arXiv subjects

M. Naito

Publications and source records attributed to M. Naito.

At least 19 recordsLinked to original sources

arXiv2018

Intrinsic charge dynamics in high-Tc AFeAs(O,F) superconductors

We report the first determination of the in-plane complex optical conductivity of 1111 high-Tc superconducting iron oxypnictide single crystals PrFeAsO0.6F0.35 and thin films SmFeAsO1-xFx by means of bulk-sensitive conventional and micro-focused infrared spectroscopy, ellipsometry, and time-domain THz transmission spectroscopy. A strong itinerant contribution is found to exhibit a dramatic difference in coherence between the crystal and the film. Using extensive temperature-dependent measurements of THz transmission we identify a previously undetected 2.5-meV collective mode in the optical conductivity of SmFeAs(O,F), which is strongly suppressed at Tc and experiences an anomalous T-linear softening and narrowing below T*=110K>>Tc. The suppression of the infrared absorption in the superconducting state reveals a large optical superconducting gap with a similar gap ratio 2D=kBTc=7 in both materials, indicating strong pairing.

cond-mat.supr-con↗

arXiv2014

Eliashberg analysis of the optical conductivity in superconducting Pr$_2$CuO$_{x}$ ($x \simeq 4$)

Superconducting Pr$_2$CuO$_x$, $x\simeq 4$ films with $T^\prime$ structure and a $T_c$ of 27 K have been investigated by millimeter-wave transmission and broadband (infrared-to-ultraviolet) reflectivity measurements in the normal and superconducting state. The results obtained by both experimental methods show a consistent picture of the superconducting condensate formation below $T_c$. An Eliashberg analysis of the data proves $d$-wave superconductivity and unitary-limit impurity scattering of the charge carriers below $T_{c}$. The derived electron-exchange boson interaction spectral function $I^2χ(ω)$ shows only marginal changes at the superconducting transition with the mass enhancement factor $λ$, the first inverse moment of $I^2χ(ω)$, being equal to 4.16 at 30 K and to 4.25 at 4 K.

cond-mat.supr-con↗

arXiv2014

Optical study of superconducting Pr$_2$CuO$_{x}$ with $x\simeq 4$

Superconducting Pr$_2$CuO$_x$, $x\simeq 4$ (PCO) films with $T^\prime$ structure and a $T_c$ of 27 K have been investigated by various optical methods in a wide frequency (7 - 55000 cm$^{-1}$) and temperature (2 to 300 K) range. The optical spectra do not reveal any indication of a normal-state gap formation. A Drude-like peak centered at zero frequency dominates the optical conductivity below 150 K. At higher temperatures, it shifts to finite frequencies. The detailed analysis of the low-frequency conductivity reveals that the Drude peak and a far-infrared (FIR) peak centered at about 300 cm$^{-1}$ persist at all temperatures. The FIR-peak spectral weight is found to grow at the expense of the Drude spectral weight with increasing temperature. The temperature dependence of the penetration depth follows a behavior typical for $d$-wave superconductors. The absolute value of the penetration depth for zero temperature is 1.6 $μ$m, indicating a rather low density of the superconducting condensate.

cond-mat.supr-con↗

arXiv2014

Bulk superconductivity in undoped T'-La$_{1.9}$Y$_{0.1}$CuO$_4$ probed by muon spin rotation

The Meissner effect has been directly demonstrated by depth-resolved muon spin rotation measurements in high-quality thin films of the T'-structured cuprate, T'-La$_{1.9}$Y$_{0.1}$CuO$_4$, to confirm bulk superconductivity ($T_c\simeq21$ K) in its {\sl undoped} state. The gradual expelling of an external magnetic field is observed over a depth range of $\sim$100 nm in films with a thickness of 275(15) nm, from which the penetration depth is deduced to be 466(22) nm. Based on this result, we argue that the true ground state of the "parent" compound of the $n$-type cuprates is not a Mott insulator but a strongly correlated metal with colossal sensitivity to apical oxygen impurities.

cond-mat.supr-con↗

arXiv2010

Anomalously Sharp Superconducting Transitions in Overdoped $La_{2-x}Sr_{x}CuO_{4}$ Films

We present measurements of $ab$-plane resistivity $ρ_{ab}(T)$ and superfluid density [$\propto λ^{-2}$, $λ$ = magnetic penetration depth] in $La_{2-x}Sr_{x}CuO_{4}$ films. As Sr concentration $x$ exceeds about 0.22, the superconducting transition sharpens dramatically, becoming as narrow as 200 mK near the super-to-normal metal quantum critical point. At the same time, $ρ_{ab}(T)$, $λ^{-2}(T)$, and transition temperature $T_c$ decrease, and upward curvature develops in $λ^{-2}(T)$. Given the sharp transitions, we interpret these results in the context of a homogeneous d-wave superconducting state, with elastic scattering that is enhanced relative to underdoped LSCO due to weaker electron correlations. This interpretation conflicts with the viewpoint that the overdoped state is inhomogeneous due to phase separation into superconducting and normal metal regions.

cond-mat.supr-con↗

arXiv2010

Superconductor-to-Metal Quantum Phase Transition in Overdoped $La_{2-x}Sr_xCuO_4$

We investigate $T_c$ and magnetic penetration depth $λ(T)$ near the superconductor-metal quantum phase transition in overdoped $La_{2-x}Sr_{x}CuO_{4}$ films. Both $T_c$ and superfluid density $n_s$, $\propto λ^{-2}$, decrease with overdoping. They obey the scaling relation $T_c \propto [λ^{-2}(0)]^α$ with $α\approx 1/2$. We discuss this result in the frameworks of disordered d-wave superconductors and of scaling near quantum critical points. Our result, and the linear scaling ($α\approx 1$) found for the more anisotropic $Tl_2Ba_2CuO_{6+δ}$, can both be understood in terms of quantum critical scaling, with different dimensionalities for fluctuations.

cond-mat.supr-con↗

arXiv2008

Synthesis of infinite-layer LaNiO2 films by metal-organic deposition

We report the synthesis of infinite-layer LaNiO2 thin films by metal organic decomposition. Our work is aimed to synthesize perovskite-like oxides with 3d9 electronic configuration, which is common to high-Tc copper oxides. The 3d9 configuration is very rare in oxides other than cuprates. Ni1+ oxides, even though Ni1+ is an unusual oxidation state, may be one of very few candidates. One example of the Ni1+ phases is infinite-layer LaNiO2. The bulk synthesis of LaNiO2 is difficult, but we demonstrate in this article that the thin-film synthesis of LaNiO2 by metal organic decomposition is rather easy. This is due to the advantage of thin films with a large-surface-to-volume ratio, which makes oxygen diffusion prompt. Resistivity measurements indicate that LaNiO2 is essentially a metal but unfortunately with no trace of superconductivity yet.

cond-mat.supr-con↗

arXiv2008

Superconductivity phase diagrams of electron doped cuprates R(2-x)CexCuO4 (R = La, Pr, Nd, Sm, and Eu)

The superconductivity phase diagrams of electron doped cuprates of the form R(2-x)CexCuO4 (with R= La, Pr, Nd, Sm, and Eu) have been determined for cerium compositions 0 < x < 0.36 in a consistent series of epitaxial thin films grown by reactive molecular beam epitaxy (MBE). The use of epitaxial thin films allows the growth of materials away from thermodynamical equilibrium expanding the accessible phase space beyond the availability of bulk material. The superconducting phase space systematically increases with the rare earth ionic size. The doping concentration where the maximal transition temperature occurs in La(2-x)CexCuO4 is considerably shifted to lower doping (x ~ 0.09) compared to La(2-x)SrxCuO4 (x ~ 0.15). At the same time, the width of the superconducting region is broadened.

cond-mat.supr-con↗

arXiv2008

Study of Intrinsic Spin Hall Effect and Orbital Hall Effect in 4d- and 5d- Transition Metals

We study the intrinsic spin Hall conductivity (SHC) in various $5d$-transition metals (Ta, W, Re, Os, Ir, Pt, and Au) and 4d-transition metals (Nb, Mo, Tc, Ru, Rh, Pd, and Ag) based on the Naval Research Laboratory tight-binding model, which enables us to perform quantitatively reliable analysis. In each metal, the obtained intrinsic SHC is independent of resistivity in the low resistive regime ($ρ< 50 μΩ\text{cm}$) whereas it decreases in proportion to $ρ^{-2}$ in the high resistive regime. In the low resistive regime, the SHC takes a large positive value in Pt and Pd, both of which have approximately nine $d$-electrons per ion ($n_d=9$). On the other hand, the SHC takes a large negative value in Ta, Nb, W, and Mo where $n_d<5$. In transition metals, a conduction electron acquires the trajectory-dependent phase factor that originates from the atomic wavefunction. This phase factor, which is reminiscent of the Aharonov-Bohm phase, is the origin of the SHC in paramagnetic metals and that of the anomalous Hall conductivity in ferromagnetic metals. Furthermore, each transition metal shows huge and positive $d$-orbital Hall conductivity (OHC), independently of the strength of the spin-orbit interaction (SOI). Since the OHC is much larger than the SHC, it will be possible to realize a {\it orbitronics device} made of transition metals.

cond-mat.str-el↗

arXiv2008

Superconductivity in undoped T'-RE2CuO4 with Tc over 30 K

In this article, we report the superconductivity in T'-RE2CuO4 (RE = Pr, Nd, Sm, Eu, and Gd), which have been for a long time believed as a Mott insulator. Our discovery was achieved by using metal-organic decomposition (MOD), an inexpensive and easy-to-implement thin-film process. The keys to prepare the superconducting films are firing with low partial-pressure of oxygen and reduction at low temperatures. The highest Tc of undoped T'-RE2CuO4 is over 30 K, substantially higher than "electron-doped" analogs. Remarkably, Gd2CuO4, even the derivatives of which have not shown superconductivity so far, gets superconducting with Tconset as high as 20 K. The implication of our discovery is briefly discussed.

cond-mat.supr-con↗

arXiv2008

Study of intrinsic spin and orbital Hall effects in Pt based on a (6s, 6p, 5d) tight-binding model

We study the origin of the intrinsic spin Hall conductivity (SHC) and the d-orbital Hall conductivity (OHC) in Pt based on a multiorbital tight-binding model with spin-orbit interaction. We find that the SHC reaches 1000 \hbar/eΩcm when the resistivity ρis smaller than ~10 μΩcm, whereas it decreases to 300 \hbar/eΩcm when ρ~ 100 μΩcm. In addition, the OHC is still larger than the SHC. The origin of huge SHE and OHE in Pt is the large ``effective magnetic flux'' that is induced by the interorbital transition between d_{xy}- and d_{x2-y2}-orbitals with the aid of the strong spin-orbit interaction.

cond-mat.mes-hall↗

arXiv2007

Spin Hall effect in Sr2RuO4 and transition metals (Nb,Ta)

We study the intrinsic spin Hall conductivity (SHC) and the $d$-orbital Hall conductivity (OHC) in metallic $d$-electron systems based on the multiorbital tight-binding model. The obtained Hall conductivities are much larger than that in $p$-type semiconductors. The origin of these huge Hall effects is the "effective Aharonov-Bohm phase" induced by the signs of inter-orbital hopping integrals as well as atomic spin-orbit interaction. Huge SHC and OHC due to this mecahnism is ubiquitous in multiorbital transition metals.

cond-mat.str-el↗

arXiv2006

Charge transfer gap for T'-RE2CuO4 and T-La2CuO4 as estimated from Madelung potential calculations

T'-RE2CuO4 (RE: rare-earth element), after appropriate "reduction", has fairly high conductivity and also exhibits clear Fermi edge in photoemission spectroscopy. To clarify the origin of conductivity in the T' mother compounds, we evaluated the unscreened charge-transfer gap for T'-RE2CuO4 and T-La2CuO4. The value of charge-transfer gap for T'-compounds almost linearly decreases with increasing the ionic radius of RE from 12.24 eV for T'-Tm2CuO4 to 9.90 eV for T'-La2CuO4. The results qualitatively explain metallic conductivity in T'-RE2CuO4 for large RE.

cond-mat.supr-con↗

arXiv2006

Doping of Ce in T-La2CuO4: Rigorous test for electron-hole symmetry for high-Tc superconductivity

We report that Ce doping was achieved in La2CuO4 with the K2NiF4 (T) structure for the first time by molecular beam epitaxy. A synthesis temperature of as low as ~ 630C and an appropriate substrate choice, i.e., (001)LaSrGaO4 (a \~ 3.843 A), enebled us to incorporate Ce into K2NiF4 lattice and to obtain Ce-doped T-La2-xCexCuO4 up to x ~ 0.06. The doping of Ce makes T-La2CuO4 more insulating, which is in sharp contrast to Sr (or Ba) doping in T-La2CuO4, which make the compound metallic and superconducting. The observed smooth increase in resistivity from the hole-doped side (T-La2-xSrxCuO4) to the electron-doped side (T-La2-xCexCuO4) indicates that the electron-hole symmetry is broken in the T-phase materials.

cond-mat.supr-con↗

arXiv2006

Fabrication and properties of MgB2/AlOx/MgB2 tunnel junctions

Sandwich-type MgB2 Josephson tunnel junctions (MgB2/AlOx/MgB2) have been fabricated using as-grown MgB2 films formed by molecular beam epitaxy. The junctions exhibited substantial supercurrent and a well-defined superconducting gap (D=2.2~2.3mV). The superconducting gap voltage (D) agrees well with those of the smaller gap in the multi-gap scenario. The IcRN product is 0.4-1.3 mV at 4.2K, but approaches to 2.0 mV at 50 mK. The Ic has peculiar temperature dependence far from the Ambegaokar-Baratoff formula. It rapidly decreases with temperature, and disappears above T = 20 K, which is much lower than the gap closing temperature. Interface microstructure between AlOx and MgB2 were investigated using cross-sectional transmission electron microscopy to clarify the problems in our tunnel junctions. There are poor-crystalline MgB2 layers and/or amorphous Mg-B composite layers of a few nanometers between AlOx barrier and the upper MgB2 layer. The poor-crystalline upper Mg-B layers seem to behave as normal metal or deteriorated superconducting layers, which may be the principal reason for all non-idealities of our MgB2/AlOx/MgB2 junctions.

cond-mat.supr-con↗

arXiv2005

Silver-assisted growth of NdBa2Cu3O7-d thin films: A novel approach for the growth of superior quality ceramic oxide films

We have grown NdBa2Cu3O7-d films under silver atomic flux by molecular beam epitaxy, which show a drastic improvement in microstructure and also crystallinity leading to 30 % enhancement in critical current density. The most remarkable point is that the final film is free from silver. The key to our process in achieving a silver-free film was the use of RF-activated oxygen that oxidizes silver, nonvolatile, to silver oxide, volatile at the deposition temperature. This process enables one to utilize the beneficial effects of silver in the growth of oxide films and at the same time ensures that the final film be free from silver, which is important for high-frequency applications. This method can be made use in the growth of thin films of other complex oxide materials.

cond-mat.supr-con↗

arXiv2005

Ce doping in T-La2CuO4 films: Broken electron-hole symmetry for high-Tc superconductivity

We attempted Ce doping in La2CuO4 with the K2NiF4 (T) structure by molecular beam epitaxy. At low growth temperature and with an appropriate substrate choice, we found that Ce can be incorporated into the K2NiF4 lattice up to x ~ 0.06, which had not yet been realized in bulk synthesis. The doping of Ce makes T-La2-xCexCuO4 more insulating, which is in sharp contrast to Ce doping in La2CuO4 with the Nd2CuO4 structure, which makes the compounds superconducting. The observed smooth increase in resistivity from hole-doped side (T-La2-xSrxCuO4) to electron-doped side (T-La2-xCexCuO4) indicates that electron-hole symmetry is broken in the T-phase materials.

cond-mat.supr-con↗

arXiv2005

Role of impurity oxygen in superconductivity of "non-doped" T'-(La,RE)2CuO4

We have systematically investigated the effect of oxygen nonstoichiometry in a nominally undoped superconductor T'-(La,Y)2CuO4+y. In the experiments, the reduction condition was changed after the sample growth by MBE. The superconductivity is very sensitive to the reduction condition. With systematically increasingly reduced atmospheres, resistivity shows a continuous drop and no discontinuity is observed even until the appearance of superconductivity. The absence of the highly insulating state expected for Mott insulators around y ~ 0 suggests that T'-(La,Y)2CuO4 has intrinsic carriers. The role of residual apical oxygen, which is detrimental to superconductivity, is also discussed based on the resistivity-temperature characteristics in insufficiently reduced samples.

cond-mat.supr-con↗