arXiv subjects

K. Iida

Publications and source records attributed to K. Iida.

At least 55 records · Page 3Linked to original sources

arXiv2013

From frustration to glassiness via quantum fluctuations and random tiling with exotic entropy

When magnetic moments (spins) are regularly arranged in a geometry of a triangular motif, the spins may not satisfy simultaneously their interactions with their neighbors. This phenomenon, called frustration, leads to numerous energetically equivalent magnetic states (ground states), which results in exotic states such as spin liquid and spin ice. Here we report an alternative situation: a system that, classically, is to be a liquid in the clean limit freezes into a glassy state induced by quantum fluctuations. The case in point is a frustrated magnet in which spins are arranged in a triangular network of bi-pyramids. When taking into account quantum corrections, the classical degeneracy is broken into a set of local minima in a rugged energy landscape, which are separated by large energy barriers, over a finite number of degenerate, periodic, ground states. The appearance of large barriers is due to the absence of local zero-energy modes that are typical in spin-liquid candidate systems. We establish this by mapping the set of local energy minima states into a tiling with colored hexagonal tiles. We show that the system exhibits a large number of aperiodic tessellations. The configuration entropy of the local minima is extremely sensitive to boundary conditions, scaling with the boundary length rather than its volume. The low temperature thermodynamics is also discussed to compare it with other glassy materials.

cond-mat.stat-mech↗

arXiv2013

Doping and critical-temperature dependence of the energy gaps in Ba(Fe_{1-x}Co_x)_2As_2 thin films

The dependence of the superconducting gaps in epitaxial Ba(Fe_{1-x}Co_{x})_2As_2 thin films on the nominal doping x (0.04 \leq x \leq 0.15) was studied by means of point-contact Andreev-reflection spectroscopy. The normalized conductance curves were well fitted by using the 2D Blonder-Tinkham-Klapwijk model with two nodeless, isotropic gaps -- although the possible presence of gap anisotropies cannot be completely excluded. The amplitudes of the two gaps Δ_{S} and Δ_{L} show similar monotonic trends as a function of the local critical temperature T_{c}^{A} (measured in the same point contacts) from 25 K down to 8 K. The dependence of the gaps on x is well correlated to the trend of the critical temperature, i.e. to the shape of the superconducting region in the phase diagram. When analyzed within a simple three-band Eliashberg model, this trend turns out to be compatible with a mechanism of superconducting coupling mediated by spin fluctuations, whose characteristic energy scales with T_{c} according to the empirical law Ω_{0}= 4.65*k_{B}*T_{c}, and with a total electron-boson coupling strength λ_{tot}= 2.22 for x \leq 0.10 (i.e. up to optimal doping) that slightly decreases to λ_{tot}= 1.82 in the overdoped samples (x = 0.15).

cond-mat.supr-con↗

arXiv2013

Surface transport properties of Fe-based superconductors: The influence of degradation and inhomogeneity

Surface properties of Co-doped BaFe2As2 epitaxial superconducting thin films were inspected by X-ray photoelectron spectroscopy, scanning spreading resistance microscopy (SSRM), and point contact spectroscopy (PCS). It has been shown that surface of Fe-based superconductors degrades rapidly if being exposed to air, what results in suppression of gap-like structure on PCS spectra. Moreover, SSRM measurements revealed inhomogeneous surface conductivity, what is consistent with strong dependence of PCS spectra on contact position. Presented results suggest that fresh surface and small probing area should be assured for surface sensitive measurements like PCS to obtain intrinsic properties of Fe-based superconductors.

cond-mat.supr-con↗

arXiv2013

Evolution of the domain topology in a ferroelectric

Topological materials, including topological insulators, magnets with Skyrmions and ferroelectrics with topological vortices, have recently attracted phenomenal attention in the materials science community. Complex patterns of ferroelectric domains in hexagonal REMnO3 (RE: rare earths) turn out to be associated with the macroscopic emergence of Z2xZ3 symmetry. The results of our depth profiling of crystals with a self-poling tendency near surfaces reveal that the partial dislocation (i.e., wall-wall) interaction, not the interaction between vortices and antivortices, is primarily responsible for topological condensation through the macroscopic breaking of the Z2-symmetry.

cond-mat.mtrl-sci↗

arXiv2013

The order-parameter symmetry and Fermi surface topology of 122 Fe-based superconductors: a point-contact Andreev-reflection study

We report on the results of directional point-contact Andreev-reflection (PCAR) measurements in Ba(Fe_{1-x}Co_x)2As2 single crystals and epitaxial c-axis oriented films with x = 0.08 as well as in Ca(Fe_{1-x}Co_x)2As2 single crystals with x = 0.06. The PCAR spectra are analyzed within the two-band 3D version of the Blonder-Tinkham-Klapwijk model for Andreev reflection we recently developed, and that makes use of an analytical expression for the Fermi surface that mimics the one calculated within the density-functional theory (DFT). The spectra in Ca(Fe_{0.94}Co_{0.06})2As2 unambiguously demonstrate the presence of nodes or zeros in the small gap. In Ba(Fe_{0.92}Co_{0.08})2As2, the ab-plane spectra in single crystals can be fitted by assuming two nodeless gaps, but this model fails to fit the c-axis ones in epitaxial films. All these results are discussed in comparison with recent theoretical predictions about the occurrence of accidental 3D nodes and the presence of "hot spots" in the gaps of 122 compounds.

cond-mat.supr-con↗

arXiv2012

Electronic phase diagram of disordered Co doped BaFe2As2

Superconducting and normal state transport properties in iron pnictides are sensitive to disorder and impurity scattering. By investigation of Ba(Fe1-xCox)2As2 thin films with varying Co concentration, we demonstrate that in the dirty limit the superconducting dome in the electronic phase diagram of Ba(Fe1-xCox)2As2 shifts towards lower doping concentrations, which differs significantly from observations in single crystals. We show that especially in the underdoped regime superconducting transition temperatures higher than 27 K are possible.

cond-mat.supr-con↗

arXiv2012

Infrared photo-response of Fe-shunted Ba-122 thin film microstructures

We present a study of the response to pulsed infrared radiation of Fe-layer shunted pnictide thin film microstructures. The thin film multilayer consisting of 20 nm thick Fe-buffer, 50 nm thick Ba(Fe,Co)2As2 film and gold protection layer were deposited on heated MgO and MgAl2O4 substrates by pulsed-laser deposition. The multilayers were patterned into 5 to 8 um wide and 5 um long microbridges by electron-beam lithography and ion-milling technique. The microbridges show Tc up to 20 K and a critical current density up to 2.56 MA/cm2 at T = 10 K. The photo-response of Fe-shunted Ba(Fe,Co)2As2 thin film microbridges to infrared radiation was studied in a wide range of incident optical power, operation temperature and bias current. We have found that the electron energy relaxation in studied multilayers is dependent on substrate material and is 1.75 times faster in case of MgAl2O4 characterized by lattice matching to pnictide film in comparison to MgO substrate.

cond-mat.supr-con↗

arXiv2012

Coexisting order and disorder hidden in a quasi-two-dimensional frustrated magnet

Frustrated magnetic interactions in a quasi-two-dimensional [111] slab of pyrochlore lattice were studied. For uniform nearest neighbor (NN) interactions, we show that the complex magnetic problem can be mapped onto a model with two independent degrees of freedom, tri-color and binary sign. This provides a systematic way to construct the complex classical spin ground states with collinear and coplanar bi-pyramid spins. We also identify `partial but extended' zero-energy excitations amongst the ground states. For nonuniform NN interactions, the coplanar ground state can be obtained from the collinear bi-pyramid spin state by collectively rotating two spins of each tetrahedron with an angle, $α$, in an opposite direction. The latter model with $α\sim 30^\circ$ fits the experimental neutron data from SCGO well.

cond-mat.str-el↗

arXiv2012

Determination of spin Hamiltonian in the Ni$_4$ magnetic molecule

Magnetic excitations in a Ni$_4$ magnetic molecule were investigated by inelastic neutron scattering and bulk susceptibility ($χ_\text{bulk}$) techniques. The magnetic excitation spectrum obtained from the inelastic neutron scattering experiments exhibits three modes at energy transfers of $\hbarω=0.5$, 1.35, and 1.6 meV. We show that the energy, momentum, and temperature dependences of the inelastic neutron scattering data and $χ_\text{bulk}$ can be well reproduced by an effective spin Hamiltonian consisted of intra-molecule exchange interactions, a single-ionic anisotropy, biquadratic interactions, and Zeeman term. Under a hydrostatic pressure, the bulk magnetization decreases with increasing pressure, which along with the biquadratic term indicates spin-lattice coupling present in this system.

cond-mat.str-el↗

arXiv2012

Two-dimensional incommensurate magnetic fluctuations in Sr$_2$(Ru$_{0.99}$Ti$_{0.01}$)O$_4$

We investigate the imaginary part of the wave vector dependent dynamic spin susceptibility in Sr$_2$(Ru$_{0.99}$Ti$_{0.01}$)O$_4$ as a function of temperature using neutron scattering. At T=5 K, two-dimensional incommensurate (IC) magnetic fluctuations are clearly observed around $\mathbf{Q}_\text{c}=(0.3,0.3,L)$ up to approximately 60 meV energy transfer. We find that the IC excitations disperse to ridges around the $(π,π)$ point. Below 50 K, the energy and temperature dependent excitations are well described by the phenomenological response function for a Fermi liquid system with a characteristic energy of 4.0(1) meV. Although the wave vector dependence of the IC magnetic fluctuations in Sr$_2$(Ru$_{0.99}$Ti$_{0.01}$)O$_4$ is similar to that in the Fermi liquid state of the parent compound, Sr$_2$RuO$_4$, the magnetic fluctuations are clearly suppressed by the Ti-doping.

cond-mat.supr-con↗

arXiv2011

Planar hybrid superconductor-normal metal-superconductor thin film junctions based on BaFe1.8Co0.2As2

To investigate the transport properties of iron based superconductors, we prepared planar hybrid superconductor-normal metal-superconductor (SNS') thin film junctions with BaFe1.8Co0.2As2 as base electrode. As counter electrode we used a lead indium alloy, while the normal metal layer was formed by thin gold films. Temperature dependent measurements of the electrical conductivity were strongly influenced by the properties of the electrodes. We developed a junction structure that allows us to characterize the electrodes, too, including the behavior of their normal state resistance in order to correct their influences on the conduction spectra. The corrected conductivity of the junction was described within an extended BTK-model and shows a behavior dominated by Andreev reflexion.

cond-mat.supr-con↗

arXiv2011

Proton-nucleus total reaction cross sections in the optical limit Glauber theory: Subtle dependence on the equation of state of nuclear matter

We calculate the proton-nucleus total reaction cross sections at different energies of incident protons within the optical limit approximation of the Glauber theory. The isospin effect has been taken into account. The nucleon distribution is obtained in the framework of macroscopic nuclear models in a way depending on the equation of state of uniform nuclear matter near the saturation density. We find that at an energy of order 40 MeV, the reaction cross section calculated for neutron- rich isotopes significantly increases as the parameter L characterizing the density dependence of the symmetry energy increases, while at energies of order 300 and 800 MeV, it is almost independent of L. This is a feature of the optical limit Glauber theory in which an exponential dependence of the reaction cross section on the neutron skin thickness remains when the total proton-neutron cross section is small enough.

nucl-th↗

arXiv2011

Inelastic neutron scattering study of the magnetic fluctuations in Sr$_2$RuO$_4$

By performing time-of-flight neutron scattering measurements on a large amount of single crystals of Sr$_2$RuO$_4$, we studied detailed structure of the imaginary part of the dynamic spin susceptibility over a wide range of phase space. In the normal state at T=5 K, strong incommensurate (IC) peaks were clearly observed at around $\mathbf{Q}_\text{c}=(0.3,0.3)$ up to at least $\hbarω=80$ meV. In addition, our data also show strong magnetic fluctuations that exist on the ridges connecting the IC peaks around the $(π,π)$ point rather than around the $Γ$ point. Our results are consistent with the semi-mean-field random phase approximation calculation for a two dimensional Fermi liquid with a characteristic energy of 5.0 meV. Furthermore, the IC fluctuations were observed even at room temperature.

cond-mat.str-el↗

arXiv2010

Influenced of Fe buffer thickness on the crystalline quality and the transport properties of Fe/Ba(Fe1-xCox)2As2 bilayers

The implementation of an Fe buffer layer is a promising way to obtain epitaxial growth of Co-doped BaFe2As2 (Ba-122). However, the crystalline quality and the superconducting properties of Co-doped Ba-122 are influenced by the Fe buffer layer thickness, dFe. The well-textured growth of the Fe/Ba-122 bilayer with dFe = 15 nm results in a high Jc of 0.45 MAcm$^{-2}$ at 12 K in self-field, whereas a low Jc value of 61000 Acm$^{-2}$ is recorded for the bilayer with dFe = 4 nm at the corresponding reduced temperature due to the presence of grain boundaries.

cond-mat.supr-con↗

arXiv2010

Highly anisotropic energy gap in superconducting Ba(Fe$_{0.9}$Co$_{0.1}$)$_{2}$As$_{2}$ from optical conductivity measurements

We have measured the complex dynamical conductivity, $σ= σ_{1} + iσ_{2}$, of superconducting Ba(Fe$_{0.9}$Co$_{0.1}$)$_{2}$As$_{2}$ ($T_{c} = 22$ K) at terahertz frequencies and temperatures 2 - 30 K. In the frequency dependence of $σ_{1}$ below $T_{c}$, we observe clear signatures of the superconducting energy gap opening. The temperature dependence of $σ_{1}$ demonstrates a pronounced coherence peak at frequencies below 15 cm$^{-1}$ (1.8 meV). The temperature dependence of the penetration depth, calculated from $σ_{2}$, shows power-law behavior at the lowest temperatures. Analysis of the conductivity data with a two-gap model, gives the smaller isotropic s-wave gap of $Δ_{A} = 3$ meV, while the larger gap is highly anisotropic with possible nodes and its rms amplitude is $Δ_{0} = 8$ meV. Overall, our results are consistent with a two-band superconductor with an $s_{\pm}$ gap symmetry.

cond-mat.supr-con↗

arXiv2010

BaFe_{1.8}Co_{0.2}As_2 thin film hybrid Josephson junctions

Josephson junctions with iron pnictides open the way for fundamental experiments on superconductivity in these materials and their application in superconducting devices. Here, we present hybrid Josephson junctions with a BaFe_{1.8}Co_{0.2}As_2 thin film electrode, an Au barrier and a PbIn counter electrode. The junctions show RSJ-like current-voltage characteristics up to the critical temperature of the counter electrode of about 7.2K. The temperature dependence of the critical current, IC, does not show an Ambegaokar-Baratoff behavior. Well-pronounced Shapiro steps are observed at microwave frequencies of 10-18GHz. Assuming an excess current, I_ex, of 200 μA at 4.2K we get an effective I_C R_N product of 6 μV.

cond-mat.supr-con↗

arXiv2010

Two-band BCS superconductivity in Ba(Fe0.9Co0.1)2As2

The conductivity and permittivity optical spectra of iron-pnictide Ba(Fe0.9Co0.1)2As2 film (Tc=20 K) are analyzed. In the superconducting state, at all temperatures up to Tc the temperature dependences of the magnetic field penetration depth and of the superconducting condensate density are well described within the generalized two-band BCS model with intraband and interband pairing interactions considered. It is shown that the smaller superconducting energy gap 2Δ = 3.7 meV develops in the electronic subsystem while the larger gap 2Δ >= 7 meV opens in the hole subsystem. The normal state parameters (plasma frequencies and scattering rates) of electron and hole conduction bands are determined. At all temperatures the obtained data are consistent with the results of electronic photoemission experiments on Ba(Fe1-x Cox)2As2.

cond-mat.supr-con↗

arXiv2010

Critical current scaling and anisotropy in oxypnictide superconductors

Investigating the anisotropy of superconductors permits an access to fundamental properties. Having succeeded in the fabrication of epitaxial superconducting LaFeAs(O,F) thin films we performed an extensive study of electrical transport properties. In face of multiband superconductivity we can demonstrate that a Blatter scaling of the angular dependent critical current densities can be adopted, although being originally developed for single band superconductors. In contrast to single band superconductors the mass anisotropy of LaFeAs(O,F) is temperature dependent. A very steep increase of the upper critical field and the irreversibility field can be observed at temperatures below 6K, indicating that the band with the smaller gap is in the dirty limit. This temperature dependence can be theoretically described by two dominating bands responsible for superconductivity. A pinning force scaling provides insight into the prevalent pinning mechanism and can be specified in terms of the Kramer model.

cond-mat.supr-con↗