Browsing by Author "KU, HC"

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ANISOTROPIC IRREVERSIBILITY LINES FOR C-AXIS ALIGNED (BI, PB)2CA2SR2CU3O10+DELTA POWDERS
(1992-04-15)
SHI, JB
;
CHIOU, BS
;
KUO, PL
;
KU, HC
;
電控工程研究所
Anisotropic irreversibility lines due to thermal fluctuation for quasi-two-dimensional high-T(c) superconductors were observed in c-axis-aligned powders of the (Bi, Pb)2Ca2Sr2Cu3O10+deltaBi(2223) compound with T(c) = 108 K. The anisotropic ratio H(r)(perpendicular-to c)/H(r)(parallel-to c) decreases sharply from 13.6 at 100 K to low values of 3.2 at 80 K and 3.1 at 70 K. The simple 3D-like power law H(r)=a.(1 - T/T(c))n was observed only in the low field region (less-than-or-equal-to 200 G) with n=2.19 for H perpendicular-to c and 2.99 for H parallel-to c. In the higher field region up to 4 kG, the temperature dependence of H(r)(T) lines changes into a 2D-like exponential function H(r) = b.exp (-T/T0) due to the breakdown of the interlayer and/or intralayer coupling of the conduction channel which consists of three Cu-0 planes, with T0 = 14.2 K for H perpendicular-to c and 13.7 K for H parallel-to c. The magnetic susceptibility ratio chi(c)/chi(ab) for this highly anisotropic superconductor in a low applied field of 8 G increases sharply from 9.8 at 5 K to 17.9 near T(c).
ANISOTROPIC IRREVERSIBILITY LINES FOR C-AXIS ALIGNED POWDERS OF THE BI(2223) COMPOUND (BI,PB)2CA2SR2CU3O10+DELTA
(1991-12-01)
SHI, JB
;
KUO, PL
;
CHIOU, BS
;
KU, HC
;
電控工程研究所
Anisotropic irreversibility lines were observed for c-axis aligned powders of the Bi(2223) compound (Bi,Pb)2Ca2Sr2Cu3O10+delta (T(c) = 108 K). The aligned sample was prepared using powders embedded in epoxy and aligned in a 9.4 T applied magnetic field at room temperature. The anisotropic irreversibility lines H(r)(T) were measured with applied field H parallel and perpendicular to the c-axis. The simple power law H(r) = a.(1 - T/T(c))n with n = 2.19 (H perpendicular-to c) and n = 2.99 (H parallel-to c) were observed only in low applied field H less-than-or-equal-to 250 G. For applied field up to 4 kG, H(r)(T) lines were better fitted with the exponential law H(r) = b.exp(-T/t).
ANISOTROPIC MAGNETIC-PROPERTIES OF BI2SR2CUO6+DELTA SINGLE-CRYSTAL
(1992-07-01)
SHI, JB
;
CHIOU, BS
;
HO, JC
;
KU, HC
;
電控工程研究所
Single crystals of Bi2Sr2CuO6+delta were synthesized and characterized by X-ray diffraction. The Bi(2201) sample remains not superconducting or magnetically ordered down to 5 K. With an applied magnetic field perpendicular or parallel to the c-axis, magnetic measurements yielded anisotropic magnetic susceptibilities chi(c)(H perpendicular-to c) and chi(ab)(H parallel-to c). Both can be fitted to the sum of a temperature-independent chi(0) and a Curie-Weiss term C/(T + theta(p)) in the temperature range between 5 and 300 K, with a common Curie constant C = 1.0 x 10(-2) cm3 K/mol and Curie-Weiss paramagnetic intercept theta(p) = + 1.1 K. Since the resulting Curie-Weiss term is the same for both directions, the overall anisotropy DELTA(chi) = chi(c) - chi(ab) = DELTA(chi-0) is also temperature independent. Its magnitude, 3.3 x 10(-5) cm3/mol, however, is lower than the corresponding values for Bi(2212) (2.3 x 10(-4) cm3/mol) and Bi(2223) (5.8 x 10(-4) cm3/mol).
ANISOTROPIC SUPERCONDUCTING PROPERTIES OF ALIGNED (BI,PB)2CAN-1SR2CUNO2N+4+DELTA POWDERS (N = 1, 2, 3) WITH TC = 32, 94, AND 110 K
(1991-06-01)
SHI, JB
;
CHIOU, BS
;
KU, HC
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
The anisotropic superconducting properties of aligned powders embedded in epoxy for bismuth copper oxides with the compositions (Bi1.6Pb0.4)(Sr1.5La0.5)CuO6+delta (n = 1, T(c) = 32 K), Bi2Ca1.2Sr1.8Cu2O8+delta (n = 2, T(c) = 94 K), and (Bi1.85Pb0.15)Ca2.2Sr1.8Cu3O10+delta (n = 3, T(c) = 110 K) are reported. These c-axis-aligned samples were prepared in a 9.4-T applied magnetic field at room temperature The temperature dependence of the anisotropy ratio chi-c(T)/chi-ab(T) in the superconducting state is derived from both zero-field-cooled and field-cooled data using a small applied field H (less than or equal to the lower critical field H(c1)) parallel and perpendicular to the orthorhombic c axis. A highly anisotropic chi-c/chi-ab ratio was observed for all three systems, the maximum value of chi-c/chi-ab = 9.8 at 5 K and 10.8 at 85 K for the 2:2:2:3 compound (Bi1.85Pb0.15)Ca2.2Sr1.8Cu3O10+delta was observed. The field dependence of the anisotropy ratio chi-c(H)/chi-ab(H) for the 2:1:2:2 compound Bi2Ca1.2Sr1.8Cu2O8+delta up to 5 kG is discussed.
ANOMALOUS PR ORDERING IN THE NEW TLBA2PRCU2O7-DELTA COMPOUND
(1992-11-01)
LAI, CC
;
CHIOU, BS
;
CHEN, YY
;
HO, JC
;
KU, HC
Magnetic measurements on the new tetragonal Tl(1212) compound TlBa2PrCu2O7-delta (a = 3.910 angstrom and c = 12.53 angstrom) reveal an anomalously high antiferromagnetic Neel temperature T(N) congruent-to 8 K. This transition temperature is higher than T(N) = 2.2 K for isostructural TlBa2GdCu2O7-delta. Corroborative evidence comes from calorimetric studies, in which a magnetic entropy was obtained DELTAS congruent-to 0.5 Rln 2 between 2 and 10 K, as well as a sizeable linear term in specific heat comparable to those found in heavy-fermion systems. Through structural consideration, these observations can not only be correlated to the almost identical but still puzzling features of PrBa2Cu3O7-y, but also remove some ambiguity in the interpretation of earlier results.
COBALT ORDERING IN LAYERED BI2SR2COO6+DELTA-SINGLE CRYSTAL
(1993-01-15)
SHI, JB
;
HO, JC
;
LEE, TJ
;
CHIOU, BS
;
KU, HC
Insulating Bi2Sr2CoO6+delta is nearly isomorphic with the Bi(2201)-type high-T(c) superconductors. Magnetic and calorimetric studies were carried out on single crystal samples of this layered compound. Based on magnetic measurements, the cobalt ions undergo a canted antiferromapetic ordering at T(N) = 159 K. A low-temperature neutron diffraction study of the magnetic structure is in progress. The molar magnetic susceptibility chi(ab)(H perpendicular-to c) above 200 K can be well fitted by the Curie-Weiss form chi(ab)(T) = chi0+C/(T-theta(p)), with chi0 = 0.56 cm3K/mol, Curie constant C = 3.2 x 10(-2) cm3K/mol and Curie-Weiss intercept theta(p) = 159 K. There is no magnetic specific heat peak, however, prevailing around T(N). This anomalous calorimetric behavior is attributed to the two-dimensional quantum spin fluctuation of Co spins above T(N) in the layered compound as previously observed for Cu spins in (Gd1.85Ce0.15)CuO4-delta and La2CuO4-delta.
COMPARATIVE-STUDIES OF GD-ORDERING IN VARIOUS CUPRATE SYSTEMS
(1994-02-01)
HO, JC
;
WU, CY
;
LAI, CC
;
SHIEH, JH
;
KU, HC
Gd-ordering near 2.2 K in both superconducting and nonsuperconducting GdBa2Cu3O7-y has been well documented. This report summarizes our results from comparative studies on other Gd-containing cuprates (with well-defined Neel temperatures in parentheses) including Bi2Sr2GdCu2O8 (1.6 K), TlBa2GdCu2O7 (2.2 K), TlSr2GdCu2O7 (2.4 K), (Pb.5Cu.5)Sr2GdCu2O7 (2.3 K), Pb2Sr2GdCu3O8 (2.3 K), Gd2CuO4 (6.6 K) and (Gd1.85Ce0.15)CuO4 (5.3 K). The Neel temperature values for the Bi-, Tl-, and Pb-based cuprates remain close to that for PrBa2Cu3O7-y having similar layered structures but somewhat different Gd-Gd distance along the c-axis, whereas the T'(214) compounds become ordered at higher temperatures. Also to be noted is the general occurrence of a broad shoulder below the heat capacity peak, which needs to be taken into account for obtaining the expected magnetic entropy DELTAS = Rln8 associated with the ordering.
CRYSTAL-STRUCTURE AND MAGNETIC-PROPERTIES OF A VANADIUM(IV) PYROPHOSPHATE - RB2V3P4O17
(1990-04-01)
LII, KH
;
WANG, YP
;
CHNEG, CY
;
WANG, SL
;
KU, HC
MAGNETIC AND CALORIMETRIC STUDIES OF GD ORDERING IN TLBA2GDCU2O7-DELTA
(1993-05-01)
CHEN, YY
;
LAI, CC
;
CHIOU, BS
;
HO, JC
;
KU, HC
Antiferromagnetic ordering among Gd3+ ions in the nonsuperconducting compound TiBa2GdCu2O7-delta occurs near 2.2-2.3 K as confirmed by magnetic and calorimetric measurements. This compound is similar in structure to that of superconducting GdBa2Cu3O7-y, but with the Cu-O chains replaced by Tl-O planes, which also exhibits a similar magnetic transition at 2.2-2.3 K. This paper further compares the Neel temperatures and magnetic specific-heat anomalies of various Gd-containing compounds and related systems. Some common features between the compounds are discussed.
MAGNETIC PHASE-DIAGRAM OF 32-K SUPERCONDUCTOR (BI1.6PB0.4)(SR1.5LA0.5)CUO6+DELTA
(1990-08-01)
KU, HC
;
SHI, JB
;
LAI, CC
;
SUN, SJ
;
電控工程研究所
MAGNETIC-BEHAVIOR IN PR-CONTAINING TL-BASED AND PB-BASED CUPRATES
(1994-02-01)
KU, HC
;
LAI, CC
;
SHIEH, JH
;
LIOU, JW
;
WU, CY
Magnetic ordering among Pr ions occurs at a relatively high temperature of 17 K in PrBa2Cu3O7, but is absent in Pr2CuO4 and Bi2Sr2PrCu2O8 above 1.6 K. This work extends the study of magnetic properties to other Pr-containing cuprates including TlBa2PrCu2O7, (Pb.6Cu.4)Sr2PrCu2O7 and Pb2Sr2PrCu3O8. Low temperature calorimetric and SQUID measurements show broad peaks in C and dchi/dT in all cases. The onset temperatures are near 10 K, while the maxima in C as well as minima in dchi/dT prevail at about 8 to 5 K. The heat capacity data provides additional information concerning the entropy associated with the magnetic anomalies and large contributions seemingly linear with temperature.
MAGNETIC-ORDERING OF PR AND CU IN TLBA2PRCU2O7-Y
(1994-05-01)
HSIEH, WT
;
CHANG, KJ
;
LI, WH
;
LEE, KC
;
LYNN, JW
Neutron-scattering measurements have been performed to study the magnetic ordering of Pr and Cu in semiconducting TlBa2PrCu2O7-y. This compound has a nuclear structure which is similar to that of YBa2Cu3O7 but with the CuO-chain layers fully replaced by TlO layers. At low temperatures the magnetic diffraction pattern reveals magnetic Bragg peaks of the {1/2 1/2 1/2} type, which are associated with the antiferromagnetic ordering of the Pr moments. The magnetic structure consists of nearest-neighbor spins along all three crystallographic directions being antiparallel. The spin direction is along the c axis, with a saturated moment of 1.05 mu(B). The temperature dependence of the intensity of these reflections indicates a Neel temperature T(N) almost-equal-to 8 K, which is about half that observed in the related PrBa2Cu3O7 system. A second set of magnetic reflections, of the {1/2 1/2 1} type, are also observed, and these are associated with the Cu magnetic sublattice. This magnetic structure is identical to the ''plane ordering'' observed in the related 1:2:3 system, where the nearest-neighbor Cu spins are again antiparallel. The spin direction is in the tetragonal ab plane, with a saturated ordered moment of 0.59 mu(B). The temperature dependence indicates a Neel temperature of almost-equal-to 370 K for the Cu spins.
MAGNETIC-PROPERTIES OF TLBA2MCU2O7 COMPOUNDS (M=Y AND RARE-EARTH ELEMENTS)
(1994-02-01)
FUJIWARA, K
;
KAWAHARA, N
;
SUZUKI, T
;
KAMAGAI, K
;
LAI, CC
Hyperfine field and quadrupole frequency at the Cu sites have been obtained by analysis of Cu-63/65-NMR spectrum in insulating cuprate TlBa2MCu2O7 (M=Y and rare earth elements). We discuss the effect of rare earth-ion-substitution on magnetic properties of the Cu planer sites.
ORIENTATION-DEPENDENT X-RAY ABSORPTION FINE-STRUCTURE (XAFS) OF THE BI-SR-CA-CU-O SYSTEM
(1992-12-01)
CHOU, CH
;
PONG, WF
;
SHI, JB
;
KU, HC
;
LIN, IN
Using the transmission and fluorescence modes at the National Synchrotron Light Source (NSLS) X-23A2 beam line, we have measured polarized XAFS spectra of the Bi, Pb L3-edges and the Cu K-edge for the (Bi1.85Pb0.15)Ca2.2Sr1.8Cu3O10+delta sample and unpolarized XAFS spectra of the Bi, Pb L3-edges and the Cu K-edge for Bi2Ca1.2Sr1.8Cu2O8+delta, Pb0.2Bi1.9Sr1.9Cu1O6+delta and (PbxBi2-x)Sr2Ca2Cu3Oy (x = 0, 0.2, 0.4) samples. All XAFS spectra were measured at room temperature. The incommensurate modulations in the Bi-O layers are discussed in terms of the XAFS technique. The XAFS spectra of Bi L3-edge do not vary substantially for different sample orientation with respect to the X-ray polarization vector E. There is evidence for the existence within the Bi-Sr-Ca-Cu-O system of Bi atoms having the oxidation number exceeding 3. Pb ions predominantly substitute for Bi ions in the Bi-Sr-Ca-Cu-O system because there is no significant difference between the XANES spectra of the Pb and Bi L3-edges, respectively. The oxidation numbers of substituted Pb in Bi-Sr-Ca-Cu-O system differ much from those of Pb in PbO2 in which the oxidation number of all Pb atoms is 4. In agreement with other investigators, our results on the polarized XANES spectra of the Cu K-edge indicate that a satellite about 7 eV above the main K-XANES peak in both polarizations is a multielectron excitation to the final-state configuration 1s3d9epsilonp1, and that a shoulder on the rising absorption edge, present for many Cu compounds and generally assigned to a shake-down multielectron excitation, is a transition to a final state of the Cu compounds. We have also deduced that the structure of the Cu-O planes in the Bi-Sr-Ca-Cu-O samples is ordered.
STRUCTURE OF THE NEW TLBA2PRCU2O7-X BY RIETVELD ANALYSIS
(1993-12-01)
LEE, TJ
;
FUN, HK
;
WANG, A
;
CHOU, CH
;
LAI, CC
The crystalline structure of new TlBa2PrCu2O7-y was obtained at room temperature from X-ray powder diffraction with CuKalpha radiation using Rietveld analysis. TlBa2PrCu2O7-x is semiconducting at room temperature with a TlBa2CaCu2O7 type (1212) structure (space group P4/mmm) and cell parameters a = 3.91836(3) angstrom, c = 12.5498(2) angstrom. The structure was refined for 15 parameters to R(wp) = 6.89 %, R(p) = 5.09 % for 6323 step intensities and R(b) = 13.24 % for 138 reflections.
STRUCTURES AND ANTIFERROMAGNETIC PR ORDERING MECHANISM OF TL(BA2-XSRX)PRCU2O7-Y COMPOUNDS BY RIETVELD ANALYSIS
(1994-04-10)
FUN, HK
;
YANG, P
;
LAI, CC
;
KU, HC
;
LEE, TJ
The crystalline structures of Tl(Ba2-xSrx)PrCu2O7-x (x=0, 0.4, 0.8, 1.0, 1.2, 1.6, 2) were obtained at room temperature (300 K) and low temperature (100 K) from X-ray powder diffraction with Cu Kalpha radiation using Rietveld analysis. Tl(Ba2-xSrx)PrCu2O7-x has an isotypical structure with TlBa2CaCu2O7(1212) at 300 and 100 K, i.e., the tetragonal system, space group P4/mmm, Z=1. The relationships between the Pr-O(1) bond lengths and the O(1)-Pr-O(1) bond angles with the antiferromagnetic ordering temperature T(n) (Neel temperature) are discussed. The mechanism of the decrease in the antiferromagnetic ordering temperature T(n) as well as the nonsuperconductivity of this system are explained using the Pr(4f) and O(2p) orbital hybridization and the Pr-O(1) bond lengths. The suggested mechanism of Pr-Pr coupling (interplanar Pr-Pr distance) for the Pr ordering by a previous study is inconsistent with our experimental results.
THE CORRELATION BETWEEN SUPERCONDUCTIVITY AND CU-O BOND LENGTH FOR THE SINGLE CU-O LAYER COMPOUNDS WITH THE BI2SR2CUO6+DELTA-TYPE (2021) STRUCTURE
(1991-12-01)
KU, HC
;
SUN, SJ
;
SHI, JB
;
電控工程研究所
;
Institute of Electrical and Control Engineering
The structure and superconductivity in the single Cu-O layer bismuth (2021) system can be stabilized also by the doping of Na1+ ions. Contrary to the La-doped Bi2(Sr,La)2CuO6+delta system with T(c) enhancement up to 35 K. T(c) remains around 10 K for the Na-doped Bi2(Sr2-xNax)CuO6+delta system (0.0 less-than-or-equal-to x less-than-or-equal-to 0.4). The variation of T(c) with the Cu-O bond length r(Cu-0) in the Cu-O layer for the Bi family which consists of the systems (Bi,Pb)2Sr2CuO6+delta, Bi2(Sr,Na)2CuO6+delta, Bi2(Sr,La)2CuO6+delta and (Bi,Pb)2(Sr,La)2CuO6+delta indicates an optimum r(Cu-0) value of 1.906-1.913 angstrom with maximum T(c) of 35 K for these Bi(2021) compounds.
THE OCCURRENCE OF SUPERCONDUCTIVITY IN THE TLBA2CUO5-DELTA-TYPE-(1021) SYSTEM
(1989-06-01)
KU, HC
;
TAI, MF
;
SHI, JB
;
SHIEH, MJ
;
HSU, SW