arXiv subjects

G. Cao

Publications and source records attributed to G. Cao.

At least 109 records · Page 6Linked to original sources

arXiv2009

Optical spectroscopic investigation on the temperature-dependent electronic structure evolution of the Jeff,1/2 Mott insulator Sr2IrO4

We investigated the temperature-dependent evolution of the electronic structure of the Jeff,1/2 Mott insulator Sr2IrO4 using optical spectroscopy. The optical conductivity spectra $σ(ω)$ of this compound has recently been found to exhibit two d-d transitions associated with the transition between the Jeff,1/2 and Jeff,3/2 bands due to the cooperation of the electron correlation and spin-orbit coupling. As the temperature increases, the two peaks show significant changes resulting in a decrease in the Mott gap. The experimental observations are compared with the results of first-principles calculation in consideration of increasing bandwidth. We discuss the effect of the temperature change on the electronic structure of Sr2IrO4 in terms of local lattice distortion, excitonic effect, electron-phonon coupling, and magnetic ordering.

cond-mat.str-el↗

arXiv2009

Giant Magneto-electric Effect in the Novel Mott Insulator, Sr2IrO4

Our magnetic, electrical, and thermal measurements on single-crystals of the novel Mott insulator, Sr2IrO4, reveal a novel giant magneto-electric effect (GME) arising from a frustrated magnetic/ferroelectric state whose signatures are: (1) a strongly enhanced electric permittivity that peaks near a newly observed magnetic anomaly at 100 K, (2) a large (~100%) magneto-dielectric shift that occurs near a metamagnetic transition, and (3) magnetic and electric polarization hysteresis. The GME and electric polarization hinge on a spin-orbit gapping of 5d-bands, rather than the magnitude and spatial dependence of magnetization, as traditionally accepted.

cond-mat.str-el↗

arXiv2009

Anisotropic, Intermediate Coupling Superconductivity in Cu0.03TaS2

The anisotropic superconducting state properties in Cu0.03TaS2 have been investigated by magnetization, magnetoresistance, and specific heat measurements. It clearly shows that Cu0.03TaS2 undergoes a superconducting transition at TC = 4.03 K. The obtained superconducting parameters demonstrate that Cu0.03TaS2 is an anisotropic type-II superconductor. Combining specific heat jump = 1.6(4), gap ratio 2/kBTC = 4.0(9) and the estimated electron-phonon coupling constant ~ 0.68, the superconductivity in Cu0.03TaS2 is explained within the intermediate coupling BCS scenario. First-principles electronic structure calculations suggest that copper intercalation of 2H-TaS2 causes a considerable increase of the Fermi surface volume and the carrier density, which suppresses the CDW fluctuation and favors the raise of TC.

cond-mat.supr-con↗

arXiv2008

Bandwidth-Controlled Insulator-Metal Transition and Correlated Metallic State in 5$d$ Transition Metal Oxides Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ ($n$=1, 2, and $\infty$)

We investigated the electronic structures of the 5$d$ Ruddlesden-Popper series Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ ($n$=1, 2, and $\infty$) using optical spectroscopy and first-principles calculations. As 5$d$ orbitals are spatially more extended than 3$d$ or 4$d$ orbitals, it has been widely accepted that correlation effects are minimal in 5$d$ compounds. However, we observed a bandwidth-controlled transition from a Mott insulator to a metal as we increased $n$. In addition, the artificially synthesized perovskite SrIrO$_{3}$ showed a very large mass enhancement of about 6, indicating that it was in a correlated metallic state.

cond-mat.str-el↗

arXiv2008

Decisive Influence of Cation Size on the Magnetic Groundstate and Non-Fermi Liquid Behavior of ARuO3 (A = Ca, Sr)

We report calorimetric, magnetic and electric transport properties of single-crystal CaRuO3 and SrRuO3 as a function of temperature T and applied magnetic field B. We find that CaRuO3 is a non-Fermi-liquid metal near a magnetic instability, as characterized by the following properties: (1) the heat capacity C(T,B) ~ -T log T is readily enhanced in low applied fields, and exhibits a Schottky peak at 2.3 K that exhibits field dependence when T is reduced; (2) the magnetic susceptibility diverges as T^-x at low temperatures with 1/2 < x < 1, depending on the applied field; and (3) the electrical resistivity exhibits a T3/2 dependence over the range 1.7 < T < 24 K. No Shubnikov-de Haas oscillations are discerned at T = 0.65 K for applied fields up to 45 T. These properties, which sharply contrast those of the itinerant ferromagnet SrRuO3, indicate CaRuO3 is a rare example of a stoichiometric oxide compound that exhibits non-Fermi-liquid behavior near a quantum critical point.

cond-mat.str-el↗

arXiv2008

Novel Jeff = 1/2 Mott State Induced by Relativistic Spin-Orbit Coupling in Sr2IrO4

We investigated electronic structure of 5d transition-metal oxide Sr2IrO4 using angle-resolved photoemission, optical conductivity, and x-ray absorption measurements and first-principles band calculations. The system was found to be well described by novel effective total angular momentum Jeff states, in which relativistic spin-orbit (SO) coupling is fully taken into account under a large crystal field. Despite of delocalized Ir 5d states, the Jeff-states form so narrow bands that even a small correlation energy leads to the Jeff = 1/2 Mott ground state with unique electronic and magnetic behaviors, suggesting a new class of the Jeff quantum spin driven correlated-electron phenomena.

cond-mat.str-el↗

arXiv2007

Ca3(Ru1-xCrx)2O7: A new paradigm for spin valves

The spin valve effect is a quantum phenomenon so far only realized in multilayer thin films or heterostructures. Here we report a strong spin valve effect existing in bulk single crystals of Ca3(Ru1-xCrx)2O7 having an anisotropic, bilayered crystal structure. This discovery opens new avenues to understand the underlying physics of spin valves, and fully realize its potential in practical devices.

cond-mat.str-el↗

arXiv2007

Non-Fermi-liquid behavior in nearly ferromagnetic metallic SrIrO3 single crystals

We report transport and thermodynamic properties of single-crystal SrIrO3 as a function of temperature T and applied magnetic field H. We find that SrIrO3 is a non-Fermi-liquid metal near a ferromagnetic instability, as characterized by the following properties: (1) small ordered moment but no evidence for long-range order down to 1.7 K; (2) strongly enhanced magnetic susceptibility that diverges as T or T1/2 at low temperatures, depending on the applied field; (3) heat capacity C(T,H) ~ -Tlog T that is readily amplified by low applied fields; (4) a strikingly large Wilson ratio at T< 4K; and (5) a T3/2-dependence of electrical resistivity over the range 1.7 < T < 120 K. A phase diagram based on the data implies SrIrO3 is a rare example of a stoichiometric oxide compound that exhibits non-Fermi-liquid behavior near a quantum critical point (T = 0 and H = 0.23 T).

cond-mat.str-el↗

arXiv2007

Studies on the Weak Itinerant Ferromagnet SrRuO3 under High Pressure to 34 GPa

The dependence of the Curie temperature Tc on nearly hydrostatic pressure has been determined to 17.2 GPa for the weak itinerant ferromagnetic SrRuO3 in both polycrystalline and single-crystalline form. Tc is found to decrease under pressure from 162 K to 42.7 K at 17.2 GPa in nearly linear fashion at the rate dTc/dP = -6.8 K/GPa. No superconductivity was found above 4 K in the pressure range 17 to 34 GPa. Room-temperature X-ray diffraction studies to 25.3 GPa reveal no structural phase transition but indicate that the average Ru-O-Ru bond angle passes through a minimum near 15 GPa. The bulk modulus and its pressure derivative were determined to be B =192(3) GPa and B' = 5.0(3), respectively. Parallel ac susceptibility studies on polycrystalline CaRuO3 at 6 and 8 GPa pressure found no evidence for either ferromagnetism or superconductivity above 4 K.

cond-mat.str-el↗

arXiv2007

Anomalous itinerant magnetism in single crystal Sr4Ru3O10: A thermodynamic and transport investigation

A thermodynamic and transport study of Sr4Ru3O10 as a function of temperature and magnetic field is presented. The central results include a growing specific heat C with increasing field B, a magnetic contribution to C/T at low temperatures proportional to -log(T), an abrupt jump and a peak in C/T at 2.90 T and 7 T for B||ab-plane and B||c-axis, respectively, and corresponding changes in the low T power laws of the resistivity. The novelty of this work lies in the fact that this system is strongly anisotropic displaying spontaneous ferromagnetism along the c-axis and an intralayer metamagnetic transition with a possibility of a nearby quantum critical point. The exotic behavior reflects new physics that is yet to be understood.

cond-mat.str-el↗

arXiv2006

Partial antiferromagnetism in spin-chain Sr5Rh4O12, Ca5Ir3O12 and Ca4IrO6 single crystals

We report a structural, thermodynamic and transport study of the newly synthesized Sr5Rh4O12, Ca5Ir3O12 and Ca4IrO6 single crystals. These quasi-one-dimensional insulators consist of a triangular lattice of spin chains running along the c-axis, and are commonly characterized by a partial antiferromagnetic (AFM) order, a small entropy removal associated with the phase transitions and a sizable low-temperature specific heat linearly proportional to temperature. Sr5Rh4O12 is defined by an AFM order below 23 K with strong evidence for an Ising character and two step-like transitions in isothermal magnetization leading to a ferrimagnetic state at 2.4 T and a ferromagnetic state at 4.8 T, respectively. Ca5Ir3O12 and Ca4IrO6 are also antiferromagnetically ordered below 7.8 K and 12 K, respectively, and show an unusually large ratio of the Curie-Weiss temperature to the Neel temperature. In particular, Ca5Ir3O12, which includes both Ir4+ and Ir5+ ions, reveals that only S=1/2 spins of the Ir4+ ions are involved in the magnetic ordering whereas S=3/2 spins of the Ir5+ ions remain disordered. All results suggest the presence of the geometrical frustration that causes incomplete long-range AFM order in these quasi-one-dimensional compounds.

cond-mat.str-el↗

arXiv2006

Specific Heat of (Ca1-xSrx)3Ru2O7 Single Crystals

We have measured the specific heat of crystals of (Ca1-xSrx)3Ru2O7 using ac- and relaxation-time calorimetry. Special emphasis was placed on the characterization of the Neel (TN=56 K) and structural (Tc = 48 K) phase transitions in the pure, x=0 material. While the latter is believed to be first order, detailed measurements under different experimental conditions suggest that all the latent heat (with L ~ 0.3 R) is being captured in a broadened peak in the effective heat capacity. The specific heat has a mean-field-like step at TN, but its magntitude (Delta cP ~ R) is too large to be associated with a conventional itinerant electron (e.g. spin-density-wave) antiferromagnetic transition, while its entropy is too small to be associated with full ordering of localized spins. The TN transition broadens with Sr substitution while its magnitude decreases slowly. On the other hand, the entropy change associated with the Tc transition decreases rapidly with Sr substitution and is not observable for our x=0.58 sample.

cond-mat.str-el↗

arXiv2006

Magnetic anisotropy and geometrical frustration in the Ising spin-chain system Sr5Rh4O12

A structural and thermodynamic study of the newly synthesized single crystal Sr5Rh4O12 is reported. Sr5Rh4O12 consists of a triangular lattice of spin chains running along the c-axis. It is antiferromagnetically ordered below 23 K with the intrachain and interchain coupling being ferromagnetic (FM) and antiferromagnetic (AFM), respectively. There is strong evidence for an Ising character in the interaction and geometrical frustration that causes incomplete long-range AFM order. The isothermal magnetization exhibits two step-like transitions leading to a ferrimagnetic state at 2.4 T and a FM state at 4.8 T, respectively. Sr5Rh4O12 is a unique frustrated spin-chain system ever found in 4d and 5d based materials without a presence of an incomplete 3d-electron shell.

cond-mat.str-el↗

arXiv2006

Electronic structures of layered perovskite Sr2MO4 (M=Ru, Rh, and Ir)

We investigated the electronic structures of the two-dimensional layered perovskite Sr$_{2}$\textit{M}O$_{4}$ (\textit{M}=4\textit{d} Ru, 4\textit{d} Rh, and 5\textit{d} Ir) using optical spectroscopy and polarization-dependent O 1\textit{s} x-ray absorption spectroscopy. While the ground states of the series of compounds are rather different, their optical conductivity spectra $σ(ω)$ exhibit similar interband transitions, indicative of the common electronic structures of the 4\textit{d} and 5\textit{d} layered oxides. The energy splittings between the two $e_{g}$ orbitals, $i.e.$, $d_{3z^{2}-r^{2}}$ and $d_{x^{2}-y^{2}}$, are about 2 eV, which is much larger than those in the pseudocubic and 3\textit{d} layered perovskite oxides. The electronic properties of the Sr$_{2}$\textit{M}O$_{4}$ compounds are discussed in terms of the crystal structure and the extended character of the 4\textit{d} and 5\textit{d} orbitals.

cond-mat.str-el↗

arXiv2006

Possible quantum criticality and coexistence of spontaneous ferromagnetism and field-induced metamagnetism in triple-layered Sr4Ru3O10

A thermodynamic and transport study of Sr4Ru3O10 as a function of temperature and magnetic field are presented. The central results include a growing specific heat C with increasing field B, a divergent magnetic contribution to C/T at low temperatures, an abrupt jump and a peak in C/T at 2.90 T and 7 T for B||ab-plane and B||c-axis, respectively, and corresponding changes in the low T power laws of the resistivity. The novelty of this work lies in the fact that the quantum fluctuations occur in a system that shows both intralayer metamagnetism and interlayer spontaneous ferromagnetism, a feature characteristically different from all other systems involving quantum criticality.

cond-mat.str-el↗

arXiv2006

Field- and pressure-induced phases in Sr$_{4}$Ru$_{3}$O$_{10}$: A spectroscopic investigation

We have investigated the magnetic-field- and pressure-induced structural and magnetic phases of the triple-layer ruthenate - Sr$_{4}$Ru$_{3}$O$_{10}$. Magnetic-field-induced changes in the phonon spectra reveal dramatic spin-reorientation transitions and strong magneto-elastic coupling in this material. Additionally, pressure-dependent Raman measurements at different temperatures reveal an anomalous negative Gruneisen-parameter associated with the B$_{1g}$ mode ($\sim$ 380 cm$^{-1}$) at low temperatures (T $<$ 75K), which can be explained consistently with the field dependent Raman data.

cond-mat.str-el↗

arXiv2006

Borderline magnetism in Sr4Ru3O10: Impact of dilute La and Ca doping on itinerant ferromagnetism and metamagnetism

An investigation of La and Ca doped Sr4Ru3O10, featuring a coexistence of interlayer ferromagnetism and intralayer metamagnetism, is presented. La doping readily changes magnetism between ferromagnetism and metamagnetism by tuning the density of states. It also results in different Curie temperatures for the c-axis and the basal plane, highlighting a rare spin-orbit coupling with the crystal field states. In contrast, Ca doping enhances the c-axis ferromagnetism and the magnetic anisotropy. La doping also induces a dimensional crossover in the interlayer transport whereas Ca doping exhibits a tunneling magnetoresistance and an extraordinary T3/2-dependence of the resisitivity. The drastic changes caused by the dilute doping demonstrate a rare borderline magnetism that is delicately linked to the interplay of the density of states and spin-orbit coupling.

cond-mat.str-el↗

arXiv2005

Observation of oscillatory magnetoresistance periodic in 1/B and B in Ca3Ru2O7

We report magnetoresistance oscillations in high magnetic fields, B, up to 45 T and over a wide range of temperature in the Mott-like system Ca3Ru2O7. For B rotating within the ac-plane, slow and strong Shubnikov-de Haas (SdH) oscillations periodic in 1/B are observed for T≤1.5 K in the presence of metamagnetism. These oscillations are highly angular dependent and intimately correlated with the spin-polarization of the ferromagnetic state. For B||[110], oscillations are also observed but periodic in B (rather than 1/B) which persist up to 15 K. While the SdH oscillations are a manifestation of the presence of small Fermi surface (FS) pockets in the Mott-like system, the B-periodic oscillations, an exotic quantum phenomenon, may be a result of anomalous coupling of the magnetic field to the t2g-orbitals that makes the extremal cross-section of the FS field-dependent.

cond-mat.str-el↗