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T. Valla

Publications and source records attributed to T. Valla.

At least 55 records · Page 3Linked to original sources
arXiv2010

Coupling of spin and orbital excitations in the iron-based superconductor FeSe(0.5)Te(0.5)

We present a combined analysis of neutron scattering and photoemission measurements on superconducting FeSe(0.5)Te(0.5). The low-energy magnetic excitations disperse only in the direction transverse to the characteristic wave vector (1/2,0,0), whereas the electronic Fermi surface near (1/2,0,0) appears to consist of four incommensurate pockets. While the spin resonance occurs at an incommensurate wave vector compatible with nesting, neither spin-wave nor Fermi-surface-nesting models can describe the magnetic dispersion. We propose that a coupling of spin and orbital correlations is key to explaining this behavior. If correct, it follows that these nematic fluctuations are involved in the resonance and could be relevant to the pairing mechanism.

cond-mat.supr-con↗
arXiv2010

Metal to insulator transition on the N = 0 Landau level in graphene

The magnetotransport in single layer graphene has been experimentally investigated in magnetic fields up to 18 T as a function of temperature. A pronounced T-dependence is observed for T < 50 K, which is either metallic, or insulating, depending on the filling factor nu. The metal-insulator transition (MIT) occurs at nu_c ~ 0.65 and in the regime of the dissipative transport, where the longitudinal resistance Rxx > R_K/2. The critical resistivity (Rxx per square) is rho_xx(nu_c) ~ R_K/2 and is correlated with the appearance of zero plateau in Hall conductivity sigma_xy(nu) and peaks in sigma_xx(nu). This leads us to construct a universal low-T (n, B) phase diagram of this quantum phase transition.

cond-mat.mes-hall↗
arXiv2009

Breakdown of the N=0 Quantum Hall State in graphene: two insulating regimes

We studied the unusual Quantum Hall Effect (QHE) near the charge neutrality point (CNP) in high-mobility graphene sample for magnetic fields up to 18 T. We observe breakdown of the delocalized QHE transport and strong increase in resistivities $ρ_{xx},|ρ_{xy}|$ with decreasing Landau level filling for $ν< 2$, where we identify two insulating regimes. For $1 \gtrsim |ν| \gtrsim 1/2$ we find an exponential increase of $ρ_{xx,xy} \sim e^{a(H-H_c)}$ within the range up to several resistance quanta $R_K$, while the Hall effect gradually disappears, consistent with the Hall insulator (HI) with local transport. Then, at $ν\approx 1/2$ a cusp in $ρ_{xx}(H)$ followed by an onset of even faster growth indicates transition to a collective insulator (CI) state. The likely candidate for this state is a pinned Wigner crystal.

cond-mat.mes-hall↗
arXiv2008

The Fermi surface of Ba(1-x)K(x)Fe2As2 and its evolution with doping

We use angle-resolved photoemission spectroscopy (ARPES) to investigate the electronic properties of the newly discovered iron-arsenic superconductor, Ba(1-x)K(x)Fe2As2 and non-supercondcuting BaFe2As2. Our study indicates that the Fermi surface of the undoped, parent compound BaFe$_2$As$_2$ consists of hole pocket(s) at Gamma (0,0) and larger electron pocket(s) at X (1,0), in general agreement with full-potential linearized plane wave (FLAPW) calculations. Upon doping with potassium, the hole pocket expands and the electron pocket becomes smaller with its bottom approaching the chemical potential. Such an evolution of the Fermi surface is consistent with hole doping within a rigid band shift model. Our results also indicate that FLAPW calculation is a reasonable approach for modeling the electronic properties of both undoped and K-doped iron arsenites.

cond-mat.supr-con↗
arXiv2008

Anisotropic Electron-Phonon Coupling and Dynamical Nesting on the Graphene Sheets in CaC6

Superconductivity in graphite intercalated compounds has been studied for more than 40 years and it is still not fully understood, despite the recent progress and the discovery of relatively high Tc superconductivity in CaC6 and YbC6. Recent studies now suggest that the electron-phonon coupling is most likely responsible for pairing and that the intercalant-derived electronic states and vibrations play the dominant role. Here, we present the first studies of electronic structure in CaC6, a superconductor with Tc=11.6 K. Using angle-resolved photoemission spectroscopy, we find that, contrary to theoretical models, the EPC on the graphene-derived Fermi sheets is surprisingly strong, reflecting the interaction with high-frequency graphene-derived vibrations. Thus, in addition to the amazing properties in the charge-neutral state, graphene sheets also show surprises in the heavily doped regime: they may support strong pairing interactions and lead to superconductivity in compounds in which they are building blocks.

cond-mat.supr-con↗
arXiv2008

Spin-orbit interaction effect in the electronic structure of \BiTe \ observed by angle-resolved photoemission spectroscopy

The electronic structure of $p$-type doped \BiTe is studied by angle resolved photoemission spectroscopy (ARPES) to experimentally confirm the mechanism responsible for the high thermoelectric figure of merit. Our ARPES study shows that the band edges are located off the $Γ$-Z line in the Brillouin zone, which provides direct observation that the spin-orbit interaction is a key factor to understand the electronic structure and the corresponding thermoelectric properties of \BiTe. Successive time dependent ARPES measurement also reveals that the electron-like bands crossing E$_F$ near the $\underlineΓ$ point are formed in an hour after cleaving the crystals. We interpret these as surface states induced by surface band bending, possibly due to quintuple inter-layer distance change of \BiTe.

cond-mat.mtrl-sci↗
arXiv2006

The Ground State of the Pseudogap in Cuprate Superconductors

We present studies of the electronic structure of La2-xBaxCuO4, a system where the superconductivity is strongly suppressed as static spin and charge orders or "stripes" develop near the doping level of x=1/8. Using angle-resolved photoemission and scanning tunneling microscopy, we detect an energy gap at the Fermi surface with magnitude consistent with d-wave symmetry and with linear density of states, vanishing only at four nodal points, even when superconductivity disappears at x=1/8. Thus, the non-superconducting, "striped" state at x=1/8 is consistent with a phase incoherent d-wave superconductor whose Cooper pairs form spin/charge ordered structures instead of becoming superconducting.

cond-mat.supr-con↗
arXiv2006

ARPES Study of the Metal-Insulator Transition in Bismuth Cobaltates

We present an angle-resolved photoemission spectroscopy (ARPES) study of a Mott-Hubbard-type bismuth cobaltate system across a metal-insulator transition. By varying the amount of Pb substitution, and by doping with Sr or Ba cation, a range of insulating to metallic properties is obtained. We observe a systematic change in the spectral weight of the coherent and incoherent parts, accompanied by an energy shift of the incoherent part. The band dispersion also shows the emergence of a weakly dispersing state at the Fermi energy with increasing conductivity. These changes correspond with the changes in the temperature-dependent resistivity behavior. We address the nature of the coherent-incoherent parts in relation to the peak-dip-hump feature seen in cuprates superconductors.

cond-mat.str-el↗
arXiv2006

High-energy kink in high-temperature superconductors

In conventional metals, electron-phonon coupling, or the phonon-mediated interaction between electrons, has long been known to be the pairing interaction responsible for the superconductivity. The strength of this interaction essentially determines the superconducting transition temperature TC. One manifestation of electron-phonon coupling is a mass renormalization of the electronic dispersion at the energy scale associated with the phonons. This renormalization is directly observable in photoemission experiments. In contrast, there remains little consensus on the pairing mechanism in cuprate high temperature superconductors. The recent observation of similar renormalization effects in cuprates has raised the hope that the mechanism of high temperature superconductivity may finally be resolved. The focus has been on the low energy renormalization and associated "kink" in the dispersion at around 50 meV. However at that energy scale, there are multiple candidates including phonon branches, structure in the spin-fluctuation spectrum, and the superconducting gap itself, making the unique identification of the excitation responsible for the kink difficult. Here we show that the low-energy renormalization at ~50 meV is only a small component of the total renormalization, the majority of which occurs at an order of magnitude higher energy (~350 meV). This high energy kink poses a new challenge for the physics of the cuprates. Its role in superconductivity and relation to the low-energy kink remains to be determined.

cond-mat.supr-con↗
arXiv2006

Unusual photoemission resonances of oxygen-dopant induced states in Bi$_{2}$Sr$_2$CaCu$_2$O$_{8+x}$

We have performed an angular-resolved photoemission study of underdoped, optimally doped and overdoped Bi$_{2}$Sr$_2$CaCu$_2$O$_{8+x}$ samples using a wide photon energy range (15 - 100 eV). We report a small and broad non-dispersive A$_{1g}$ peak in the energy distribution curves whose intensity scales with doping. We attribute it to a local impurity state similar to the one observed recently by scanning tunneling spectroscopy and identified as the oxygen dopants. Detailed analysis of the resonance profile and comparison with the single-layered Bi$_{2}$Sr$_2$CuO$_{6+x}$ suggest a mixing of this local state with Cu via the apical oxygens.

cond-mat.supr-con↗
arXiv2006

Charge order, metallic behavior and superconductivity in La_{2-x}Ba_xCuO_4 with x=1/8

The ab-plane optical properties of a cleaved single crystal of La_{2-x}Ba_xCuO_4 for x=1/8 (T_c ~ 2.4 K) have been measured over a wide frequency and temperature range. The low-frequency conductivity is Drude-like and shows a metallic response with decreasing temperature. However, below ~ 60 K, corresponding to the onset of charge-stripe order, there is a rapid loss of spectral weight below about 40 meV. The gapping of single-particle excitations looks surprisingly similar to that observed in superconducting La_{2-x}Sr_{x}CuO_4, including the presence of a residual Drude peak with reduced weight; the main difference is that the lost spectral weight moves to high, rather than zero, frequency, reflecting the absence of a bulk superconducting condensate.

cond-mat.supr-con↗
arXiv2005

Fine Details of the Nodal Electronic Excitations in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

Very high energy resolution photoemission experiments on high quality samples of optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ show new features in the low-energy electronic excitations. A marked change in the binding energy and temperature dependence of the near-nodal scattering rates is observed near the superconducting transition temperature, $T_C$. The temperature slope of the scattering rate measured at low energy shows a discontinuity at ~$T_C$. In the superconducting state, coherent excitations are found with the scattering rates showing a cubic dependence on frequency and temperature. The superconducting gap has a d-wave magnitude with negligible contribution from higher harmonics. Further, the bi-layer splitting has been found to be finite at the nodal point.

cond-mat.supr-con↗
arXiv2005

Scaling of the superfluid density in high-temperature superconductors

A scaling relation ρ_s \simeq 35σ_{dc}T_c has been observed in the copper-oxide superconductors, where ρ_s is the strength of the superconducting condensate, T_c is the critical temperature, and σ_{dc} is the normal-state dc conductivity close to T_c. This scaling relation is examined within the context of a clean and dirty-limit BCS superconductor. These limits are well established for an isotropic BCS gap 2Δand a normal-state scattering rate 1/τ; in the clean limit 1/τ\ll 2Δ, and in the dirty limit 1/τ> 2Δ. The dirty limit may also be defined operationally as the regime where ρ_s varies with 1/τ. It is shown that the scaling relation ρ_s \propto σ_{dc}T_c is the hallmark of a BCS system in the dirty-limit. While the gap in the copper-oxide superconductors is considered to be d-wave with nodes and a gap maximum Δ_0, if 1/τ> 2Δ_0 then the dirty-limit case is preserved. The scaling relation implies that the copper-oxide superconductors are likely to be in the dirty limit, and that as a result the energy scale associated with the formation of the condensate is scaling linearly with T_c. The a-b planes and the c axis also follow the same scaling relation. It is observed that the scaling behavior for the dirty limit and the Josephson effect (assuming a BCS formalism) are essentially identical, suggesting that in some regime these two effects may be viewed as equivalent. This raises the possibility that electronic inhomogeneities in the copper-oxygen planes may play an important role in the nature of the superconductivity in the copper-oxide materials.

cond-mat.supr-con↗
arXiv2004

Extracting the electron--boson spectral function $α^2$F($ω$) from infrared and photoemission data using inverse theory

We present a new method of extracting electron-boson spectral function $α^2$F($ω$) from infrared and photoemission data. This procedure is based on inverse theory and will be shown to be superior to previous techniques. Numerical implementation of the algorithm is presented in detail and then used to accurately determine the doping and temperature dependence of the spectral function in several families of high-T$_c$ superconductors. Principal limitations of extracting $α^2$F($ω$) from experimental data will be pointed out. We directly compare the IR and ARPES $α^2$F($ω$) and discuss the resonance structure in the spectra in terms of existing theoretical models.

cond-mat.supr-con↗
arXiv2004

Response to comment by Cuk et al

In a recent study of the optical conductivity of the Bi-2212 high temperature superconductor, Hwang, et al. have confirmed the mass-renormalization changes observed in ARPES but find that such changes are no longer observed in the highly overdoped regime. In a comment on this study, Cuk, et al. challenge this conclusion and present ARPES evidence for a kink in the superconducting state in the highly overdoped regime. We show, however that two doping dependent properties in the data of Cuk et al., the frequencies of the superconducting gap and the Van Hove singularity, suggest a Tc that is higher than the 58 K quoted, and that their sample is not as highly overdoped as claimed. As a result, these data can not refute the claim of Hwang et al. that the kink is absent in the highly overdoped region.

cond-mat.supr-con↗
arXiv2004

Doping of a One-Dimensional Mott Insulator: Photoemision and Optical Studies of Sr$_2$CuO$_{3+δ}$

The spectral properties of a one-dimensional (1D) single-chain Mott insulator Sr$_2$CuO$_{3}$ have been studied in angle-resolved photoemission and optical spectroscopy, at half filling and with small concentrations of extra charge doped into the chains via high oxygen pressure growth. The single- particle gap is reduced with oxygen doping, but the metallic state is not reached. The bandwidth of the charge-transfer band increases with doping, while the state becomes narrower, allowing unambiguous observation of separated spinon and holon branches in the doped system. The optical gap is not changed upon doping, indicating that a shift of chemical potential rather than decrease of corelation gap is responsible for the apparent reduction of the photoemission gap.

cond-mat.str-el↗
arXiv2003

Quasi-Particle Spectra, Charge-Density-Wave, Superconductivity and Electron-Phonon Coupling in 2H-NbSe2

High-resolution photoemission has been used to study the electronic structure of the charge density wave (CDW) and superconducting (SC) dichalcogenide, 2H- NbSe2. From the extracted self-energies, important components of the quasiparticle (QP) interactions have been identified. In contrast to previously studied TaSe2, the CDW transition does not affect the electronic properties significantly. The electron-phonon coupling is identified as a dominant contribution to the QP self-energy and is shown to be very anisotropic (k-dependent) and much stronger than in TaSe2.

cond-mat.str-el↗