arXiv subjects

J. T. Park

Publications and source records attributed to J. T. Park.

43 records · Page 3Linked to original sources

arXiv2009

Large single crystal growth of BaFe1.87Co0.13As2 using a nucleation pole

Co-doped iron arsenic single crystal of BaFe1.87Co0.13As2 with dimension up to 20 x 10 x 2 mm3 were grown by a nucleation pole: an alumina stick served as nucleation center during growth. The high quality of crystalline was illustrated by the measurements of neutron rocking curve and X-ray diffraction pattern. A very sharp superconducting transition temperature Tc~25 K was revealed by both resistivity and susceptibility measurements. A nearly 100% shielding fraction and bulk nature of the superconductivity for the single crystal were confirmed using magnetic susceptibility data.

cond-mat.supr-con↗

arXiv2009

Normal-State Spin Dynamics and Temperature-Dependent Spin Resonance Energy in an Optimally Doped Iron Arsenide Superconductor

The proximity of superconductivity and antiferromagnetism in the phase diagram of iron arsenides, the apparently weak electron-phonon coupling and the "resonance peak" in the superconducting spin excitation spectrum have fostered the hypothesis of magnetically mediated Cooper pairing. However, since most theories of superconductivity are based on a pairing boson of sufficient spectral weight in the normal state, detailed knowledge of the spin excitation spectrum above the superconducting transition temperature Tc is required to assess the viability of this hypothesis. Using inelastic neutron scattering we have studied the spin excitations in optimally doped BaFe1.85Co0.15As2 (Tc = 25 K) over a wide range of temperatures and energies. We present the results in absolute units and find that the normal state spectrum carries a weight comparable to underdoped cuprates. In contrast to cuprates, however, the spectrum agrees well with predictions of the theory of nearly antiferromagnetic metals, without complications arising from a pseudogap or competing incommensurate spin-modulated phases. We also show that the temperature evolution of the resonance energy follows the superconducting energy gap, as expected from conventional Fermi-liquid approaches. Our observations point to a surprisingly simple theoretical description of the spin dynamics in the iron arsenides and provide a solid foundation for models of magnetically mediated superconductivity.

cond-mat.supr-con↗

arXiv2009

Magnetic-field-enhanced incommensurate magnetic order in the underdoped high-temperature superconductor YBa(2)Cu(3)O(6.45)

We present a neutron-scattering study of the static and dynamic spin correlations in the underdoped high-temperature superconductor YBa(2)Cu(3)O(6.45) in magnetic fields up to 15 T. The field strongly enhances static incommensurate magnetic order at low temperatures and induces a spectral-weight shift in the magnetic-excitation spectrum. A reconstruction of the Fermi surface driven by the field-enhanced magnetic superstructure may thus be responsible for the unusual Fermi surface topology revealed by recent quantum-oscillation experiments.

cond-mat.str-el↗

arXiv2009

Two-gap superconductivity in Ba_1-xK_xFe_2As_2: A complementary study of the magnetic penetration depth by \muSR and ARPES

We investigate the magnetic penetration depth λin superconducting Ba_1-xK_xFe_2As_2 (T_c\simeq32K) with muon-spin rotation (\muSR) and angle-resolved photoemission (ARPES). Using \muSR, we find the penetration-depth anisotropy γ_λ=λ_c/λ_{ab} and the second-critical-field anisotropy γ_{H_c2} to show an opposite T-evolution below T_c. This dichotomy resembles the situation in the two-gap superconductor MgB_2. A two-gap scenario is also suggested by an inflection point in the in-plane penetration depth λ_ab around 7K. The complementarity of \muSR and ARPES allows us to pinpoint the values of the two gaps and to arrive to a remarkable agreement between the two techniques concerning the full T-evolution of λ_ab. This provides further support for the described scenario and establishes ARPES as a tool to assess macroscopic properties of the superconducting condensate.

cond-mat.supr-con↗

arXiv2009

Suppression of the structural phase transition and lattice softening in slightly underdoped Ba(1-x)K(x)Fe2As2 with electronic phase separation

We present x-ray powder diffraction (XRPD) and neutron diffraction measurements on the slightly underdoped iron pnictide superconductor Ba(1-x)K(x)Fe2As2, Tc = 32K. Below the magnetic transition temperature Tm = 70K, both techniques show an additional broadening of the nuclear Bragg peaks, suggesting a weak structural phase transition. However, macroscopically the system does not break its tetragonal symmetry down to 15 K. Instead, XRPD patterns at low temperature reveal an increase of the anisotropic microstrain proportionally in all directions. We associate this effect with the electronic phase separation, previously observed in the same material, and with the effect of lattice softening below the magnetic phase transition. We employ density functional theory to evaluate the distribution of atomic positions in the presence of dopant atoms both in the normal and magnetic states, and to quantify the lattice softening, showing that it can account for a major part of the observed increase of the microstrain.

cond-mat.supr-con↗

arXiv2008

(pi,pi)-electronic order in iron arsenide superconductors

The distribution of valence electrons in metals usually follows the symmetry of an ionic lattice. Modulations of this distribution often occur when those electrons are not stable with respect to a new electronic order, such as spin or charge density waves. Electron density waves have been observed in many families of superconductors[1-3], and are often considered to be essential for superconductivity to exist[4]. Recent measurements[5-9] seem to show that the properties of the iron pnictides[10, 11] are in good agreement with band structure calculations that do not include additional ordering, implying no relation between density waves and superconductivity in those materials[12-15]. Here we report that the electronic structure of Ba1-xKxFe2As2 is in sharp disagreement with those band structure calculations[12-15], instead revealing a reconstruction characterized by a (pi,pi) wave vector. This electronic order coexists with superconductivity and persists up to room temperature.

cond-mat.supr-con↗

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↗