Search Results - (Author, Cooperation:M. Greven)

2018-10-18

American Physical Society (APS)

1098-0121

1095-3795

Physics

Electronic structure and strongly correlated systems

2012-03-31

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2018-02-24

American Association for the Advancement of Science (AAAS)

2375-2548

Natural Sciences in General

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] High-transition-temperature (high-Tc) superconductivity develops near antiferromagnetic phases, and it is possible that magnetic excitations contribute to the superconducting pairing mechanism. To assess the role of antiferromagnetism, it is essential to understand the doping and ...

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] Quantum criticality is associated with a system composed of a nearly infinite number of interacting quantum degrees of freedom at zero temperature, and it implies that the system looks on average the same regardless of the time- and length scale on which it is observed. Electrons on the atomic ...

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] Since the discovery of superconductivity at elevated temperatures in the copper oxide materials there has been a considerable effort to find universal trends and correlations amongst physical quantities, as a clue to the origin of the superconductivity. One of the earliest patterns that emerged ...

Electronic Resource

0921-4526

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Physics

Electronic Resource

0921-4526

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Physics

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] One challenge in condensed-matter physics is to unravel the interplay between magnetism and superconductivity in copper oxides with a high critical temperature (Tc). Kang et al. claim to have revealed a quantum phase transition from the superconducting to an antiferromagnetic state ...

Electronic Resource

1434-6036

PACS. 75.10.Jm Quantized spin models - 75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.) - 75.40.Mg Numerical simulation studies

Springer Online Journal Archives 1860-2000

Physics

Abstract: Antiferromagnetic Heisenberg spin chains with various spin values (S=1/2,1,3/2,2,5/2) are studied numerically with the quantum Monte-Carlo method. Effective spin S chains are realized by ferromagnetically coupling n=2S antiferromagnetic spin chains with S=1/2. The temperature dependence of the uniform susceptibility, the staggered susceptibility, and the static structure factor peak intensity are computed down to very low temperatures, . The correlation length at each temperature is deduced from numerical measurements of the instantaneous spin-spin correlation function. At high temperatures, very good agreement with exact results for the classical spin chain is obtained independent of the value of S. For the S=2 chain which has a gap , the correlation length and the uniform susceptibility in the temperature range are well predicted by the semi-classical theory of Damle and Sachdev.

Electronic Resource

1434-6036

75.10.Jm ; 75.25.tZ ; 75.40.−S ; 75.30.Ds

Springer Online Journal Archives 1860-2000

Physics

Abstract The static and dynamic spin fluctuations in the spinS=1, two-dimensional (2D) square-lattice antiferromagnet La2NiO4 have been studied over a wide temperature range using neutron scattering techniques. The spin correlations in La2NiO4 exhibit a crossover from two- to three-dimensional (3D) behavior as the Néel temperature is approached from above. Critical slowing down of the low-energy spin fluctuations is also observed just aboveT N . The correlation length, ξ(T), and the static structure factor,S(0), have been measured and are compared with recent theoretical calculations for the quantum 2D Heisenberg antiferromagnet using microscopic parameters determined from previous spin-wave measurements. Good agreement for ξ(T) is found with the exact low-temperature result of Hasenfratz and Niedermeyer provided that 2π p s is renormalized by ≈20% from the spin-wave value.

Electronic Resource

1434-6036

PACS. 75.10.Jm Quantized spin models - 75.25.+z Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source X-ray scattering, etc.) - 75.30.Gw Magnetic anisotropy

Springer Online Journal Archives 1860-2000

Physics

Abstract: We report a neutron scattering study of the instantaneous spin correlations in the two-dimensional spin S =5/2 square-lattice Heisenberg antiferromagnet Rb2MnF4. The measured correlation lengths are quantitatively described, with no adjustable parameters, by high-temperature series expansion results and by a theory based on the quantum self-consistent harmonic approximation. Conversely, we find that the data, which cover the range from about 1 to 50 lattice constants, are outside of the regime corresponding to renormalized classical behavior of the quantum non-linear model. In addition, we observe a crossover from Heisenberg to Ising critical behavior near the Néel temperature; this crossover is well described by a mean-field model with no adjustable parameters.

Electronic Resource

1434-6036

Springer Online Journal Archives 1860-2000

Physics

Abstract We report a neutron scattering investigation of a La1.65Nd0.35CuO4 single crystal which undergoes a second order structural transition with decreasing temperature from a high temperature orthorhombic phase (space group Bmab) to a distinct low temperature orthorhombic phase (space group Pccn) atT s =76 K. The transition is induced by the softening of the phonon mode that involves rotations of the tilt axis of the CuO6 octahedra. Antiferromagnetic long range order is established belowT N =316 K, and a hysteretic reorientation to a noncollinear spin structure occurs belowT s . The low energy spin dynamics of La1.65Nd0.35CuO4 aboveT s are closely similar to those of stoichiometric La2CuO4; at 100 K the gaps for in-plane and out-of-plane polarized spin waves are 2.3 meV and 5 meV, respectively. Out-of-plane polarized magnons are little affected by the structural transition, but the gap for in-plane polarized magnons increases markedly belowT s .

Electronic Resource

1434-6036

75.10.Jm ; 75.25.+Z ; 75.40.−S

Springer Online Journal Archives 1860-2000

Physics

Abstract We have carried out a neutron scattering investigation of the static structure factorS(q 2D ) (q 2D is the in-plane wave vector) in the two-dimensional spinS=1/2 square-lattice Heisenberg antiferromagnet Sr2CuO2Cl2. For the spin correlation length ξ we find quantitative agreement with Monte Carlo results over a wide range of temperature. The combined Sr2CuO2Cl2-Monte Carlo data, which cover the length scale from ≈1 to 200 lattice constants, are predicted without adjustable parameteres by renormalized classical theory for the quantum nonlinear sigma model. For the structure factor peakS(0), on the other hand, we findS(0)∼ξ 2 for the reduced temperature range 0.16〈T/2πρ s 〈0.36, whereas current theories predict that at low temperaturesS(0)∼T 2 ξ 2. This discrepancy has important implications for the interpretation of many derivative quantities such as NMR relaxation rates. In the ordered phase, we have measured the temperature dependence of the out-of-plane spin-wave gap. Its low-temperature value of 5.0 meV corresponds to an XY anisotropyJ XY /J=1.4×10−4. From measurements of the sublattice mangetization we obtain β=0.22±0.01 for the order parameter exponent. This may either reflect tricricality as in La2CuO4, or it may indicate finite-size two-dimensional XY behavior as suggested by Bramwell and Holdsworth. As in theS=1 system K2NiF4, the gap energy in Sr2CuO2Cl2 scales linearly with the order parameter up to the Néel temperature. We also reanalyze static structure factor data for K2NiF4 using the exact low temperature result for the correlation length of Hasenfratz and Niedermayer and including the Ising anisotropy explicitly. Excellent agreement between experiment and theory is obtained for the correlation length, albeit with the spin-stiffnessρ s reduced by ≈20% from the spin-wave value. As in Sr2CuO2Cl2 we find thatS(0)∼ξ 2 for the reduced temperature range 0.22〈T/2πρ s 〈0.47.

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