154 research outputs found
Complete bond-operator theory of the two-chain spin ladder
The discovery of the almost ideal, two-chain spin-ladder material
(C_5H_12N)_2CuBr_4 has once again focused attention on this most fundamental
problem in low-dimensional quantum magnetism. Within the bond-operator
framework, three qualitative advances are introduced which extend the theory to
all finite temperatures and magnetic fields in the gapped regime. This
systematic description permits quantitative and parameter-free experimental
comparisons, which are presented for the specific heat, and predictions for
thermal renormalization of the triplet magnon excitations.Comment: 12 pages, 10 figure
Dimensional reduction by pressure in the magnetic framework material CuF(DO)pyz: from spin-wave to spinon excitations
Metal organic magnets have enormous potential to host a variety of electronic
and magnetic phases that originate from a strong interplay between the spin,
orbital and lattice degrees of freedom. We control this interplay in the
quantum magnet CuF(DO)pyz by using high pressure to drive the
system through a structural and magnetic phase transition. Using neutron
scattering, we show that the low pressure state, which hosts a two-dimensional
square lattice with spin-wave excitations and a dominant exchange coupling of
0.89 meV, transforms at high pressure into a one-dimensional spin-chain
hallmarked by a spinon continuum and a reduced exchange interaction of 0.43
meV. This direct microscopic observation of a magnetic dimensional crossover as
a function of pressure opens up new possibilities for studying the evolution of
fractionalised excitations in low dimensional quantum magnets and eventually
pressure-controlled metal--insulator transitions
Ehrenfest relations and magnetoelastic effects in field-induced ordered phases
Magnetoelastic properties in field-induced magnetic ordered phases are
studied theoretically based on a Ginzburg-Landau theory. A critical field for
the field-induced ordered phase is obtained as a function of temperature and
pressure, which determine the phase diagram. It is found that magnetic field
dependence of elastic constant decreases discontinuously at the critical field,
Hc, and that it decreases linearly with field in the ordered phase (H>Hc). We
found an Ehrenfest relation between the field dependence of the elastic
constant and the pressure dependence of critical field. Our theory provides the
theoretical form for magnetoelastic properties in field- and pressure-induced
ordered phases.Comment: 7 pages, 3 figure
Quantum and classical criticality in a dimerized quantum antiferromagnet
A quantum critical point (QCP) is a singularity in the phase diagram arising
due to quantum mechanical fluctuations. The exotic properties of some of the
most enigmatic physical systems, including unconventional metals and
superconductors, quantum magnets, and ultracold atomic condensates, have been
related to the importance of the critical quantum and thermal fluctuations near
such a point. However, direct and continuous control of these fluctuations has
been difficult to realize, and complete thermodynamic and spectroscopic
information is required to disentangle the effects of quantum and classical
physics around a QCP. Here we achieve this control in a high-pressure,
high-resolution neutron scattering experiment on the quantum dimer material
TlCuCl3. By measuring the magnetic excitation spectrum across the entire
quantum critical phase diagram, we illustrate the similarities between quantum
and thermal melting of magnetic order. We prove the critical nature of the
unconventional longitudinal ("Higgs") mode of the ordered phase by damping it
thermally. We demonstrate the development of two types of criticality, quantum
and classical, and use their static and dynamic scaling properties to conclude
that quantum and thermal fluctuations can behave largely independently near a
QCP.Comment: 6 pages, 4 figures. Original version, published version available
from Nature Physics websit
Quantum Statistics of Interacting Dimer Spin Systems
The compound TlCuCl3 represents a model system of dimerized quantum spins
with strong interdimer interactions. We investigate the triplet dispersion as a
function of temperature by inelastic neutron scattering experiments on single
crystals. By comparison with a number of theoretical approaches we demonstrate
that the description of Troyer, Tsunetsugu, and Wuertz [Phys. Rev. B 50, 13515
(1994)] provides an appropriate quantum statistical model for dimer spin
systems at finite temperatures, where many-body correlations become
particularly important.Comment: 4 pages, 4 figures, to appear in Physical Review Letter
Microscopic model for the magnetization plateaus in NH4CuCl3
A simple model consisting of three distinct dimer sublattices is proposed to
describe the magnetism of NH4CuCl3. It explains the occurrence of magnetization
plateaus only at 1/4 and 3/4 of the saturation magnetization. The field
dependence of the excitation modes observed by ESR measurements is also
explained by the model. The model predicts that the magnetization plateaus
should disappear under high pressure.Comment: 4 pages, 5 figures, REVTeX
Multiple Magnon Modes and Consequences for the Bose-Einstein Condensed Phase in BaCuSi2O6
The compound BaCuSi2O6 is a quantum magnet with antiferromagnetic dimers of S
= 1/2 moments on a quasi-2D square lattice. We have investigated its spin
dynamics by inelastic neutron scattering experiments on single crystals with an
energy resolution considerably higher than in an earlier study. We observe
multiple magnon modes, indicating clearly the presence of magnetically
inequivalent dimer sites. This more complex spin Hamiltonian leads to a
distinct form of magnon Bose-Einstein condensate (BEC) phase with a spatially
modulated condensate amplitude.Comment: 5 pages, 4 figures, to be published in Phys. Rev. Let
Spinon localization in the heat transport of the spin-1/2 ladder compound (CHN)CuBr
We present experiments on the magnetic field-dependent thermal transport in
the spin-1/2 ladder system (CHN)CuBr. The thermal
conductivity is only weakly affected by the field-induced
transitions between the gapless Luttinger-liquid state realized for and the gapped states, suggesting the absence of a direct
contribution of the spin excitations to the heat transport. We observe,
however, that the thermal conductivity is strongly suppressed by the magnetic
field deeply within the Luttinger-liquid state. These surprising observations
are discussed in terms of localization of spinons within finite ladder segments
and spinon-phonon umklapp scattering of the predominantly phononic heat
transport.Comment: 4 pages, 3 figure
Pressure-induced electronic phase separation of magnetism and superconductivity in CrAs
The recent discovery of pressure induced superconductivity in the binary
helimagnet CrAs has attracted much attention. How superconductivity emerges
from the magnetic state and what is the mechanism of the superconducting
pairing are two important issues which need to be resolved. In the present
work, the suppression of magnetism and the occurrence of superconductivity in
CrAs as a function of pressure () were studied by means of muon spin
rotation. The magnetism remains bulk up to ~kbar while its volume
fraction gradually decreases with increasing pressure until it vanishes at
7~kbar. At 3.5 kbar superconductivity abruptly appears with its
maximum ~K which decreases upon increasing the pressure. In the
intermediate pressure region (~kbar) the
superconducting and the magnetic volume fractions are spatially phase separated
and compete for phase volume. Our results indicate that the less conductive
magnetic phase provides additional carriers (doping) to the superconducting
parts of the CrAs sample thus leading to an increase of the transition
temperature () and of the superfluid density (). A scaling of
with as well as the phase separation between magnetism and
superconductivity point to a conventional mechanism of the Cooper-pairing in
CrAs.Comment: 9 pages, 8 figure
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