arXiv subjects

Y. Matsuda

Publications and source records attributed to Y. Matsuda.

At least 271 records · Page 15Linked to original sources

arXiv2007

Flux Line Lattice Melting and the Formation of a Coherent Quasiparticle Bloch State in the Ultraclean URu$_2$Si$_2$ Superconductor

We find that in ultraclean heavy-fermion superconductor URu$_2$Si$_2$ ($T_{c0}=1.45$ K) a distinct flux line lattice melting transition with outstanding characters occurs well below the mean-field upper critical fields. We show that a very small number of carriers with heavy mass in this system results in exceptionally large thermal fluctuations even at subkelvin temperatures, which are witnessed by a sizable region of the flux line liquid phase. The uniqueness is further highlighted by an enhancement of the quasiparticle mean free path below the melting transition, implying a possible formation of a quasiparticle Bloch state in the periodic flux line lattice.

cond-mat.supr-con↗

arXiv2007

$ΛN$ correlations from the stopped $K^-$ reaction on ${}^4$He

We have investigated correlations of coincident $ΛN$ pairs from the stopped $K^-$ reaction on ${}^4$He, and clearly observed $Λp$ and $Λn$ branches of the two-nucleon absorption process in the $ΛN$ invariant mass spectra. In addition, non-mesonic reaction channels, which indicate possible exotic signals for the formation of strange multibaryon states, have been identified.

nucl-ex↗

arXiv2007

Vortex Redistribution below the First-Order Transition Temperature in the β-Pyrochlore Superconductor KOs_2O_6

A miniature Hall sensor array was used to detect magnetic induction locally in the vortex states of the $β$-pyrochlore superconductor KOs$_2$O$_6$. Below the first-order transition at $T_{\rm p}\sim 8$ K, which is associated with a change in the rattling motion of K ions, the lower critical field and the remanent magnetization both show a distinct decrease, suggesting that the electron-phonon coupling is weakened below the transition. At high magnetic fields, the local induction shows an unexpectedly large jump at $T_{\rm p}$ whose sign changes with position inside the sample. Our results demonstrate a novel redistribution of vortices whose energy is reduced abruptly below the first-order transition at $T_{\rm p}$.

cond-mat.supr-con↗

arXiv2007

FFLO state in thin superconducting films

We present the analysis of the inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state in thin superconducting films in the parallel magnetic field. For the tetragonal crystal symmetry (relevant to CeCoIn$_{5}$ - the most probable candidate for the FFLO state formation) we predict a very peculiar in-plane angular dependence of the FFLO critical field due to the orbital effect. In the uniform superconducting state the critical field should be isotropic. The magnetic field pins also the direction of the FFLO modulation permitting thus to study the critical current anisotropy. Our calculations reveal a strong critical current anisotropy in the FFLO state in sharp contrast with the usual superconducting state. The predicted characteristic anisotropies of the critical field and critical current may provide an unambiguous probe of the FFLO phase formation.

cond-mat.supr-con↗

arXiv2007

Thermoelectric response near a quantum critical point: the case of CeCoIn5

We present a study of thermoelectric coefficients in CeCoIn_5 down to 0.1 K and up to 16 T in order to probe the thermoelectric signatures of quantum criticality. In the vicinity of the field-induced quantum critical point, the Nernst coefficient nu exhibits a dramatic enhancement without saturation down to lowest measured temperature. The dimensionless ratio of Seebeck coefficient to electronic specific heat shows a minimum at a temperature close to threshold of the quasiparticle formation. Close to T_c(H), in the vortex-liquid state, the Nernst coefficient behaves anomalously in puzzling contrast with other superconductors and standard vortex dynamics.

cond-mat.str-el↗

arXiv2007

Search for strange tribaryon states in the inclusive ^4He(K^{-}_{stopped}, p) reaction

A search for tribaryon states was performed at KEK-PS. We adopted the $^4$He($K^-_{stopped}, p)$ reaction to populate the states with strangeness -1, charge 0 and isospin 1. No significant narrow structure was observed in the mass region from 3000 to 3200 MeV/c^2 in an inclusive missing mass spectrum. The upper limit of the formation branching ratio was determined to be ($1\sim3)\times 10^{-4}$, ($0.7\sim2)\times 10^{-3}$ and ($2\sim8) \times 10^{-3}$/($stopped K^{-}$) with 95 % confidence level for narrow states with an assumed width of 0, 20 and 40 MeV/c^2, respectively.

nucl-ex↗

arXiv2007

Exotic Superconducting Properties in the Electron-Hole Compensated Heavy Fermion `Semimetal' URu2Si2

We show that the charge and thermal transport measurements on ultraclean crystals of URu2Si2 reveal a number of unprecedented superconducting properties. The uniqueness is best highlighted by the peculiar field dependence of thermal conductivity including the first order transition at Hc2 with a reduction of entropy flow. This is a consequence of multi-band superconductivity with compensated electronic structure in the hidden order state of this system. We provide strong evidence for a new type of unconventional superconductivity with two distinct gaps having different nodal topology.

cond-mat.supr-con↗

arXiv2007

High-Resolution Submillimeter Imaging of the Ly-alpha Blob1 in SSA 22

We present ~2" resolution submillimeter observations of the submillimeter luminous giant Ly-alpha blob (LAB1) in the SSA 22 protocluster at redshift z=3.1 with the Submillimeter Array (SMA). Although the expected submillimeter flux density is 16 mJy at 880 micron, no emission is detected with the 2".4 x 1".9 (18 x 14 kpc) beam at the 3 sigma level of 4.2 mJy beam^{-1} in the SMA field of view of 35". This is in contrast to the previous lower angular resolution (15") observations where a bright (17 mJy) unresolved submillimeter source was detected at 850 micron toward the LAB1 using the Submillimeter Common-User Bolometer Array on the James Clerk Maxwell Telescope. The SMA non-detection suggests that the spatial extent of the submillimeter emission of LAB1 should be larger than 4" (>30 kpc). The most likely interpretation of the spatially extended submillimeter emission is that starbursts occur throughout the large area in LAB1. Some part of the submillimeter emission may come from spatially extended dust expelled from starburst regions by galactic superwind. The spatial extent of the submillimeter emission of LAB1 is similar to those of high redshift radio galaxies rather than submillimeter galaxies.

astro-ph↗

arXiv2007

Precision measurement of the $3d \to 2p$ x-ray energy in kaonic $^4$He

We have measured the Balmer-series x-rays of kaonic $^4$He atoms using novel large-area silicon drift x-ray detectors in order to study the low-energy $\bar{K}$-nucleus strong interaction. The energy of the $3d \to 2p$ transition was determined to be 6467 $\pm$ 3 (stat) $\pm$ 2 (syst) eV. The resulting strong-interaction energy-level shift is in agreement with theoretical calculations, thus eliminating a long-standing discrepancy between theory and experiment.

nucl-ex↗

arXiv2007

Search for a kaonic nuclear state via $^4$He$(K^-, N)$

Very recently, we have performed a couple of experiments, {\it{KEK PS-E549/E570}}, for the detailed study of the strange tribaryon $S^0(3115)$ obtained in {\it{KEK PS-E471}}. These experiments were performed to accumulate much higher statistics with improved experimental apparatusespecially for the better proton spectroscopy of the $^4$He({\it{stopped K}}$^-$, {\it{N}}) reaction. In contrast to the previous proton spectrum, no narrow ($\sim$ 20 MeV) peak structure was found either in the inclusive $^4$He({\it{stopped K}}$^-$, {\it{p}}) or in the semi-inclusive $^4$He({\it{stopped K}}$^-$, {\it{p}}$X^\pm$) reaction channel, which is equivalent to the previous $E471$ event trigger condition. Detailed analysis of the present data and simulation shows that the peak, corresponding to $S^0(3115)$, has been an experimental artifact. Present analysis does not exclude the possible existence of a much wider structure. To be sensitive to such structure and for better understanding of the non-mesonic $K^-$ absorption reaction channel, detailed analysis of the data is in progress.

nucl-ex↗

arXiv2007

Effects of Rattling Phonons on the Quasiparticle Excitation and Dynamics in the Superconducting $β$-Pyrochlore KOs$_2$O$_6$

Microwave penetration depth $λ$ and surface resistance at 27 GHz are measured in high quality crystals of KOs$_2$O$_6$. Firm evidence for fully-gapped superconductivity is provided from $λ(T)$. Below the second transition at $T_{\rm p}\sim 8$ K, the superfluid density shows a step-like change with a suppression of effective critical temperature $T_{\rm c}$. Concurrently, the extracted quasiparticle scattering time shows a steep enhancement, indicating a strong coupling between the anomalous rattling motion of K ions and quasiparticles. The results imply that the rattling phonons help to enhance superconductivity, and that K sites freeze to an ordered state with long quasiparticle mean free path below $T_{\rm p}$.

cond-mat.supr-con↗

arXiv2007

Competition between unconventional superconductivity and incommensurate antiferromagnetic order in CeRh1-xCoxIn5

Elastic neutron diffraction measurements were performed on the quasi-two dimensional heavy fermion system CeRh1-xCoxIn5, ranging from an incommensurate antiferromagnet for low x to an unconventional superconductor on the Co-rich end of the phase diagram. We found that the superconductivity competes with the incommensurate antiferromagnetic (AFM) order characterized by qI=(1/2, 1/2, delta) with delta=0.298, while it coexists with the commensurate AFM order with qc=(1/2, 1/2, 1/2). This is in sharp contrast to the CeRh1-xIrxIn5 system, where both the commensurate and incommensurate magnetic orders coexist with the superconductivity. These results reveal that particular areas on the Fermi surface nested by qI play an active role in forming the superconducting state in CeCoIn5.

cond-mat.str-el↗

arXiv2007

Fulde-Ferrell-Larkin-Ovchinnikov State in Heavy Fermion Superconductors

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is a novel superconducting state in a strong magnetic field characterized by the formation of Cooper pairs with nonzero total momentum (k \uparrow, -k+q \downarrow), instead of the ordinary BCS pairs (k \uparrow, -k \downarrow). A fascinating aspect of the FFLO state is that it exhibits inhomogeneous superconducting phases with a spatially oscillating order parameter and spin polarization. The FFLO state has been of interest in various research fields, not only in superconductors in solid state physics, but also in neutral Fermion superfluid of ultracold atomic gases and in color superconductivity in high energy physics. In spite of extensive studies of various superconductors, there has been no undisputed experimental verification of the FFLO state, mainly because of the very stringent conditions required of the superconducting materials. Among several classes of materials, certain heavy fermion and organic superconductors are believed to provide conditions that are favorable to the formation of the FFLO state. This review presents recent experimental and theoretical developments of the FFLO state mainly in heavy fermion superconductors. In particular we address the recently discovered quasi-two-dimensional superconductor CeCoIn_5, which is a strong candidate for the formation of the FFLO state.

cond-mat.supr-con↗

arXiv2006

Nodal Structure of Unconventional Superconductors Probed by the Angle Resolved Thermal Transport Measurements

Over the past two decades, unconventional superconductivity with gap symmetry other than s-wave has been found in several classes of materials, including heavy fermion (HF), high-T_c, and organic superconductors. Unconventional superconductivity is characterized by anisotropic superconducting gap functions, which may have zeros (nodes) along certain directions in the Brillouin zone. The nodal structure is closely related to the pairing interaction, and it is widely believed that the presence of nodes is a signature of magnetic or some other exotic, rather than conventional phonon-mediated, pairing mechanism. Therefore experimental determination of the gap function is of fundamental importance. However, the detailed gap structure, especially the direction of the nodes, is an unresolved issue in most unconventional superconductors. Recently it has been demonstrated that the thermal conductivity and specific heat measurements under magnetic field rotated relative to the crystal axes are a powerful method for determining the shape of the gap and the nodal directions in the bulk. Here we review the theoretical underpinnings of the method and the results for the nodal structure of several unconventional superconductors, including borocarbide YNi$_2$B$_2$C, heavy fermions UPd$_2$Al$_3$, CeCoIn$_5$, and PrOs$_4$Sb$_{12}$, organic superconductor, $κ$-(BEDT-TTF)$_2$Cu(NCS)$_2$, and ruthenate Sr$_2$RuO$_4$, determined by angular variation of the thermal conductivity and heat capacity.

cond-mat.supr-con↗

arXiv2006

Uncommonly High Upper Critical Field in the Superconducting KOs$_2$O$_6$ Pyrochlore

The entire temperature dependence of the upper critical field $H_{\rm c2}$ in the $β$-pyrochlore KOs$_2$O$_6$ is obtained from high-field resistivity and magnetic measurements. Both techniques identically give $H_{\rm c2}(T \simeq 0 {\rm K})$ not only surprisingly high ($\sim 33$ T), but also the approach to it unusually temperature-\emph{linear} all the way below $T_{\rm c}$ (= 9.6 K). We show that, while $H_{\rm c2}(0)$ exceeds a simple spin-singlet paramagnetic limit $H_{\rm P}$, it is well below an $H_{\rm P}$ enhanced due to the missing spatial inversion symmetry reported recently in KOs$_2$O$_6$, ensuring that the pair-breaking here is executed by orbital degrees. {\it Ab initio} calculations of orbital $H_{\rm c2}$ show that the unusual temperature dependence is reproduced if dominant s-wave superconductivity resides on the smaller closed Fermi surfaces.

cond-mat.supr-con↗

arXiv2006

Non-Fermi liquid behavior in the magnetotransport of CeMIn5 (M: Co and Rh): Striking similarity between quasi 2D heavy fermion and high-Tc cuprates

We present a systematic study of the dc-resistivity, Hall effect, and magnetoresistance in the normal state of quasi 2D heavy fermion superconductors CeMIn5 (M: Rh and Co) under pressure. Here the electronic system evolves with pressure from an antiferromagnetic (AF) metal, through a highly unconventional non-Fermi liquid, and finally into a Fermi-liquid state. The amplitude of the Hall coefficient increases dramatically with decreasing T, reaching at low temperatures a value significantly larger than 1/ne. Furthermore, the magnetoresistance is characterized by T- and H-dependence which clearly violate Kohler's rule. We found that the Hall angle cotΘvaries as T^2, and the magnetoresistance is well scaled by the Hall angle as Δρ_{xx}/ρ_{xx}\propto \tan^2Θ. These non-Fermi liquid properties in the electron transport are remarkably pronounced when the AF fluctuations are enhanced in the vicinity of the QCP. We lay particular emphasis on the striking resemblance of these anomalous magnetotransport with those of the high-Tc cuprates. We argue that features commonly observed in quasi 2D heavy fermion and cuprates very likely capture universal features of strongly correlated electron systems.

cond-mat.str-el↗

arXiv2006

The End of the Reionization Epoch Probed by Ly-alpha Emitters at z=6.5 in the Subaru Deep Field

We report an extensive search for Lyman-alpha emitters (LAEs) at z=6.5 in the Subaru Deep Field. Subsequent spectroscopy with Subaru and Keck identified eight more LAEs, giving a total of 17 spectroscopically confirmed LAEs at z=6.5. Based on this spectroscopic sample of 17, complemented by a photometric sample of 58 LAEs, we have derived a more accurate Lyman-alpha luminosity function of LAEs at z=6.5, which reveals an apparent deficit at the bright end of ~0.75 mag fainter L*, compared with that observed at z=5.7. The difference in the LAE luminosity functions between z=5.7 and 6.5 is significant at the 3-sigma level, which is reduced to 2-sigma when cosmic variance is taken into account. This result may imply that the reionization of the universe has not been completed at z=6.5. We found that the spatial distribution of LAEs at z=6.5 was homogeneous over the field. We discuss the implications of these results for the reionization of the universe.

astro-ph↗

arXiv2006

Thermal Conductivity of the Pyrochlore Superconductor KOs2O6: Strong Electron Correlations and Fully Gapped Superconductivity

To elucidate the nature of the superconducting ground state of the geometrically frustrated pyrochlore KOs2O6 (Tc=9.6K), the thermal conductivity was measured down to low temperatures (~Tc/100). We found that the quasiparticle mean free path is strikingly enhanced below a transition at Tp=7.5K, indicating enormous electron inelastic scattering in the normal state. In a magnetic field the conduction at T ->0K is nearly constant up to ~0.4Hc2, in contrast with the rapid growth expected for superconductors with an anisotropic gap. This unambiguously indicates a fully gapped superconductivity, in contrast to the previous studies. These results highlight that KOs2O6 is unique among superconductors with strong electron correlations.

cond-mat.supr-con↗