Quantum magnetism and quantum phase transitions

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Neutron-scattering studies of the S = 2 antiferromagnetic chain MnCl3(C10D8N2)

Applied Physics A Materials Science and Processing 74:SUPPL.I (2002) S868-S870

Authors:

GE Granroth, SE Nagler, R Coldea, RS Eccleston, BH Ward, DR Talham, MW Meisel

Abstract:

Quasi-elastic and inelastic neutron scattering studies of the quasi-one-dimensional S = 2 antiferromagnet McCl3(C10D8N2) are reported. The quasi-elastic measurements exhibit a broad peak at Q ≈ 0.69 Å-1, which is consistent with short-range antiferromagnetic coupling between neighboring Mn3+ ions. Inelastic experiments, at 150 mK and Q = 0.70 Å-1, reveal decreased magnetic scattering at energies less than 0.2 meV when compared to similar studies at 20 K. These results provide microscopic evidence for the presence of a Haldane gap and are consistent with the bulk magnetization measurements of Granroth et al.

Quantum critical fluctuations in heavy fermion compounds

International Journal of Modern Physics B 16:20-22 (2002) 3031-3036

Authors:

A Schroeder, G Aeppli, P Coleman, R Ramazashvili, R Coldea, M Adams, E Bucher, DF Mcmorrow, HV Löhneysen, O Stockert

Abstract:

The electronic properties of heavy fermion alloys are dominated by spin fluctuations which are expected to become critical when tuned by pressure to a quantum critical point (QCP), entering a magnetic ordered state. Apart from the onset of exotic superconductivity, unexpected "normal conducting" behavior is found close to the QCP, which does not seem only to escape the conventional view of metals (Fermi liquids) but also the "conventional view" of an antiferromagnetic quantum phase transition in these f-metals. So far only few compounds have been investigated by neutron scattering to directly reveal the critical fluctuations spectrum. In CeCu59Au01 the fluctuations develop an unusual energy dependence, characterized by an exponent α = 0.75, which persist over the entire Brillouin zone, provoking an unexpected local non Fermi liquid behavior. The same unusual exponent derived from E/T scaling determines the H/T scaling of the uniform magnetization. Recent neutron scattering data in magnetic fields further confirm this picture of nearly free local magnetic moments (modified by α) emerging at the antiferromagnetic QCP in this strongly correlated electron system.

Comment on "spin dynamics of the 2D spin 1/2 quantum antiferromagnet copper deuteroformate tetradeuterate (CFTD)" (multiple letters)

Physical Review Letters 89:7 (2002) 079702/1

Authors:

P Kopietz, I Spremo, HM Ronnow, DF McMorrow, R Coldea, A Harrison, ID Youngson, TG Perring, G Aeppli, O Syljuåsen, K Lefmann, C Rischel

QUANTUM CRITICAL FLUCTUATIONS IN HEAVY FERMION COMPOUNDS

World Scientific Publishing (2002) 121-126

Authors:

A SCHROEDER, G AEPPLI, P COLEMAN, R RAMAZASHVILI, R COLDEA, M ADAMS, E BUCHER, DF MCMORROW, HV LÖHNEYSEN, O STOCKERT

Direct measurement of the spin Hamiltonian and observation of condensation of magnons in the 2D frustrated quantum magnet Cs2CuCl4.

Phys Rev Lett 88:13 (2002) 137203

Authors:

R Coldea, DA Tennant, K Habicht, P Smeibidl, C Wolters, Z Tylczynski

Abstract:

We propose a method for measuring spin Hamiltonians and apply it to the spin- 1/2 Heisenberg antiferromagnet Cs2CuCl4, which shows a 2D fractionalized resonating valence bond state at low fields. By applying strong fields we fully align the spin moment of Cs2CuCl4, transforming it into an effective ferromagnet. In this phase the excitations are conventional magnons and their dispersion relation measured using neutron scattering give the exchange couplings directly, which are found to form an anisotropic triangular lattice with small Dzyaloshinskii-Moriya terms. Using the field to control the excitations we observe Bose condensation of magnons into an ordered ground state.