arXiv subjects

Kouji Segawa

Publications and source records attributed to Kouji Segawa.

87 records · Page 5Linked to original sources

arXiv2002

Magnetoresistance of Untwinned YBa_{2}Cu_{3}O_{y} Single Crystals in a Wide Range of Doping: Anomalous Hole-Doping Dependence of the Coherence Length

Magnetoresistance (MR) in the a-axis resistivity of untwinned YBa_{2}Cu_{3}O_{y} single crystals is measured for a wide range of doping (y = 6.45 - 7.0). The y-dependence of the in-plane coherence length ξ_{ab} estimated from the fluctuation magnetoconductance indicates that the superconductivity is anomalously weakened in the 60-K phase; this gives evidence, together with the Hall coefficient and the a-axis thermopower data that suggest the hole doping to be 12% for y = 6.65, that the origin of the 60-K plateau is the 1/8 anomaly. At high temperatures, the normal-state MR data show signatures of the Zeeman effect on the pseudogap in underdoped samples.

cond-mat.supr-con↗

arXiv2002

Electrical Resistivity Anisotropy from Self-Organized One-Dimensionality in High-Temperature Superconductors

We investigate the manifestation of the stripes in the in-plane resistivity anisotropy in untwinned single crystals of La_{2-x}Sr_{x}CuO_{4} (x = 0.02 - 0.04) and YBa_{2}Cu_{3}O_{y} (y = 6.35 - 7.0). It is found that both systems show strongly temperature-dependent in-plane anisotropy in the lightly hole-doped region and that the anisotropy in YBa_{2}Cu_{3}O_{y} grows with decreasing y below about 6.60 despite the decreasing orthorhombicity, which gives most direct evidence that electrons self-organize into a macroscopically anisotropic state. The transport is found to be easier along the direction of the spin stripes already reported, demonstrating that the stripes are intrinsically conducting in cuprates.

cond-mat.supr-con↗

arXiv2001

Magnetotransport properties of untwinned YBa_{2}Cu_{3}O_{y} single crystals: Novel 60-K-phase anomalies in the charge transport

We present the result of our accurate measurements of the a- and b-axis resistivity (ρ_a and ρ_b), magnetoconductivity (Δσ/ σ), Hall coefficient R_H, and the a-axis thermopower S_a in untwinned YBa_{2}Cu_{3}O_{y} single crystals in a wide range of doping (6.45 < y < 7.0). The systematics of our data reveals a number of novel 60-K-phase anomalies in the charge transport: (i) Temperature dependences of ρ_a show anomalous overlap below ~130 K for 6.65 < y < 6.80, (ii) Hall mobility μ_H shows an enhancement near y ~ 6.65, which is reflected in an anomalous y dependence of σ_{xy}, (iii) With decreasing temperature R_H shows a marked drop upon approaching T_c only in samples with 6.70 < y < 6.85, (iv) Superconducting fluctuation magnetoconductivity is anomalously enhanced near y ~ 6.7, and (v) H_{c2} is anomalously reduced near y ~ 6.70. We discuss that the fluctuating charge stripes might be responsible for these anomalies in the charge transport.

cond-mat.supr-con↗

arXiv2001

Mobility of the Doped Holes and the Antiferromagnetic Correlations in Underdoped High-T_c Cuprates

The emergence and the evolution of the metallic charge transport in the La_{2-x}Sr_{x}CuO_{4} system from lightly- to optimally-doped samples (x = 0.01 - 0.17) are studied. We demonstrate that in high-quality single crystals the in-plane resistivity shows a metallic behavior for ALL values of x at moderate temperatures and that the hole mobility at 300 K changes only by a factor of 3 from x = 0.01 to 0.17, where its x-dependence is found to be intriguingly similar to that of the inverse antiferromagnetic correlation length. We discuss an incoherent-metal picture and a charged-stripe scenario as candidates to account for these peculiar features.

cond-mat.supr-con↗

arXiv2001

Transport Anomalies and the Role of Pseudogap in the "60-K Phase" of YBa_{2}Cu_{3}O_{7-δ}

We report the result of our accurate measurements of the a- and b-axis resistivity, Hall coefficient, and the a-axis thermopower in untwinned YBa_{2}Cu_{3}O_{y} single crystals in a wide range of doping. It is found that both the a-axis resistivity and the Hall conductivity show anomalous dependences on the oxygen content y in the "60-K phase" below the pseudogap temperature T^*. The complete data set enables us to narrow down the possible pictures of the 60-K phase, with which we discuss a peculiar role of the pseudogap in the charge transport.

cond-mat.supr-con↗

arXiv2000

Ando, Lavrov, and Segawa Reply

Authors' Reply to the Comment by Janossy et al. [cond-mat/0005275] on our article, "Magnetoresistance Anomalies in Antiferromagnetic YBa_{2}Cu_{3}O_{6+x}: Fingerprints of Charged Stripes" [cond-mat/9905071, Phys. Rev. Lett. 83, 2813 (1999)].

cond-mat.supr-con↗

arXiv2000

Magnetotransport study of the charged stripes in high-T_c cuprates

We present a study of the in-plane and out-of-plane magnetoresistance (MR) in heavily-underdoped, antiferromagnetic YBa_{2}Cu_{3}O_{6+x}, which reveals a variety of striking features. The in-plane MR demonstrates a "d-wave"-like anisotropy upon rotating the magnetic field H within the ab plane. With decreasing temperature below 20-25 K, the system acquires memory: exposing a crystal to the magnetic field results in a persistent in-plane resistivity anisotropy. The overall features can be explained by assuming that the CuO_2 planes contain a developed array of stripes accommodating the doped holes, and that the MR is associated with the field-induced topological ordering of the stripes.

cond-mat.supr-con↗

arXiv2000

Carrier concentrations in Bi_{2}Sr_{2-z}La_{z}CuO_{6+δ} single crystals and their relation to Hall coefficient and thermopower

We measured the thermopower S and the Hall coefficients R_H of Bi_{2}Sr_{2-z}La_{z}CuO_{6+δ} (BSLCO) single crystals in a wide doping range, in an effort to identify the actual hole concentrations per Cu, p, in this system. It is found that the "universal" relation between the room-temperature thermopower and T_c does not hold in the BSLCO system. Instead, comparison of the temperature-dependent R_H data with other cuprate systems is used as a tool to identify the actual p value. To justify this approach, we compare normalized R_H(T) data of BSLCO, La_{2-x}Sr_{x}CuO_{4} (LSCO), YBa_{2}Cu_{3}O_{y}, and Tl_{2}Ba_{2}CuO_{6+δ}, and demonstrate that the R_H(T) data of the LSCO system can be used as a template for the estimation of p. The resulting phase diagram of p vs T_c for BSLCO suggests that T_c is anomalously suppressed in the underdoped samples, becoming zero at around p ~ 0.10, while the optimum T_c is achieved at p ~ 0.16 as expected.

cond-mat.supr-con↗

arXiv1999

The Hall effect in Zn-doped YBa_{2}Cu_{3}O_{7-δ} revisited: Hall angle and the pseudogap

The temperature dependence of the Hall coefficient is measured with a high accuracy in a series of Zn-doped YBCO_{6.78} crystals with the Zn concentration from 0% to 1.3%. We found that the cotangent of the Hall angle, cot(θ_H), starts to deviate upward from the T^2 dependence below T_0 (which is roughly 130 K), regardless of the Zn concentration. We discuss that this deviation is caused by the pseudogap; the direction of the deviation and its insensitivity to the Zn doping suggest that the pseudogap affects cot(θ_H) through a change in the effective mass, rather than through a change in the Hall scattering rate.

cond-mat.supr-con↗

arXiv1999

Manifestations of the Charged Stripes in the Magnetoresistance of Heavily Underdoped YBa_2Cu_3O_{6+x}

We present a study of the in-plane and out-of-plane magnetoresistance (MR) in heavily-underdoped, antiferromagnetic YBa_2Cu_3O_{6+x}, which reveals a variety of striking features. The in-plane MR demonstrates a "d-wave"-like anisotropy upon rotating the magnetic field H within the ab plane. With decreasing temperature below 20-25 K the system acquires memory: exposing a crystal to the magnetic field results in a persistent in-plane resistivity anisotropy. The overall features can be explained by assuming that the CuO_2 planes contain a developed array of stripes accommodating the doped holes, and that the MR is associated with the field-induced topological ordering of the stripes.

cond-mat.supr-con↗

arXiv1999

Magnetoresistance Anomalies in Antiferromagnetic YBa_2Cu_3O_{6+x}: Fingerprints of Charged Stripes

We report novel features in the in-plane magnetoresistance (MR) of heavily underdoped YBa_2Cu_3O_{6+x}, which unveil a developed ``charged stripe'' structure in this system. One of the striking features is an anisotropy of the MR with a "d-wave" symmetry upon rotating the magnetic field H within the ab plane, which is caused by the rotation of the stripes with the external field. With decreasing temperature, a hysteresis shows up below ~20 K in the MR curve as a function of H and finally below 10 K the magnetic-field application produces a persistent change in the resistivity. This "memory effect" is caused by the freezing of the directionally-ordered stripes.

cond-mat.supr-con↗

arXiv1999

Unusual Charge Localization in Zn-doped and Heavily Underdoped YBa2Cu3O7-d at Low Temperatures

The in-plane normal-state resistivity of Zn-doped YBa2Cu3O7-d and heavily underdoped pure YBCO single crystals is measured down to low temperatures under magnetic fields up to 18 T. We found that the temperature dependence of the normal-state rho_ab does not obey log(1/T) and tends not to diverge in the low temperature limit. The result suggests that the ``ground state'' of the normal state of YBCO is metallic.

cond-mat.supr-con↗

arXiv1999

Magnetoresistance in Heavily Underdoped YBa_2Cu_3O_{6+x}: Antiferromagnetic Correlations and Normal-State Transport

We report on a contrasting behavior of the in-plane and out-of-plane magnetoresistance (MR) in heavily underdoped antiferromagnetic (AF) YBa_2Cu_3O_{6+x} (x<0.37). The out-of-plane MR (I//c) is positive over most of the temperature range and shows a sharp increase, by about two orders of magnitude, upon cooling through the Neel temperature T_N. A contribution associated with the AF correlations is found to dominate the out-of-plane MR behavior for H//c from far above T_N, pointing to the key role of spin fluctuations in the out-of-plane transport. In contrast, the transverse in-plane MR (I//a(b);H//c) appears to be small and smooth through T_N, implying that the development of the AF order has little effect on the in-plane resistivity.

cond-mat.supr-con↗

arXiv1999

Specific-heat evidence for strong electron correlations in the thermoelectric material (Na,Ca)Co_{2}O_{4}

The specific heat of (Na,Ca)Co_{2}O_{4} is measured at low-temperatures to determine the magnitude of the electronic specific-heat coefficient γ, in an attempt to gain an insight into the origin of the unusually large thermoelectric power of this compound. It is found that γis as large as 48 mJ/molK^2, which is an order of magnitude larger than γof simple metals. This indicates that (Na,Ca)Co_{2}O_{4} is a strongly-correlated electron system, where the strong correlation probably comes from the low-dimensionality and the frustrated spin structure. We discuss how the large thermopower and its dependence on Ca doping can be understood with the strong electron correlations.

cond-mat.str-el↗

arXiv1998

Charge Localization from Local Destruction of Antiferromagnetic Correlation in Zn-doped YBa2Cu3O7-d

The in-plane normal-state resistivity of Zn-doped YBa2Cu3O7-d single crystals is measured down to low temperatures by suppressing superconductivity with magnetic fields up to 18 T. Substitution of Cu with Zn in the CuO2 planes is found to induce carrier localization at low temperatures in "clean" samples with kF l > 5, where the mean free path l is larger than the electron wave length and thus localization is not normally expected. The destruction of the local antiferromagnetic correlation among Cu spins by Zn is discussed to be the possible origin of this unusual charge localization.

cond-mat.supr-con↗