arXiv subjects

G. Cao

Publications and source records attributed to G. Cao.

At least 91 records · Page 5Linked to original sources

arXiv2013

Tuning electronic structures via epitaxial strain in Sr2IrO4 thin films

We have synthesized epitaxial Sr2IrO4 thin-films on various substrates and studied their electronic structures as a function of lattice-strains. Under tensile (compressive) strains, increased (decreased) Ir-O-Ir bond-angles are expected to result in increased (decreased) electronic bandwidths. However, we have observed that the two optical absorption peaks near 0.5 eV and 1.0 eV are shifted to higher (lower) energies under tensile (compressive) strains, indicating that the electronic-correlation energy is also affected by in-plane lattice-strains. The effective tuning of electronic structures under lattice-modification provides an important insight into the physics driven by the coexisting strong spin-orbit coupling and electronic correlation.

cond-mat.str-el↗

arXiv2013

Superconductivity and Strong Non-Fermi-Liquid Behavior in Single-Crystal Ir3Te8

We observe superconductivity below a critical temperature TC = 1.8 K in single-crystal Ir3Te8, which also exhibits normal-state diamagnetism and a linear temperature dependence of electrical resistivity for a wide temperature interval, 20 K < T < 700 K. Single-crystal Ir3Te8 also undergoes a structural phase transition at TS = 350 K from a cubic (above TS) to a rhombohedral lattice below TS. Our first-principles electronic structure calculations reveal two bands crossing the Fermi level; despite the three-dimensional lattice, one band is quasi-two-dimensional, and is responsible for the observed diamagnetism and structure transition. The strong non-Fermi-liquid behavior characterized by the observed linearity in resistivity in such a nonmagnetic state suggests novel physics in this newly discovered superconductor.

cond-mat.supr-con↗

arXiv2012

Dimensionality controlled Mott transition and correlation effects in single- and bi-layer perovskite iridates

We studied Sr2IrO4 and Sr3Ir2O7 using angle-resolved photoemission spectroscopy (ARPES), making direct experimental determinations of intra- and inter-cell coupling parameters as well as Mott correlations and gap sizes. The results are generally consistent with LDA+U+Spin-orbit coupling (SOC) calculations, though the calculations missed the momentum positions of the dominant electronic states and neglected the importance of inter-cell coupling on the size of the Mott gap. The calculations also ignore the correlation-induced spectral peak widths, which are critical for making a connection to activation energies determined from transport experiments. The data indicate a dimensionality-controlled Mott transition in these 5d transition-metal oxides (TMOs).

cond-mat.str-el↗

arXiv2012

Testing the validity of the strong spin-orbit-coupling limit for octahedrally coordinated iridates in a model system Sr$_3$CuIrO$_6$

The electronic structure of Sr$_3$CuIrO$_6$, a model system for the 5d Ir ion in an octahedral environment, is studied through a combination of resonant inelastic x-ray scattering (RIXS) and theoretical calculations. RIXS spectra at the Ir L$_3$-edge reveal an Ir $t_{2g}$ manifold that is split into three levels, in contrast to the expectations of the strong spin-orbit-coupling limit. Effective Hamiltonian and $ab inito$ quantum chemistry calculations find a strikingly large non-cubic crystal field splitting comparable to the spin-orbit coupling, which results in a strong mixing of the $j_{\mathsf{eff}}=1/2$ and $j_{\mathsf{eff}}=3/2$ states and modifies the isotropic wavefunctions on which many theoretical models are based.

cond-mat.str-el↗

arXiv2012

Spin-orbit tuned metal-insulator transitions in single-crystal Sr2Ir1-xRhxO4 (0\leqx\leq1)

Sr2IrO4 is a magnetic insulator driven by spin-orbit interaction (SOI) whereas the isoelectronic and isostructural Sr2RhO4 is a paramagnetic metal. The contrasting ground states have been shown to result from the critical role of the strong SOI in the iridate. Our investigation of structural, transport, magnetic and thermal properties reveals that substituting 4d Rh4+ (4d5) ions for 5d Ir4+(5d5) ions in Sr2IrO4 directly reduces the SOI and rebalances the competing energies so profoundly that it generates a rich phase diagram for Sr2Ir1-xRhxO4 featuring two major effects: (1) Light Rh doping (0\leqx\leq0.16) prompts a simultaneous and precipitous drop in both the electrical resistivity and the magnetic ordering temperature TC, which is suppressed to zero at x = 0.16 from 240 K at x=0. (2) However, with heavier Rh doping (0.24< x<0.85 (\pm0.05)) disorder scattering leads to localized states and a return to an insulating state with spin frustration and exotic magnetic behavior that only disappears near x=1. The intricacy of Sr2Ir1-xRhxO4 is further highlighted by comparison with Sr2Ir1-xRuxO4 where Ru4+(4d4) drives a direct crossover from the insulating to metallic states.

cond-mat.str-el↗

arXiv2012

Field-induced magnetic behavior of the bilayer iridate Sr3Ir2O7

We have performed resonant magnetic x-ray scattering (RMXS) measurements on single crystal samples of the bilayer iridate Sr3Ir2O7. We observe the development of antiferromagnetic order below T* ~ 260 K, which persists down to T = 4 K under both field-cooled (fc) and zero-field-cooled (zfc) conditions. The temperature dependence of the fc and zfc magnetic Bragg peaks suggests the presence of competition between canted and collinear antiferromagnetic ground states. Under zfc conditions, we observe a suppression of magnetic peak intensity at T_D ~ 50 K which can be attributed to a spin reorientation transition. Under fc conditions, we find that the canted antiferromagnetic state is stabilized over a much wider range of temperatures, with significantly enhanced magnetic correlation lengths within the Ir-O layers. The field dependence of the magnetic Bragg peaks provides no evidence of field-induced phase transitions for H < 4 T.

cond-mat.str-el↗

arXiv2012

Direct evidence of a zigzag spin chain structure in the honeycomb lattice: A neutron and x-ray diffraction investigation on single crystal $\rm Na_2IrO_3$

We have combined single crystal neutron and x-ray diffractions to investigate the magnetic and crystal structures of the honeycomb lattice $\rm Na_2IrO_3$. The system orders magnetically below $18.1(2)$ K with Ir$^{4+}$ ions forming zigzag spin chains within the layered honeycomb network with ordered moment of $\rm 0.22(1) μ_B$/Ir site. Such a configuration sharply contrasts the N{é}el or stripe states proposed in the Kitaev-Heisenberg model. The structure refinement reveals that the Ir atoms form nearly ideal 2D honeycomb lattice while the $\rm IrO_6$ octahedra experience a trigonal distortion that is critical to the ground state. The results of this study provide much-needed experimental insights into the magnetic and crystal structure crucial to the understanding of the exotic magnetic order and possible topological characteristics in the 5$d$-electron based honeycomb lattice.

cond-mat.str-el↗

arXiv2012

Observation of a Slater-type metal-to-insulator transition in Sr$_2$IrO$_4$ from time-resolved photo-carrier dynamics

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 $J_{eff}$=1/2 Mott insulator. By studying the recovery dynamics of photo-carriers excited across the Mott gap, we find that upon cooling through the Néel temperature $T_N$ the system evolves continuously from a metal-like phase with fast ($\sim$50 fs) and excitation density independent relaxation dynamics to a gapped phase characterized by slower ($\sim$500 fs) excitation density dependent bimolecular recombination dynamics. The development of the insulating gap is accompanied by a transfer of in-gap spectral weight to energies far in excess of the gap and occurs over an unusually broad temperature window, which suggests Sr$_2$IrO$_4$ to be a Slater- rather than Mott-Hubbard type insulator and naturally explains the absence of anomalies at $T_N$ in transport and thermodynamic measurements.

cond-mat.str-el↗

arXiv2012

Strong magnetic instability in correlated metal Bi2Ir2O7

The interplay of spin-orbit interactions and electronic correlations dominates the physical properties of pyrochlore iridates, R2Ir2O7 (R = Y, rare earth element), which are typically magnetic insulators. We report an experimental/theoretical study of single-crystal Bi2Ir2O7 where substitutions of Bi for R sensitively tips the balance between competing interactions so as to favor a metallic state with a strongly exchange enhanced paramagnetism. The ground state is characterized by the following features: (1) A divergent low-temperature magnetic susceptibility that indicates no long-range order down to 50 mK; (2) strongly field-dependent coefficients of the low-temperature T- and T3-terms of the specific heat; (3) a conspicuously large Wilson ratio R_W \approx 53.5; and (4) unusual temperature and field dependences of the Hall resistivity that abruptly change below 80 K, without any clear correlation with the magnetic behavior. All these unconventional properties suggest the existence of an exotic ground state in Bi2Ir2O7.

cond-mat.str-el↗

arXiv2012

Magnetic and Orbital Orders Coupled to Negative Thermal Expansion in Mott Insulators, Ca2Ru1-xMxO4 (M = Mn and Fe)

Ca2RuO4 is a structurally-driven Mott insulator with a metal-insulator transition at TMI = 357K, followed by a well-separated antiferromagnetic order at TN = 110 K. Slightly substituting Ru with a 3d transition metal ion M effectively shifts TMI by weakening the orthorhombic distortion and induces either metamagnetism or magnetization reversal below TN. Moreover, M doping for Ru produces negative thermal expansion in Ca2Ru1-xMxO4 (M = Cr, Mn, Fe or Cu); the lattice volume expands on cooling with a total volume expansion ratio, ΔV/V, reaching as high as 1%. The onset of the negative thermal expansion closely tracks TMI and TN, sharply contrasting classic negative thermal expansion that shows no relevance to electronic properties. In addition, the observed negative thermal expansion occurs near room temperature and extends over a wide temperature interval up to 300 K. These findings underscores new physics driven by a complex interplay between orbital, spin and lattice degrees of freedom.

cond-mat.str-el↗

arXiv2011

Lattice-Driven Magnetoresistivity and Metal-Insulator Transition in Single-Layered Iridates

Sr2IrO4 exhibits a novel insulating state driven by spin-orbit interactions. We report two novel phenomena, namely a large magnetoresistivity in Sr2IrO4 that is extremely sensitive to the orientation of magnetic field but exhibits no apparent correlation with the magnetization, and a robust metallic state that is induced by dilute electron (La3+) or hole (K+) doping for Sr2+ ions in Sr2IrO4. Our structural, transport and magnetic data reveal that a strong spin-orbit interaction alters the balance between the competing energies so profoundly that (1) the spin degree of freedom alone is no longer a dominant force; (2) underlying transport properties delicately hinge on the Ir-O-Ir bond angle via a strong magnetoelastic coupling; and (3) a highly insulating state in Sr2IrO4 is proximate to a metallic state, and the transition is governed by lattice distortions. This work suggests that a novel class of lattice-driven electronic materials can be developed for applications.

cond-mat.str-el↗

arXiv2011

Measurement of the internal magnetic field in the correlated iridates Ca$_4$IrO$_6$, Ca$_5$Ir$_3$O$_{12}$, Sr$_3$Ir$_2$O$_7$ and Sr$_2$IrO$_4$

Oxides containing iridium ions display a range of magnetic and conducting properties that depend on the delicate balance between interactions and are controlled, at least in part, by the details of the crystal architecture. We have used muon-spin rotation ($μ$SR) to study the local field in four iridium oxides, Ca$_4$IrO$_6$, Ca$_5$Ir$_3$O$_{12}$, Sr$_3$Ir$_2$O$_7$ and Sr$_2$IrO$_4$, which show contrasting behavior. Our $μ$SR data on Ca$_4$IrO$_6$ and Ca$_5$Ir$_3$O$_{12}$ are consistent with conventional antiferromagnetism where quasistatic magnetic order develops below $T_{\rm N}=13.85(6)$ K and 7.84(7) K respectively. A lower internal field is observed for Ca$_5$Ir$_3$O$_{12}$, as compared to Ca$_4$IrO$_6$ reflecting the presence of both Ir$^{4+}$ and Ir$^{5+}$ ions, resulting in a more magnetically dilute structure. Muon precession is only observed over a restricted range of temperature in Sr$_3$Ir$_2$O$_7$, while the Mott insulator Sr$_2$IrO$_4$ displays more complex behavior, with the $μ$SR signal containing a single, well-resolved precession signal below $T_{\rm N}=230$\,K, which splits into two precession signals at low temperature following a reorientation of the spins in the ordered state.

cond-mat.str-el↗

arXiv2010

Molecule States in a Gate Tunable Graphene Double Quantum Dot

We have measured a graphene double quantum dot device with multiple electrostatic gates that are used to enhance control to investigate it. At low temperatures the transport measurements reveal honeycomb charge stability diagrams which can be tuned from weak to strong interdot tunnel coupling regimes. We precisely extract a large interdot tunnel coupling strength for this system allowing for the observation of tunnel-coupled molecular states extending over the whole double dot. This clean, highly controllable system serves as an essential building block for quantum devices in a nuclear-spin-free world.

cond-mat.mes-hall↗

arXiv2010

Giant Magnetoelectric Effect in Antiferromagnetic BaMnO3-δand Its Derivatives

Hexagonal perovskite 15R-BaMnO2.99 with a ratio of cubic to hexagonal layers of 1/5 in the unit cell is an antiferromagnetic insulator that orders at a Néel temperature TN = 220 K. Here we report structural, magnetic, dielectric and thermal properties of single crystal BaMnO2.99 and its derivatives BaMn0.97Li0.03O3 and Ba0.97K0.03MnO3. The central findings of this work are: (1) these materials possess a usually large, high-temperature magnetoelectric effect that amplifies the dielectric constant by more than an order of magnitude near their respective Néel temperature; (2) Li and K doping can readily vary the ratio of cubic to hexagonal layers and cause drastic changes in dielectric and magnetic properties; in particular, a mere 3% Li substitution for Mn significantly weakens the magnetic anisotropy and relaxes the lattice; consequently, the dielectric constant for both the a- and c-axis sharply rises to 2500 near the Néel temperature. This lattice softening is also accompanied by weak polarization. These findings provide a new paradigm for developing novel, high-temperature magnetoelectric materials that may eventually contribute to technology.

cond-mat.str-el↗

arXiv2010

Belle II Technical Design Report

The Belle detector at the KEKB electron-positron collider has collected almost 1 billion Y(4S) events in its decade of operation. Super-KEKB, an upgrade of KEKB is under construction, to increase the luminosity by two orders of magnitude during a three-year shutdown, with an ultimate goal of 8E35 /cm^2 /s luminosity. To exploit the increased luminosity, an upgrade of the Belle detector has been proposed. A new international collaboration Belle-II, is being formed. The Technical Design Report presents physics motivation, basic methods of the accelerator upgrade, as well as key improvements of the detector.

physics.ins-det↗

arXiv2010

Ca2Ru1-xCrxO4 (0 < x < 0.13): Negative volume thermal expansion via orbital and magnetic orders

Ca2RuO4 undergoes a metal-insulator transition at TMI = 357 K, followed by a well-separated transition to antiferromagnetic order at TN = 110 K. Dilute Cr doping for Ru reduces the temperature of the orthorhombic distortion at TMI and induces ferromagnetic behavior at TC. The lattice volume V of Ca2Ru1-xCrxO4 (0 < x < 0.13) abruptly expands with cooling at both TMI and TC, giving rise to a total volume expansion ΔV/V {\simeq} 1 %, which sharply contrasts the smooth temperature dependence of the few known examples of negative volume thermal expansion driven by anharmonic phonon modes. In addition, the near absence of volume thermal expansion between TC and TMI represents an Invar effect. The two phase transitions suggest an exotic ground state driven by an extraordinary coupling between spin, orbit and lattice degrees of freedom.

cond-mat.str-el↗

arXiv2010

Electron-Doped Sr2IrO4-delta (0 <= delta <= 0.04): Evolution of a Disordered Jeff = 1/2 Mott Insulator into an Exotic Metallic State

Stoichiometric Sr2IrO4 is a ferromagnetic Jeff = 1/2 Mott insulator driven by strong spin-orbit coupling. Introduction of very dilute oxygen vacancies into single-crystal Sr2IrO4-delta with delta < 0.04 leads to significant changes in lattice parameters and an insulator-to-metal transition at TMI = 105 K. The highly anisotropic electrical resistivity of the low-temperature metallic state for delta ~ 0.04 exhibits anomalous properties characterized by non-Ohmic behavior and an abrupt current-induced transition in the resistivity at T* = 52 K, which separates two regimes of resisitive switching in the nonlinear I-V characteristics. The novel behavior illustrates an exotic ground state and constitutes a new paradigm for devices structures in which electrical resistivity is manipulated via low-level current densities ~ 10 mA/cm2 (compared to higher spin-torque currents ~ 107-108 A/cm2) or magnetic inductions ~ 0.1-1.0 T.

cond-mat.str-el↗

arXiv2009

Pressure-Induced Insulating State in Ba1-xRExIrO3 (RE = Gd, Eu) Single Crystals

BaIrO3 is a novel insulator with coexistent weak ferromagnetism, charge and spin density wave. Dilute RE doping for Ba induces a metallic state, whereas application of modest pressure readily restores an insulating state characterized by a three-order-of-magnitude increase of resistivity. Since pressure generally increases orbital overlap and broadens energy bands, a pressure-induced insulating state is not commonplace. The profoundly dissimilar responses of the ground state to light doping and low hydrostatic pressures signal an unusual, delicate interplay between structural and electronic degrees of freedom in BaIrO3.

cond-mat.str-el↗