Publications (251)895.94 Total impact
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ABSTRACT: The energy spectrum of incommensurate spinfluctuations in superconducting Srdoped La2CuO4 has beenstudied by inelastic neutron scattering experiments. An energy gap inthe spin excitation spectrum is observed in the superconducting stateof optimally doped (x=0.15) and slightly overdoped (x=0.18)samples. At temperatures well below T c, the incommensurate peaksdiminish rapidly in intensity with decreasing energy below ˜8 meV andmerge into the background below ˜3.5 meV for x=0.15 and ˜4.5 meVfor x=0.18. Forboth samples, the energy spectrum of the qintegrated spin susceptibility,chi''(omega), exhibits an enhancement around 7 meV, which is caused by abroadeningin the momentum width of the incommensurate peaks. The gaplike structure inthe energy spectrum and the enhancement of chi''(omega)survive at T c. On the other hand, for both underdoped(x=0.10) and heavily overdoped(x=0.25) samples there is neither a clear energy gap nor anenhancement of chi'' (omega) below T c.Journal of the Physical Society of Japan 04/2000; 69(4):11701176. DOI:10.1143/JPSJ.69.1170 · 1.59 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The optical absorption spectrum from 0.1 to 1.5 eV photon energy ispresented for extremely lightly oxygen doped crystals of La2CuO4. Crystals grown by two different methods have different low temperaturespectra and different temperature dependencies. However, at roomtemperature the spectra are nearly identical, suggesting that theyresult from the optical absorption of free isolated holes in thetwo dimensional antiferromagnetic CuO2 square lattice.Journal of the Physical Society of Japan 03/2000; 69(3):749753. DOI:10.1143/JPSJ.69.749 · 1.59 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present results for dark current measurements on large closepacked arrays of CdSe nanocrystals. The nanocrystals are chemically synthesized in solution and then deposited between gold electrodes onto a silicon substrate. In response to an applied voltage step, we observe a powerlaw decay of the current over five orders of magnitude in time and four orders of magnitude in current; we do not observe steadystate dark current for fields up to 10^6 V/cm and times out to 5 × 10^4 seconds. We see clear evidence for injection of electrons into the array, but do not observe hole injection. The number of electrons in the array can be increased by cooling the sample with a positive gate voltage. However, as the number of electrons is increased, the magnitude of the current transient decreases substantially. Preliminary results suggest that the array is much more conductive at very short times ( < 1ms after the voltage step). We believe our results can be understood in the context of the Coulomb glass model, where transport is dominated by the longrange electrostatic interactions between electrons at different sites.  [Show abstract] [Hide abstract]
ABSTRACT: A spinglass compound, La1.98Sr0.02CuO4, shows quasithreedimensional magnetic ordering below ~40 K. A remarkable feature is that the magnetic correlation length along the orthorhombic aortho axis is much longer than that along the bortho axis, suggesting that the spin structure is closely related to the diagonal stripe structure. The spinglass state can be expressed as a random freezing of quasithreedimensional spin clusters with anisotropic spin correlations (ξ'a~160 Å , ξ'b~25 Å , and ξ'c~4.7 Å at 1.6 K). The new magnetic state is important as an intermediate phase between the threedimensional antiferromagnetic ordered phase in La2CuO4 and the incommensurate phase in La1.95Sr0.05CuO4 in which the positions of the incommensurate peaks are rotated by 45° in reciprocal space about (π,π) from those observed in the superconducting La2CuO4 compounds.Physical Review B 02/2000; 61(6):43264333. DOI:10.1103/PhysRevB.61.4326 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present an experimental study of the uctuations of Coulomb blockade peak positions of a quantum dot. The dot is deÿned by patterning the twodimensional electron gas of a silicon MOSFET structure using stacked gates. The ratio of charging energy to singleparticle energy is considerably larger than in comparable GaAs=AlGaAs quantum dots. The statistical distribution of the conductance peak spacings in the Coulomb blockade regime was found to be unimodal and does not follow the Wigner surmise. The uctuations of the spacings are much larger than the typical singleparticle level spacing and thus clearly contradict the expectation of random matrix theory. Measurements of the natural line width of a set of several adjacent conductance peaks suggest that all of the peaks in the set are dominated by electrons being transported through a singlebroad energy level. ? 2000 Elsevier Science B.V. All rights reserved.Physica E Lowdimensional Systems and Nanostructures 02/2000; 6(14):382387. DOI:10.1016/S13869477(99)001861 · 2.00 Impact Factor 
Article: Direct observation of a onedimensional static spin modulation in insulating La1.95Sr0.05CuO4
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ABSTRACT: We report the results of an extensive elastic neutronscattering study of the incommensurate (IC) static spin correlations in La1.95Sr0.05CuO4, which is an insulating spin glass at low temperatures. Recent work by Wakimoto et al. has revealed the presence of new twodimensional satellite peaks in La1.95Sr0.05CuO4 at positions rotated by ∼45° in reciprocal space from those found in superconducting samples. The present neutronscattering experiments on the same x=0.05 crystal employ a narrower instrumental Q resolution and thereby have revealed that the crystal has only two, rather than four, orthorhombic twins at low temperatures with relative populations of 2:1. This has made possible the precise characterization of the IC elastic peaks around (1,0,0) and (0,1,0) (orthorhombic notation) in each domain separately. We find that, in a single twin, only two satellites are observed at (1,±0.064,L)ortho and (0,1±0.064,L)ortho, that is, the modulation vector is only along the orthorhombic b* axis. This demonstrates unambiguously that La1.95Sr0.05CuO4 has a onedimensional rather than twodimensional static diagonal spin modulation at low temperatures, consistent with certain stripe models. From the L dependence we conclude that the spin correlations are predominantly two dimensional. We have also reexamined the x=0.04 crystal that previously was reported to show a single commensurate peak. By mounting the sample in the (H,K,0) zone, we have discovered that the x=0.04 sample in fact has the same IC structure as the x=0.05 sample. The incommensurability parameter δ for x=0.04 and 0.05, where δ is the distance from (1/2,1/2) in tetragonal reciprocal lattice units, follows the linear relation δ≃x. These results demonstrate that the insulator to superconductor transition in the underdoped regime (0.05<~x<~0.06) in La2xSrxCuO4 is coincident with a transition from diagonal to collinear static stripes at low temperatures thereby evincing the intimate coupling between the onedimensional spin density modulation and the superconductivity.Physical Review B 02/2000; 61(5):36993706. DOI:10.1103/PhysRevB.61.3699 · 3.74 Impact Factor 
Article: ARPES features of the AF insulators Sr 2CuO 2Cl 2 and Ca 2CuO 2Cl 2 close to the AF zone boundary
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ABSTRACT: ARPES studies on Sr2CuO2Cl2 and Ca2CuO2Cl2 are presented. The integrated spectral weight of the low energy feature shows a drop across the AF zone boundary, suggesting a remnant Fermi surface. Along the remnant Fermi surface the dispersion fits the functional from of cos(kxa)  cos(kya) remarkably well. However, additional experiments with improved angular resolution demonstrate that the dispersion near (π/2, π/2) does not contain a cusp as would be expected from a simple dwave theory predicting the above functional form. The rounded node region and the total dispersion of approximately 350meV is shown to be independent of photon energy. 
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ABSTRACT: The energy spectrum of incommensurate spin fluctuations in superconducting Srdoped La2CuO4 has been studied by inelastic neutron scattering experiments. An energy gap in the spin excitation spectrum is observed in the superconducting state of optimally doped (x = 0.15) and slightly overdoped (x = 0.18) samples. At temperatures well below Tc, the incommensurate peaks diminish rapidly in intensity with decreasing energy below ∼8 meV and merge into the background below ∼3.5meV for x = 0.15 and ∼4.5meV for x = 0.18. For both samples, the energy spectrum of the qintegrated spin susceptibility. χx″(ω), exhibits an enhancement around 7 meV, which is caused by a broadening in the momentum width of the incommensurate peaks. The gaplike structure in the energy spectrum and the enhancement of χ″(ω) survive at Tc. On the other hand, for both underdoped (x = 0.10) and heavily overdoped (x = 0.25) samples there is neither a clear energy gap nor an enhancement of χ″(ω) below Tc.Journal of the Physical Society of Japan 12/1999; 69(4). · 1.59 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We have observed asymmetric Fano resonances in the conductance of a single electron transistor resulting from interference between a resonant and a nonresonant path through the system. The resonant component shows all the features typical of quantum dots, but the origin of the nonresonant path is unclear. A unique feature of this experimental system, compared to others that show Fano line shapes, is that changing the voltages on various gates allows one to alter the interference between the two paths. Comment: 8 pages, 6 figures. Submitted to PRBPhysical review. B, Condensed matter 12/1999; 62(3). DOI:10.1103/PhysRevB.62.2188 · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Recent neutronscattering experiments on La2−xSrxCuO4 single crystals by Wakimoto et al. (unpublished) have revealed that elastic magnetic peaks appear at low temperatures in both insulating (x=0.02–0.05) and superconducting (x=0.06) samples. We have carried out further investigations particularly on the elastic incommensurate peaks for x=0.06, and found that the integrated intensity drastically changes across the low temperature insulator–superconductor boundary; the intensity of x=0.06 is four times smaller than that of x=0.05, whereas the intensity in the insulating region stays constant.Journal of Physics and Chemistry of Solids 09/1999; 60(89):10791081. DOI:10.1016/S00223697(99)000529 · 1.85 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Elastic magnetic peaks around (π,π) have been studied in La2−xSrxCuO4 (LSCO) by neutron scattering. Recently in superconducting LSCO of x=0.12, we found that the incommensurate elastic peak (ICpeak) appears around 31K which is very close to Tc. The line width reaches almost resolution limit, which indicates that the static magnetic correlation length exceeds 200Å−1 isotropically in the CuO2 planes. Furthermore recently in the course of studying the ICpeak, we found the new experimental condition where the signals can be observed more clearly. The signaltonoise ratio (S/N) is much more improved (∼1.4) than that at the previous experiment (∼0.6).Journal of Physics and Chemistry of Solids 09/1999; 60(8):10671070. DOI:10.1016/S00223697(99)000499 · 1.85 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report neutronscattering measurements of spindensity wave order within the superconducting state of a single crystal of predominately stage4 La{sub 2}CuO{sub 4+y} with a T{sub c} (onset) of 42 K. The lowtemperature elastic magnetic scattering is incommensurate with the lattice and is characterized by longrange order in the copperoxide plane with the spin direction identical to that in the insulator. Between neighboring planes, the spins exhibit shortrange correlations with a stacking arrangement reminiscent of that in the undoped antiferromagnetic insulator. The elastic magnetic peak intensity appears at the same temperature within the errors as the superconductivity, suggesting that the two phenomena are strongly correlated. These observations directly reveal the persistent influence of the antiferromagnetic order as the doping level increases from the insulator to the superconductor. In addition, our results confirm that spindensity wave order for incommensurabilities near 1/8 is a robust feature of the La{sub 2}CuO{sub 4}based superconductors. {copyright} {ital 1999} {ital The American Physical Society}Physical review. B, Condensed matter 08/1999; 60(5). DOI:10.1103/PhysRevB.60.3643 · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Neutronscattering experiments have been performed on lightly doped La{sub 2x}SrâCuOâ single crystals in both the insulating (x=0.03,0.04,0.05) and superconducting (x=0.06) regions. Elastic magnetic peaks are observed at low temperatures in all samples with the maximum peak linewidth occurring at the critical concentration x{sub c}=0.05. Incommensurate peaks are observed only at x=0.05, the positions of which are rotated by 45Â° in reciprocal space about (Ï,Ï) from those observed for xâ¥0.06 in the superconducting phase. {copyright} {ital 1999} {ital The American Physical Society}Physical review. B, Condensed matter 07/1999; 60(2). DOI:10.1103/PhysRevB.60.R769 · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Sr2Cu3O4Cl2 has CuI and CuII subsystems, forming interpenetrating S = 1/2 square lattice Heisenberg antiferromagnets. The classical ground state is degenerate, due to frustration of the intersubsystem interactions. Magnetic neutron scattering experiments show that quantum fluctuations cause a two dimensional Ising ordering of the CuII's, lifting the degeneracy, and a dramatic increase of the CuI outofplane spinwave gap, unique for order out of disorder. The spinwave energies are quantitatively predicted by calculations which include quantum fluctuations.Physical Review Letters 07/1999; 83(4):852855. DOI:10.1103/PhysRevLett.83.852 · 7.51 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We have carried out a neutronscattering study of the instantaneous spinspin correlations in La2CuO4 (TN=325 K) over the temperature range 337824 K. Incident neutron energies varying from 14.7115 meV have been employed in order to guarantee that the energy integration is carried out properly. The results so obtained for the spincorrelation length as a function of temperature when expressed in reduced units agree quantitatively both with previous results for the twodimensional (2D) tetragonal material Sr2CuO2Cl2 and with quantum Monte Carlo results for the nearestneighbor square lattice S=12 Heisenberg model. All of the experimental and numerical results for the correlation length are well described without any adjustable parameters by the behavior predicted for the quantum nonlinear sigma model in the lowtemperature renormalized classical regime. The amplitude, on the other hand, deviates subtly from the predicted lowtemperature behavior. These results are discussed in the context of recent theory for the 2D quantum Heisenberg model.Physical Review B 06/1999; 59(21):127604313794. DOI:10.1103/PhysRevB.59.13788 · 3.74 Impact Factor 
Article: Fielddependent antiferromagnetism and ferromagnetism of the two copper sublattices in Sr2Cu3O4Cl2
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ABSTRACT: The Cu3O4 layer in Sr2Cu3O4Cl2 is a variant of the square CuO2 lattice of the hightemperature superconductors, in which the center of every second plaquette contains an extra Cu2+ ion. The ions that make up the conventional CuO2 network, called CuI, have CuICuI exchange energy ~130 meV, and order antiferromagnetically at about 380 K; the CuIICuII exchange is only ~10 meV, and the CuII's order at ~40 K. A study is reported here of the dependence of the magnetization on field, temperature, and crystallographic orientation for this interesting system. We show that the small permanent ferromagnetic moment, that appears when the CuI spins order, and the unusual spin rotation transitions seen most clearly for one particular direction of the magnetic field, are the result of several small bonddependent anisotropic terms in the spin Hamiltonian that are revealed because of the frustration of the isotropic Heisenberg interaction between CuI and CuII spins. These include a term which favors collinearity of the CuI and CuII spins, which originates from quantum fluctuations, and also the pseudodipolar interaction. Some of these small interactions also come into play in other lamellar cuprates, connected with the highTc superconductivity materials, and in many spinchain and spinladder compounds.Physical review. B, Condensed matter 06/1999; 59(22):1470214711. DOI:10.1103/PhysRevB.59.14702 · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The Cu3O4 layer in Sr2Cu3O4Cl2 is a variant of the square CuO2 lattice of the hightemperature superconductors, in which the center of every second plaquette contains an extra Cu2+ ion. The ions that make up the conventional CuO2 network, called CuI, have CuICuI exchange energy ≈130meV, and order antiferromagnetically at about 380 K; the CuIICuII exchange is only ≈10meV, and the CuII’s order at ≈40K. A study is reported here of the dependence of the magnetization on field, temperature, and crystallographic orientation for this interesting system. We show that the small permanent ferromagnetic moment, that appears when the CuI spins order, and the unusual spin rotation transitions seen most clearly for one particular direction of the magnetic field, are the result of several small bonddependent anisotropic terms in the spin Hamiltonian that are revealed because of the frustration of the isotropic Heisenberg interaction between CuI and CuII spins. These include a term which favors collinearity of the CuI and CuII spins, which originates from quantum fluctuations, and also the pseudodipolar interaction. Some of these small interactions also come into play in other lamellar cuprates, connected with the highTc superconductivity materials, and in many spinchain and spinladder compounds.  [Show abstract] [Hide abstract]
ABSTRACT: We report a synchrotron xrayscattering study of the magnetoresistive manganite La0.875Sr0.125MnO3. At low temperatures, this material undergoes an xrayinduced structural transition at which charge ordering of Mn3+ and Mn4+ ions characteristic to the lowtemperature state of this compound is destroyed. The transition is persistent but the chargeordered state can be restored by heating above the chargeordering transition temperature and subsequently cooling. The chargeordering diffraction peaks, which are broadened at all temperatures, broaden more upon xray irradiation, indicating the finite correlation length of the chargeordered state. Together with the recent reports on xrayinduced transitions in Pr1xCaxMnO3, our results demonstrate that the photoinduced structural change is a common property of the chargeordered perovskite manganites.Physical review. B, Condensed matter 03/1999; 59(10). DOI:10.1103/PhysRevB.59.R6581 · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report a quantum Monte Carlo study of the thermodynamic properties of arrays of spin ladders with various widths ($n$), coupled via a weak interladder exchange coupling $\alpha J$, where $J$ is the intraladder coupling both along and between the chains. This coupled ladder system serves as a simplified model for the magnetism of presumed ordered spin and charge stripes in the twodimensional CuO$_2$ planes of holedoped copper oxides. Our results for $n=3$ with weak interladder coupling $\alpha=0.05$, estimated from the $tt't''J$ model, show good agreement with the ordering temperature of the recently observed spin density wave condensation in La$_2$CuO$_{4+y}$. We show that there exists a quantum critical point at $\alpha_c \simeq 0.07$ for $n=4$, and determine the phase diagram. Our data at this quantum critical point agree quantitatively with the universal scaling predicted by the quantum nonlinear $\sigma$ model. We also report results on random mixtures of $n=2$ and $n=3$ ladders, which correspond to the doping region near but above 1/8. Our study on the magnetic static structure factor reveals a saturation of the incommensurability of the spin correlations around 1/8, while the incommensurability of the charge stripes grows linearly with hole concentration. The implications of this result for the interpretation of neutron scattering experiments on the dynamic spin fluctuations in La$_{2x}$Sr$_x$CuO$_4$ are discussed. Comment: 12 pages, LaTeX, 11 eps figuresPhysical review. B, Condensed matter 02/1999; 60(5). DOI:10.1103/PhysRevB.60.3294 · 3.66 Impact Factor
Publication Stats
16k  Citations  
895.94  Total Impact Points  
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Institutions

19822012

Massachusetts Institute of Technology
 • Department of Physics
 • Department of Materials Science and Engineering
 • Research Laboratory of Electronics
Cambridge, Massachusetts, United States


2002

Columbia University
 Department of Physics
New York, New York, United States


2001

Rutgers, The State University of New Jersey
НьюБрансуик, New Jersey, United States


19892001

Tel Aviv University
 Department of Physics and Astronomy
Tell Afif, Tel Aviv, Israel


1988

Nippon Telegraph and Telephone
Edo, Tōkyō, Japan 
Boston University
 Center for Polymer Studies
Boston, Massachusetts, United States
