrared study on 1T-TiSe_2, the parent compound of the newly discovered superconductor Cu_xTiSe_2. Previous studies of this compound have not conclusively resolved whether it is a semimetal or a semiconductor—information that is important in determining the origin of its unconventional charge density wave (CDW) transition. Here we present optical spectroscopy results that clearly reveal that the compound is metallic in both the high-temperature normal phase and the low-temperature CDW phase. The carrier scattering rate is dramatically different in the normal and CDW phases and the carrier density is found to change with temperature. We conclude that the observed properties can be explained within the scenario of an Overhauser-type CDW mechanism.

We utilize fine-tuned polarization selection coupled with excitation-energy variation of photoelectron signal to image the complete d-band dispersion relation in sodium cobaltates. A hybridization gap anticrossing is observed along the Brillouin zone corner and the full quasiparticle band is found to emerge as a many-body entity lacking a pure orbital polarization. At low dopings, the quasiparticle bandwidth (Fermion scale, many-body E(F) approximately 0.25 eV) is found to be smaller than most known oxide metals. The low-lying density of states is found to be in agreement with bulk-sensitive thermodynamic measurements for nonmagnetic dopings where the 2D Luttinger theorem is also observed to be satisfied.