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Hans P. Jenssen

Bio: Hans P. Jenssen is an academic researcher from Massachusetts Institute of Technology. The author has contributed to research in topics: Barium titanate & Photorefractive effect. The author has an hindex of 23, co-authored 68 publications receiving 2244 citations.


Papers
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TL;DR: Enhancement of the conductivity in the ferromagnetic phase demonstrates a strong connection between the magnetism and charge transport.
Abstract: Measurements of the magnetic moment of antiferromagnetic ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ at high fields reveal a new phase boundary originating from a previously undetected canting of the ${\mathrm{Cu}}^{2+}$ spins out of the Cu${\mathrm{O}}_{2}$ planes. This canting, together with the exponential temperature dependence of the two-dimensional correlation length, accounts quantitatively for the susceptibility peak at the N\'eel temperature. Enhancement of the conductivity in the ferromagnetic phase demonstrates a strong connection between the magnetism and charge transport.

285 citations

Journal ArticleDOI
TL;DR: This large energy scale for the spin fluctuations gives credence to models of the superconductivity in doped La/sub 2/CuO/sub 4/ in which the pairing is magnetic in origin.
Abstract: We report elastic, quasielastic (F dE), and inelastic neutron-scattering studies of the instantaneous and dynamic spin fluctuations in as-grown and doped ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$. Four samples have been studied: (A) as-grown ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ with ${T}_{N}$=195 K, (B) oxygenated ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ with ${\mathit{T}}_{\mathit{N}}$\ensuremath{\simeq}100 K, (C) ${\mathrm{La}}_{2}$${\mathrm{Cu}}_{0.95}$${\mathrm{Li}}_{0.05}$${\mathrm{O}}_{4}$, and (D) ${\mathrm{La}}_{1.97}$${\mathrm{Sr}}_{0.03}$${\mathrm{Cu}}_{0.95}$${\mathrm{Li}}_{0.05}$${\mathrm{O}}_{4}$. All crystals exhibit variable-range-hopping conductivity behavior. At room temperature each sample exhibits two-dimensional (2D) antiferromagnetic instantaneous correlations in the ${\mathrm{CuO}}_{2}$ sheets with correlation length varying from \ensuremath{\sim}200 A\r{} in crystal A to \ensuremath{\sim}14 A\r{} in crystal D. The integrated intensity and therefore the effective moment is, however, constant to within the experimental error. In samples A and B the 2D correlation length becomes sufficiently large with decreasing temperature that the interplanar coupling is able to drive a transition to 3D long-range order. The spin dynamics have been studied in detail in crystals A and B and quite unusual behavior is observed. In contrast to previously studied planar antiferromagnets, there is no significant E\ensuremath{\simeq}0 component for temperatures \ensuremath{\ge}${T}_{N}$ and instead the 2D response function is highly inelastic. The effective dispersion of the spin excitations is \ensuremath{\ge}0.4 eV A\r{}. This large energy scale for the spin fluctuations gives credence to models of the superconductivity in doped ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ in which the pairing is magnetic in origin.

246 citations

Journal ArticleDOI
TL;DR: It is shown that the Neel state is destroyed by the doping and spin-spin correlation lenght xi is quite short, varying from approx.35 to approx.8 A as x varies between 0.02 and 0.18; the local order is the same as in pure La/sub 2/CuO/sub 4/.
Abstract: We have carried out elastic, quasielastic (..integral.. dE), and inelastic neutron-scattering studies of the antiferromagnetic spin correlations in La/sub 2-//sub x/Sr/sub x/CuO/sub 4/ with x varying between 0.02 and 0.18. The crystals, which were grown in three different laboratories, exhibit behavior that varies smoothly with x. In all cases, antiferromagnetic correlations with a scattering amplitude corresponding to a fully occupied Cu/sup 2+/ square lattice are observed. However, the Neel state is destroyed by the doping and spin-spin correlation lenght xi is quite short, varying from approx.35 to approx.8 A as x varies between 0.02 and 0.18; the local order is, however, the same as in pure La/sub 2/CuO/sub 4/. The fluctuations are dynamic in character as in La/sub 2/CuO/sub 4/ above the Neel temperature T/sub N/. To a first approximation, xi = 3.8/ ..sqrt..x A, the average separation between the holes introduced by the Sr/sup 2+/ doping. The x = 0.08 sample exhibits superconductivity with T/sub c/ = 10 K and with a Meissner fraction exceeding 15% at 5 K; no important differences in the magnetic scattering are observed in the normal and superconducting states. In an appendix we present additional data on the spin dynamics in pure La/sub 2/CuO/submore » 4/ at T = 300 K in a sample with T/sub N/ = 235 K.« less

212 citations

Journal ArticleDOI
TL;DR: The tetragonal-to-orthorhombic phase transition is caused by a softening of a transverse-optic-phonon mode at the $X$ point and the rotational nature of the soft mode leads to moderate electron- phonon coupling and the mode is unlikely to enhance significantly conventional phonon mediated superconductivity.
Abstract: The dispersion of the low-lying phonon branches of several doped and undoped single crystals of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ has been investigated by using inelastic-neutron-scattering techniques. The zone-center modes are in reasonable agreement with Raman measurements. The reported peaks in the phonon density of states show up at energies that correspond to extrema in the dispersion curves of the transverse and longitudinal acoustic branches near the zone boundary. The tetragonal-to-orthorhombic phase transition is caused by a softening of a transverse-optic-phonon mode at the $X$ point. The rotational nature of the soft mode leads to moderate electron-phonon coupling and the mode is unlikely to enhance significantly conventional phonon mediated superconductivity. We did not observe any evidence for the predicted breathing-mode instability near the zone boundary.

127 citations

Journal ArticleDOI
TL;DR: The resistivity of nonmetallic La/sub 2-sub y/Sr/sub y,Cu/sub 1-//sub x/Li/sub X/O/sub 4-sub delta/ single crystals and ceramics accurately follows the functional form exp((T/sub 0/T)/sup 1/4/), characteristic of variable-range hopping as mentioned in this paper.
Abstract: The resistivity of nonmetallic La/sub 2-//sub y/Sr/sub y/Cu/sub 1-//sub x/Li/sub x/O/sub 4-//sub delta/ single crystals and ceramics accurately follows the functional form exp((T/sub 0//T)/sup 1/4/), characteristic of variable-range hopping. For each of the crystals grown from Li-containing flux and CuO flux, pure reduced ceramics, and ceramics containing 0.025 to 0.2 mol % Li, the values of T/sub 0/ are in the range 0.3--7 x 10/sup 6/ K. The hopping conductivity shows that the crystals, which also manifest the two-dimensional quantum spin fluid state, antiferromagnetism, and the tetragonal-to-orthorhombic transition, are nonmetallic because the electronic states at the Fermi energy are localized. No evidence of a large gap is observed, and all samples, including the reduced ceramic, are p type, leading to the suggestion that in the nonmetallic state as well as the superconductor, the Fermi energy lies near the top of the band of singly occupied states (lower Hubbard band). The suppression of superconductivity by Li impurities is discussed.

126 citations


Cited by
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TL;DR: The transport properties of disordered solids have been the subject of much work since at least the 1950s, but with a new burst of activity during the 1980s which has survived up to the present day as mentioned in this paper.
Abstract: The transport properties of disordered solids have been the subject of much work since at least the 1950s, but with a new burst of activity during the 1980s which has survived up to the present day. There have been numerous reviews of a more or less specialized nature. The present review aims to fill the niche for a non-specialized review of this very active area of research. The basic concepts behind the theory are introduced with more detailed sections covering experimental results, one-dimensional localization, scaling theory, weak localization, magnetic field effects and fluctuations.

1,466 citations

Journal ArticleDOI
TL;DR: In this paper, a review of the results of the density-functional type of electronic structure calculations is presented, and their results are compared with the relevant experimental data, showing that the important electronic states are dominated by the copper and oxygen orbitals, with strong hybridization between them.
Abstract: Since the discovery of superconductivity above 30 K by Bednorz and M\"uller in the La copper oxide system, the critical temperature has been raised to 90 K in Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and to 110 and 125 K in Bi-based and Tl-based copper oxides, respectively. In the two years since this Nobel-prize-winning discovery, a large number of electronic structure calculations have been carried out as a first step in understanding the electronic properties of these materials. In this paper these calculations (mostly of the density-functional type) are gathered and reviewed, and their results are compared with the relevant experimental data. The picture that emerges is one in which the important electronic states are dominated by the copper $d$ and oxygen $p$ orbitals, with strong hybridization between them. Photon, electron, and positron spectroscopies provide important information about the electronic states, and comparison with electronic structure calculations indicates that, while many features can be interpreted in terms of existing calculations, self-energy corrections ("correlations") are important for a more detailed understanding. The antiferromagnetism that occurs in some regions of the phase diagram poses a particularly challenging problem for any detailed theory. The study of structural stability, lattice dynamics, and electron-phonon coupling in the copper oxides is also discussed. Finally, a brief review is given of the attempts so far to identify interaction constants appropriate for a model Hamiltonian treatment of many-body interactions in these materials.

988 citations

Journal ArticleDOI
TL;DR: This review article highlights the current methods to realize the white light emission in a single-phase host, including doping a single rare earth ion into appropriate single- phase hosts and codoping different ions in one host to control emission color via energy transfer processes.
Abstract: White light-emitting diodes (WLEDs) as new solid-state light sources have a greatly promising application in the field of lighting and display. So far much effort has been devoted to exploring novel luminescent materials for WLEDs. Currently the major challenges in WLEDs are to achieve high luminous efficacy, high chromatic stability, brilliant color-rending properties, and price competitiveness against fluorescent lamps, which rely critically on the phosphor properties. In recent years, numerous efforts have been made to develop single-phase white-light-emitting phosphors for near-ultraviolet or ultraviolet excitation to solve the above challenges with certain achievements. This review article highlights the current methods to realize the white light emission in a single-phase host, including: (1) doping a single rare earth ion (Eu3+, Eu2+ or Dy3+) into appropriate single-phase hosts; (2) co-doping various luminescent ions with different emissions into a single matrix simultaneously, such as Tm3+/Tb3+/Eu3+, Tm3+/Dy3+, Yb3+/Er3+/Tm3+etc.; (3) codoping different ions in one host to control emission color via energy transfer processes; and (4) controlling the concentration of the defect and reaction conditions of defect-related luminescent materials.

979 citations

Journal ArticleDOI
TL;DR: The literature on 4f n ↔4f n −1 5d transitions of the trivalent lanthanides in inorganic compounds has been collected as mentioned in this paper from critically analyzing fd-excitation, absorption, reflection and df-emission spectra, values for the spectroscopic red shift of 5d levels and the stokes shift were determined.

853 citations

Journal ArticleDOI
TL;DR: In this paper, the absorption and emission cross sections of the transition between the ground spin-orbit multiplet and the lowest excited multiplet were measured for Er/sup 3+/, Tm/sup3+/, and Ho/Sup 3+/ ions in a variety of crystalline hosts.
Abstract: The absorption and emission cross sections of the transition between the ground spin-orbit multiplet and the lowest excited multiplet were measured for Er/sup 3+/, Tm/sup 3+/, and Ho/sup 3+/ ions in a variety of crystalline hosts. The materials that were investigated include LiYF/sub 4/, BaY/sub 2/F/sub 8/, Y/sub 3/Al/sub 5/O/sub 12/, LaF/sub 3/, KCaF/sub 3/, YAlO/sub 3/, and La/sub 2/Be/sub 2/O/sub 5/. The absolute magnitudes of the emission cross sections were determined from the absorption spectra, with the aid of the principle of reciprocity. The calculated radiative emission lifetimes derived from these measured cross sections agree well with the measured emission decay times for most materials. The potential use of these rare-earth-doped materials in pulsed laser applications requires that the ground state exhibit adequate splitting to minimize the detrimental effects of the ground state thermal population, and also that the emission cross section be sufficiently large to permit efficient extraction energy. The systems based on Ho/sup 3+/ in the eightfold coordinated sites of LiYF/sub 4/, BaY/sub 2/F/sub 8/, and Y/sub 3/Al/sub 5/O/sub 12/ appear to be the most promising. >

839 citations