M. A. Kastner

Massachusetts Institute of Technology, Cambridge, MA, United States

Are you M. A. Kastner?

Claim your profile

Publications (323)1038.82 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have measured the specific heat and magnetization versus temperature in a single crystal sample of superconducting La2CuO4.11 and in a sample of the same material after removing the excess oxygen, in magnetic fields up to 15 T. Using the deoxygenated sample to subtract the phonon contribution, we find a broad peak in the specific heat, centered at 50 K. This excess specific heat is attributed to fluctuations of the Cu spins possibly enhanced by an interplay with the charge degrees of freedom, and appears to be independent of magnetic field, up to 15 T. Near the superconducting transition Tc(H=0)=43K, we find a sharp feature that is strongly suppressed when the magnetic field is applied parallel to the crystallographic c axis. A model for three-dimensional vortex fluctuations is used to scale magnetization measured at several magnetic fields. When the magnetic field is applied perpendicular to the c axis, the only observed effect is a slight shift in the superconducting transition temperature.
    Physical Review B 09/2003; 69(17). · 3.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report neutron scattering measurements of the structure and magnetism of stage-4 La_2CuO_{4+y} with T_c ~42 K. Our diffraction results on a single crystal sample demonstrate that the excess oxygen dopants form a three-dimensional ordered superlattice within the interstitial regions of the crystal. The oxygen superlattice becomes disordered above T ~ 330 K, and a fast rate of cooling can freeze-in the disordered-oxygen state. Hence, by controlling the cooling rate, the degree of dopant disorder in our La_2CuO_{4+y} crystal can be varied. We find that a higher degree of quenched disorder reduces T_c by ~ 5 K relative to the ordered-oxygen state. At the same time, the quenched disorder enhances the spin density wave order in a manner analogous to the effects of an applied magnetic field. Comment: 4 figures included in text; submitted to PRB
    Physical Review B 09/2003; · 3.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A model of transport is proposed to explain power-law current transients and memory phenomena observed in partially ordered arrays of semiconducting nanocrystals. The model describes electron transport by a stationary Lévy process of transmission events and thereby requires no time dependence of system properties. The waiting time distribution with a characteristic long tail gives rise to a nonstationary response in the presence of a voltage pulse. We report on noise measurements that agree well with the predicted non-Poissonian fluctuations in current, and discuss possible mechanisms leading to this behavior.
    Physical review. B, Condensed matter 08/2003; 72(7). · 3.77 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A new method based on electric-force microscopy is used to image the charge distribution in arrays of CdSe nanocrystal quantum dots as a function of time. The CdSe nanocrystals are deposited on top of microfabricated electrodes and self-assemble into artificial solids. These nanocrystal solids can be charged with electrons by applying voltages to nearby metallic electrodes. We present the time evolution of the charge distribution as the array of nanocrystals is charged and discharged at room temperature. Excellent agreement is obtained between the data and a simple diffusion model for discharging of the nanocrystals at times t > 1 min after the voltage on the electrode has been turned off. From fits to the data, we find that the diffusion constant is D 3× 10-3 fracmum^2s for the as-deposited nanocrystal arrays. This corresponds to electron tunneling rates of 1ms.
    03/2003;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have measured the enhancement of the static incommensurate spin-density wave (SDW) order by an applied magnetic field in stage-4 and stage-6 samples of superconducting La2CuO4+y. We show that the stage-6 La2CuO4+y (Tc=32K) forms static long-range SDW order with the same wave vector as that in the previously studied stage-4 material. We have measured the field dependence of the SDW magnetic Bragg peaks in both stage-4 and stage-6 materials at fields up to 14.5 T. A recent model of competing SDW order and superconductivity describes these data well.
    Physical Review B 09/2002; 67(5). · 3.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report sharp peaks in the differential conductance of a single-electron transistor (SET) at low temperature, for gate voltages at which charge fluctuations are suppressed. For odd numbers of electrons we observe the expected Kondo peak at zero bias. For even numbers of electrons we generally observe Kondo-like features corresponding to excited states. For the latter, the excitation energy often decreases with gate voltage until a new zero-bias Kondo peak results. We ascribe this behavior to a singlet-triplet transition in zero magnetic field driven by the change of shape of the potential that confines the electrons in the SET. Comment: 4 p., 4 fig., 5 new ref. Rewrote 1st paragr. on p. 4. Revised author list. More detailed fit results on page 3. A plotting error in the horizontal axis of Fig. 1b and 3 was corrected, and so were the numbers in the text read from those fig. Fig. 4 was modified with a better temperature calibration (changes are a few percent). The inset of this fig. was removed as it is unnecessary here. Added remarks in the conclusion. Typos are corrected
    Physical Review B 08/2002; · 3.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Transport properties of artificial solids composed of colloidal CdSe nanocrystals (NCs) are studied from 6 K to 250 K, before and after annealing. Annealing results in greatly enhanced dark and photocurrent in NC solids, while transmission electron microscopy (TEM) micrographs show that the inter-dot separation decreases. The increased current can be attributed to the enhancement of inter-dot tunneling caused by the decreased separation between NCs and by chemical changes in their organic cap. In addition, the absorption spectra of annealed solids are slightly red-shifted and broadened. These red-shifts may result from the change of the dielectric environment around the NCs. Our measurements also indicate that Coulomb interactions between charges on neighboring NCs play an important role in the tunneling current. Comment: 24 pages,4 figures, 1 table
    Journal of Applied Physics 04/2002; · 2.21 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present results for electronic transport measurements on large three-dimensional arrays of CdSe nanocrystals. In response to a step in the applied voltage, we observe a power-law decay of the current over five orders of magnitude in time. Furthermore, we observe no steady-state dark current for fields up to 10^6 V/cm and times as long as 2x10^4 seconds. Although the power-law form of the decay is quite general, there are quantitative variations with temperature, applied field, sample history, and the material parameters of the array. Despite evidence that the charge injected into the film during the measurement causes the decay of current, we find field-scaling of the current at all times. The observation of extremely long-lived current transients suggests the importance of long-range Coulomb interactions between charges on different nanocrystals. Comment: 11 pages, 10 figures
    Physical Review B 04/2002; · 3.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Temperature-dependent angle-resolved photoemission experiments were performed on overdoped single layer Bi 2 Sr 2 CuO 6 (T c 4 K) and on the parent compounds Sr 2 CuO 2 Cl 2 and Ca 2 CuO 2 Cl 2 . Contrary to the more conventional results obtained on Bi 2 Sr 2 CuO 6 , the parent compounds show strong temperature depen-dence over a wide energy range that is two orders of magnitude larger than the temperature scale involved. The doping dependence strongly suggests that the anomalous temperature dependence has its origin in the novel many-body physics in the Mott insulators. We consider the observed temperature dependence using a heuristic model with multiple initial and final states in the photoemission process. The nature of the multiple states and broad photoemission line shape in parent compounds is also given.
    Physical review. B, Condensed matter 04/2002; 65(17). · 3.77 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: One of the major themes in the study of the CMR manganites and high-Tc superconductors is the role of electronic phase separation. Here we report on Bi_2Sr_2CoO_6+delta, the Co analogue of the high-Tc superconductor Bi_2Sr_2CuO_6+delta , which appears to exhibit electronic phase separation when delta is between 0.25 and 0.5. Bi_2Sr_2CoO_6+delta evolves from an antiferromagnetic (AF) insulator at delta ~ 0.5 to an unusual ferromagnetic (FM) insulator at delta ~ 0.25. Single crystal neutron scattering, magnetization and resistivity measurements have been used to characterize the evolution of the magnetic and transport properties between these two doping limits. For crystals with 0.25 < delta < 0.5, both FM and AF Bragg peaks are seen with neutrons above a critical field, H_c. Field-dependent neutron scattering measurements confirm that the FM peaks result from FM domains which coexist with AF domains and have an overt moment above H_c. The suppression of Neel order and accompanying increase in the volume of the FM domains with decreasing delta is measured for samples with 0.25 < delta < 0.5. We discuss this behavior in the context of coexistence of the electronically distinct delta ~ .25 FM phase and delta ~ 0.5 AF phase.
    03/2002;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We study the line shape of the Kondo zero-bias anomaly in the differential conductance of a small ( 100 nm in diameter) semiconductor quantum dot. The number of electrons on the dot can be changed by varying the voltage VG on a nearby gate electrode. When the coupling of the dot to the leads is weak, Kondo peaks with a full width at half maximum (FWHM) of less than 20 microvolts can be observed. The FWHM decreases as VG is tuned away from the location of the Coulomb blockade peak in the linear conductance. Furthermore, the FWHM is a sensitive function of temperature T for T
    03/2002;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Large arrays of artificial atoms in which particle size, inter-particle separation and chemical composition are controlled can be realized from chemically synthesized nanocrystals. We show that electronic transport properties can be tuned by annealing, and present the I-V characteristics of artificial solids composed of colloidal CdSe nanocrystals (NCs) or dots from 6 K to 400 K. Annealing of the artificial solids at moderate temperature (300 C) results in greatly enhanced dark and photocurrent. Transmission electron microscopy (TEM) and absorption data provide evidence that dots remain distinct after annealing. Absorption spectra of annealed solids broaden and undergo a red shift compared to the original solids. TEM data suggests that the inter-dot separation is decreased upon annealing, but this decrease cannot account for the observed changes in transport properties. The observed changes are due to the chemical transformation of the organic caps between the dots, which lowers the barrier for electron tunneling.
    03/2002;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report the results of elastic neutron scattering, transport, NMR and NQR measurements on oxygen-doped La_2CuO_4+y with a superconducting (SC) transition temperature Tc = 42 K. Static incommensurate spin-density wave (SDW) order appears in this material at a temperature which coincides with T_c. The SDW order manifests itself as Bragg peaks at incommensurate positions near the antiferromagnetic zone center. The peak widths, which are resolution limited, demonstrate long-range order of the static SDW in our crystals. An applied magnetic field, although much smaller than H_c2(0), reduces Tc while leading to a significant increase of the SDW Bragg peaks intensity. At 9 T the measured peak intensities increase by up to 100%. This behavior suggests competition between the SC and SDW order parameters. We use NMR and NQR measurements to demonstrate that our samples are as homogeneous as the best samples of La_2-xSr_xCuO4 with similar doping levels. The measured field dependence of the SDW peak intensity excludes a purely magnetic mechanism for the enhancement. We discuss possible mechanisms for the field dependence, and the coexistence and competition between the SDW and SC orders.
    03/2002;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper reports muon spin relaxation (MuSR) measurements of two single crystals of the title high-Tc cuprate systems where static incommensurate magnetism and superconductivity coexist. By zero-field MuSR measurements and subsequent analyses with simulations, we show that (1) the maximum ordered Cu moment size (0.36 Bohr magneton) and local spin structure are identical to those in prototypical stripe spin systems with the 1/8 hole concentration; (2) the static magnetism is confined to less than a half of the volume of the sample, and (3) regions with static magnetism form nano-scale islands with the size comparable to the in-plane superconducting coherence length. By transverse-field MuSR measurements, we show that Tc of these systems is related to the superfluid density, in the same way as observed in cuprate systems without static magnetism. We discuss a heuristic model involving percolation of these nanoscale islands with static magnetism as a possible picture to reconcile heterogeneity found by the present MuSR study and long-range spin correlations found by neutron scattering. Comment: 19 pages, 15 figures, submitted to Phys. Rev. B. E-mail: tomo@lorentz.phys.columbia.edu
    02/2002;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report muon spin relaxation (mSR) measurements using single crystals of oxygen-intercalated stage-4 La2CuO4.11 (LCO:4.11) and La1.88Sr0.12CuO4 (LSCO:0.12), in which neutron scattering studies have found incommensurate magnetic Bragg reflections. In both systems, zero-field mSR measurements show muon spin precession below the Neel temp. TN with frequency 3.6 MHz at T -> 0, having a Bessel function line shape, characteristic of spin-d.-wave systems. The amplitude of the oscillating and relaxing signals of these systems is less than half the value expected for systems with static magnetic order in 100% of the vol. Our results are consistent with a simulation of local fields for a heuristic model with (a) incommensurate spin amplitude modulation with the max. ordered Cu moment size of .apprx.0.36mB, (b) static Cu moments on the CuO2 planes forming \"islands\" having typical radius 15-30 .ANG., comparable to the in-plane superconducting coherence length, and (c) the measured vol. fraction of magnetic muon sites Vm increasing progressively with decreasing temp. below TN towards Vm .apprx. 40% for LCO:4.11 and 18% for LSCO:0.12 at T -> 0. These results may be compared with correlation lengths in excess of 600 .ANG. and a long range ordered moment of 0.15 +- 0.05mB measured with neutron scattering techniques. In this paper we discuss a model that reconciles these apparently contradictory results. In transverse magnetic field mSR measurements, sensitive to the in-plane magnetic field penetration depth lab, the results for LCO:4.11 and LSCO:0.12 follow correlations found for underdoped, overdoped and Zn-doped high-Tc cuprate systems in a plot of Tc vs. the superconducting relaxation rate s(T -> 0). This indicates that the vol.-integrated value of ns/m* (superconducting carrier d. / effective mass) is a detg. factor for Tc, not only in high-Tc cuprate systems without static magnetism, but also in the present systems where supercond. coexists with static spin-d.-wave spin order. [on SciFinder (R)]
    Physical Review B 01/2002; 66:014524/1-014524/14. · 3.66 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present results of a study of the magnetism in Bi2Sr2CoO6+δ, the Co analog of the high Tc superconductor Bi2Sr2CuO6+δ. This system evolves from an antiferromagnetic (AF) insulator to an unusual ferromagnetic (FM) insulator as δ is reduced from ∼0.5 to ∼0.25. When δ is close to 0.5, the Co ions have formal oxidation state 3+ and order antiferromagnetically at TN∼250 K. The δ∼0.25 crystal has equal numbers of Co2+ and Co3+ and exhibits FM behavior with a moment ∼1.5μb/Co at 5 T and a Curie temperature Tc ∼100 K. Single crystal neutron scattering (both polarized and unpolarized), magnetization, and resistivity measurements have been used to characterize the evolution of the magnetic and transport properties between these two doping limits. For crystals with 0.25<δ<0.5, both FM and AF Bragg peaks are observed with neutrons, above a critical field Hc. Field-dependent neutron diffraction measurements confirm that the FM peaks result from ferromagnetic domains, which coexist with antiferromagnetic domains, and have a net moment above the critical field. The suppression of Néel order and accompanying increase in the volume of the FM domains with decreasing δ is measured for samples with 0.25<δ<0.5. We discuss this behavior in the context of phase separation resulting in a hole rich, Co3+ AF phase and a hole poor, Co2+–Co3+ FM phase. In addition, the rich phenomenology of the interacting magnetic domains can be explained by mapping to a form of the random field Ising model.
    Physical Review B 01/2002; 66(5). · 3.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report a detailed study, using neutron scattering, transport and magnetization measurements, of the interplay between superconducting (SC) and spin density wave (SDW) orders in La2CuO4+y. Both kinds of order set in below the same critical temperature. However, the SDW order grows with applied magnetic field, whereas SC order is suppressed. Most importantly, the field dependence of the SDW Bragg peak intensity has a cusp at zero field, as predicted by a recent theory of competing SDW and SC orders. This leads us to conclude that there is a repulsive coupling between the two order parameters. The question of whether the two kinds of order coexist or microscopically phase separate is discussed.
    Physical Review B 12/2001; 66(1). · 3.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report a neutron scattering study of the long-wavelength dynamic spin correlations in the model two-dimensional S = 1/2 square lattice Heisenberg antiferromagnets Sr2CuO2Cl2 and Sr2Cu3O4Cl2. The characteristic energy scale, omega(0)(T/J), is determined by measuring the quasielastic peak width in the paramagnetic phase over a wide range of temperature ( 0.2 less similarT/J less similar0.7). The obtained values for omega(0)(T/J) agree quantitatively between the two compounds and also with values deduced from quantum Monte Carlo simulations. The combined data show scaling behavior, omega approximately xi(-z), over the entire temperature range with z = 1.0(1), in agreement with dynamic scaling theory.
    Physical Review Letters 05/2001; 86(14):3144-7. · 7.73 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report single-crystal growth and magnetic susceptibility and neutron diffraction studies of the S=12 quasi-one-dimensional antiferromagnet CaCu2O3. The structure of this material is similar to that of the prototype two-leg spin-ladder compound SrCu2O3. However, the Cu-O-Cu bond angle in the ladder rungs in CaCu2O3 is equal to 123°, and therefore the magnetic interaction along the rung is expected to be much weaker in this material. At high temperatures, the magnetic susceptibility of CaCu2O3 can be decomposed into a contribution from one-dimensional antiferromagnetic chains or finite-size chain segments together with a weak Curie contribution. The intrachain magnetic exchange constant J||, determined from the magnetic susceptibility measurements, is 2000+/-300 K. CaCu2O3 undergoes a Néel transition at TN=25 K with ordering wave vector of [0.429(5), 12, 12]. The magnetic structure is incommensurate in the direction of the frustrated interchain interaction. Weak commensurate magnetic Bragg peaks with the reduced wave vector (12, 12, 12) are also observed below TN. Application of a magnetic field induces a metamagnetic transition at which the incommensurability of the magnetic structure is substantially reduced. Above the transition field, the material possesses only short-range magnetic order, and no well defined temperature-driven transition is observed.
    Physical Review B 04/2001; · 3.66 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The Cu3O4 layer in Sr2Cu3O4Cl2 is a variant of the square CuO2 lattice of the high-temperature superconductors, in which the center of every second plaquette contains an extra Cu2+ ion. Whereas the ordering of the spins in the ground-state and the spin-wave excitations of this frustrated spin system are both well understood, we find peculiar behavior resulting from antiferromagnetic domain walls. Pseudodipolar coupling between the two sets of Cu2+ ions results in a ferromagnetic moment, the direction of which reflects the direction of the antiferromagnetic staggered moment, allowing us to probe the antiferromagnetic domain structure. After an excursion to the high fields (>1 T), as the field is lowered, we observe the growth of domains with ferromagnetic moment perpendicular to the field. This gives rise to a finite domain wall susceptibility at small fields, which diverges near 100 K, indicating a phase transition. We also find that the shape of the sample influences the domain-wall behavior.
    Phys. Rev. B. 03/2001; 63(13).

Publication Stats

12k Citations
1,038.82 Total Impact Points

Institutions

  • 1978–2012
    • Massachusetts Institute of Technology
      • • Department of Physics
      • • Department of Materials Science and Engineering
      • • Research Laboratory of Electronics
      Cambridge, MA, United States
  • 2001
    • Harvard University
      • Department of Physics
      Boston, MA, United States
  • 1989–2001
    • Tel Aviv University
      • Department of Physics and Astronomy
      Tell Afif, Tel Aviv, Israel
  • 1992
    • National Institute of Standards and Technology
      • NIST Center for Neutron Research
      Gaithersburg, MD, United States
  • 1988
    • Nippon Telegraph and Telephone
      Edo, Tōkyō, Japan