arXiv subjects

V. Hinkov

Publications and source records attributed to V. Hinkov.

50 records · Page 3Linked to original sources

arXiv2008

Electronic phase separation in the slightly underdoped iron pnictide superconductor Ba(1-x)K(x)Fe(2)As(2)

Here we present a combined study of the slightly underdoped novel pnictide superconductor Ba(1-x)K(x)Fe(2)As(2) by means of X-ray powder diffraction, neutron scattering, muon spin rotation (muSR), and magnetic force microscopy (MFM). Commensurate static magnetic order sets in below Tm ~ 70 K as inferred from the emergence of the magnetic (1 0 -3) reflection in the neutron scattering data and from the observation of damped oscillations in the zero-field-muSR asymmetry. Transverse-field muSR below Tc shows a coexistence of magnetically ordered and non-magnetic states, which is also confirmed by MFM imaging. We explain such coexistence by electronic phase separation into antiferromagnetic and superconducting/normal state regions on a lateral scale of several tens of nanometers. Our findings indicate that such mesoscopic phase separation can be considered an intrinsic property of some iron pnictide superconductors.

cond-mat.supr-con↗

arXiv2008

Electronic liquid crystal state in the high-temperature superconductor YBCO(6.45)

Electronic phases with symmetry properties matching those of conventional liquid crystals have recently been discovered in transport experiments on semiconductor heterostructures and metal oxides at milli-Kelvin temperatures. We report the spontaneous onset of a onedimensional, incommensurate modulation of the spin system in the high-temperature superconductor YBa2Cu3O6.45 upon cooling below ~150 K, while static magnetic order is absent above 2 K. The evolution of this modulation with temperature and doping parallels that of the in-plane anisotropy of the resistivity, indicating an electronic nematic phase that is stable over a wide temperature range. The results suggest that soft spin fluctuations are a microscopic route towards electronic liquid crystals, and nematic order can coexist with high-temperature superconductivity in underdoped cuprates.

cond-mat.str-el↗

arXiv2008

Spin dynamics in the pseudogap state of a high-temperature superconductor

The pseudogap is one of the most pervasive phenomena of high temperature superconductors. It is attributed either to incoherent Cooper pairing setting in above the superconducting transition temperature Tc, or to a hidden order parameter competing with superconductivity. Here we use inelastic neutron scattering from underdoped YBa(2)Cu(3)O(6.6) to show that the dispersion relations of spin excitations in the superconducting and pseudogap states are qualitatively different. Specifically, the extensively studied "hour glass" shape of the magnetic dispersions in the superconducting state is no longer discernible in the pseudogap state and we observe an unusual "vertical" dispersion with pronounced in-plane anisotropy. The differences between superconducting and pseudogap states are thus more profound than generally believed, suggesting a competition between these two states. Whereas the high-energy excitations are common to both states and obey the symmetry of the copper oxide square lattice, the low-energy excitations in the pseudogap state may be indicative of collective fluctuations towards a state with broken orientational symmetry predicted in theoretical work.

cond-mat.supr-con↗

arXiv2007

Momentum and excitation energy dependence of the "waterfalls" in cuprates

Using high-resolution angle-resolved photoemission spectroscopy we have studied the momentum and photon energy dependence of the anomalous high-energy dispersion, termed "waterfalls", between the Fermi level and 1 eV binding energy in several high-Tc superconductors. We observe strong changes of the dispersion between different Brillouin zones and a strong dependence on the photon energy around 75 eV, which we associate with the resonant photoemission at the Cu 3p3d_{x^2-y^2} edge. We conclude that the high-energy "waterfall" dispersion results from a strong suppression of the photoemission intensity at the center of the Brillouin zone due to matrix element effects and is, therefore, not an intrinsic feature of the spectral function. This indicates that the new high energy scale in the electronic structure of cuprates derived from the "waterfall"-like dispersion may be incorrect.

cond-mat.supr-con↗

arXiv2007

Disentangling surface and bulk photoemission using circularly polarized light

We show that in the angle resolved photoemission spectroscopy (ARPES) near-surface induced fields can be useful for disentangling the surface and bulk related emission. The jump of the dielectric function at the interface results in a nonzero term $\operatorname{div}\textbf{A}$ in the photoemission matrix element. The term happens to be significant approximately within the first unit cell and leads to the circular dichroism for the states localized therein. As an example we use ARPES spectra of an YBa$_2$Cu$_3$O$_{7-δ}$ crystal to distinguish between the overdoped surface related component and its bulk counterparts.

cond-mat.supr-con↗

arXiv2006

X-ray and neutron diffraction studies of coupled structural phase transitions in DyBaCo$_{2}$O$_{5.5}$

A structural transition at $T\approx 322$ K from the $Pmmm$ to $Pmma$ phase is found to coincide with an anomaly of resistivity. Another structural phase transition doubling the lattice parameter $c$, which has been postulated earlier to accompany a low-temperature magnetic transition in TbBaCo$_{2}$O$_{5.5}$, is observed in a single crystal DbBaCo$_{2}$O$_{5.5}$ by means of the X-ray and neutron diffraction. The low temperature phase does not belong to the space group $Pcca$ that has been chosen earlier as the highest subgroup of the $Pmma$. The transition is of the first order with the temperature hysteresis, between $T\approx 100$ and $T\approx 200$ K, which probably explains anomalous magnetic properties in this temperature range.

cond-mat.str-el↗

arXiv2006

Coexistence of metallicity and superconductivity in adjacent bilayers of a high-Tc superconductor

Experimental studies of the electronic structure remain the basic means for understanding the nature of high-temperature superconductivity (HTSC) and testing relevant theoretical models. Appreciable contributions to establish the overall picture in HTSC have recently been made by investigations on the charge dynamics in BSCCO (ref. 1) and the spin dynamics in YBCO, using Angle Resolved Photoemission Spectroscopy (ARPES) and Inelastic Neutron Scattering (INS), respectively. Concentration of each of these techniques on a "suitable" compound turns out to be a barrier on the way to a crucial quantitative test allowing to support or discard spin fluctuations (interaction between the charge and spin degrees of freedom) as a possible origin for the pairing in doped cuprates. Here we solve the long-standing puzzle of ARPES on YBCO by showing that the photoelectron spectrum of YBCO generally includes two components: One from the topmost anomalously overdoped metallic CuO2 bilayer and the other from the next superconducting bilayer that retains the bulk properties. Our findings clearly show the opening of a large superconducting gap and, for the first time, demonstrate the anisotropic renormalization in the bulk component of YBCO, supporting the universality of these effects for different cuprate families. With our study we re-open this cuprate family for new systematic ARPES investigations.

cond-mat.supr-con↗

arXiv2005

Doping Dependence of Bilayer Resonant Spin Excitations in $\bf (Y,Ca)Ba_2Cu_3O_{6+x}$

Resonant magnetic modes with odd and even symmetries were studied by inelastic neutron scattering experiments in the bilayer high-$T_c$ superconductor $\rm Y_{1-x}Ca_{x}Ba_2Cu_3O_{6+y}$ over a wide doping range. The threshold of the spin excitation continuum in the superconducting state, deduced from the energies and spectral weights of both modes, is compared with the superconducting d-wave gap, measured on the same samples by electronic Raman scattering in the $B_{1g}$ symmetry. Above a critical doping level of $δ\simeq 0.19$, both mode energies and the continuum threshold coincide. We find a simple scaling relationship between the characteristic energies and spectral weights of both modes, which indicates that the resonant modes are bound states in the superconducting energy gap, as predicted by the spin-exciton model of the resonant mode.

cond-mat.str-el↗

arXiv2005

"Kinks", Nodal Bilayer Splitting and Interband Scattering in YBCO

We apply the new-generation ARPES methodology to the most widely studied cuprate superconductor YBCO. Considering the nodal direction, we found noticeable renormalization effects known as "kinks" both in the quasiparticle dispersion and scattering rate, the bilayer splitting and evidence for strong interband scattering -- all the characteristic features of the nodal quasiparticles detected earlier in BSCCO. The typical energy scale and the doping dependence of the "kinks" clearly point to their intimate relation with the spin-1 resonance seen in the neutron scattering experiments. Our findings strongly suggest a universality of the electron dynamics in the bilayer superconducting cuprates and a dominating role of the spin-fluctuations in the formation of the quasiparticles along the nodal direction.

cond-mat.supr-con↗

arXiv2005

Magnetic order in the pseudogap phase of high-$T_C$ superconductors

One of the leading issues in high-$T_C$ superconductors is the origin of the pseudogap phase in underdoped cuprates. Using polarized elastic neutron diffraction, we identify a novel magnetic order in the YBa$_2$Cu$_3$O$_{6+x}$ system. The observed magnetic order preserves translational symmetry as proposed for orbital moments in the circulating current theory of the pseudogap state. To date, it is the first direct evidence of an hidden order parameter characterizing the pseudogap phase in high-$T_C$ cuprates.

cond-mat.str-el↗

arXiv2004

Ni impurity induced enhancement of the pseudogap in cuprate high T_c superconductors

The influence of magnetic Ni and non-magnetic Zn impurities on the normal state pseudogap (PG) in the c-axis optical conductivity of NdBa$_{2}$\{Cu$% _{1-y}$(Ni,Zn)$_{y}\}_{3}$O$_{7-δ}$ crystals was studied by spectral ellipsometry. We find that these impurities which strongly suppress superconductivity have a profoundly different impact on the PG. Zn gives rise to a gradual and inhomogeneous PG suppression while Ni strongly enhances the PG. Our results challenge theories that relate the PG either to precursor superconductivity or to other phases with exotic order parameters, such as flux phase or d-density wave states, that should be suppressed by potential scattering. The apparent difference between magnetic and non-magnetic impurities instead points towards an important role of magnetic correlations in the PG state.

cond-mat.supr-con↗

arXiv2004

Two-Dimensional Geometry of Spin Excitations in the High Temperature Superconductor YBa2Cu3O6+x

The fundamental building block of the copper oxide superconductors is a Cu4O4 square plaquette. In most of these materials, the plaquettes are slightly distorted and form a rectangular lattice. An influential theory predicts that high-temperature superconductivity is nucleated in "stripes" aligned along one of the axes of this lattice. This theory had received strong support from experiments that appeared to indicate a one-dimensional character of the magnetic excitations in the high temperature superconductor YBa2Cu3O6.6. Here we report neutron scattering data on "untwinned" YBa2Cu3O6+x crystals, in which the orientation of the rectangular lattice is maintained throughout the entire volume. Contrary to the earlier claim, we demonstrate that the geometry of the magnetic fluctuations is two-dimensional. Rigid stripe arrays therefore appear to be ruled out over a wide range of doping levels in YBa2Cu3O6+x, but the data may be consistent with liquid-crystalline stripe order.

cond-mat.str-el↗

arXiv2004

Resonant magnetic excitations at high energy in superconducting $\bf YBa_2Cu_3O_{6.85}$

A detailed inelastic neutron scattering study of the high temperature superconductor $\rm YBa_2Cu_3O_{6.85}$ provides evidence of new resonant magnetic features, in addition to the well known resonant mode at 41 meV: (i) a commensurate magnetic resonance peak at 53 meV with an even symmetry under exchange of two adjacent $\rm CuO_2$ layers; and (ii) high energy incommensurate resonant spin excitations whose spectral weight is around 54 meV. The locus and the spectral weight of these modes can be understood by considering the momentum shape of the electron-hole spin-flip continuum of d-wave superconductors. This provides new insight into the interplay between collective spin excitations and the continuum of electron-hole excitations.

cond-mat.supr-con↗

arXiv2003

Two resonant magnetic modes in an overdoped high-$\bf T_c$ superconductor

A detailed inelastic neutron scattering study of the overdoped high temperature copper oxide superconductor ${Y_{0.9}Ca_{0.1}Ba_{2}Cu_3O_{7}}$ reveals two distinct magnetic resonant modes in the superconducting state. The modes differ in their symmetry with respect to exchange between adjacent copper oxide layers. Counterparts of the mode with odd symmetry, but not the one with even symmetry, had been observed before at lower doping levels. The observation of the even mode resolves a long-standing puzzle, and the spectral weight ratio of both modes yields an estimate of the onset of particle-hole spin-flip excitations.

cond-mat.supr-con↗