270 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
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
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
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
Bound states and field-polarized Haldane modes in a quantum spin ladder
The challenge of one-dimensional systems is to understand their physics
beyond the level of known elementary excitations. By high-resolution neutron
spectroscopy in a quantum spin ladder material, we probe the leading
multiparticle excitation by characterizing the two-magnon bound state at zero
field. By applying high magnetic fields, we create and select the singlet
(longitudinal) and triplet (transverse) excitations of the fully spin-polarized
ladder, which have not been observed previously and are close analogs of the
modes anticipated in a polarized Haldane chain. Theoretical modelling of the
dynamical response demonstrates our complete quantitative understanding of
these states.Comment: 6 pages, 3 figures plus supplementary material 7 pages 5 figure
Thermodynamics of the Spin Luttinger-Liquid in a Model Ladder Material
The phase diagram in temperature and magnetic field of the metal-organic,
two-leg, spin-ladder compound (C5H12N)2CuBr4 is studied by measurements of the
specific heat and the magnetocaloric effect. We demonstrate the presence of an
extended spin Luttinger-liquid phase between two field-induced quantum critical
points and over a broad range of temperature. Based on an ideal spin-ladder
Hamiltonian, comprehensive numerical modelling of the ladder specific heat
yields excellent quantitative agreement with the experimental data across the
complete phase diagram.Comment: 4 pages, 4 figures, updated refs and minor changes to the text,
version accepted for publication in Phys. Rev. Let
Field- and pressure-induced magnetic quantum phase transitions in TlCuCl_3
Thallium copper chloride is a quantum spin liquid of S = 1/2 Cu^2+ dimers.
Interdimer superexchange interactions give a three-dimensional magnon
dispersion and a spin gap significantly smaller than the dimer coupling. This
gap is closed by an applied hydrostatic pressure of approximately 2kbar or by a
magnetic field of 5.6T, offering a unique opportunity to explore the both types
of quantum phase transition and their associated critical phenomena. We use a
bond-operator formulation to obtain a continuous description of all disordered
and ordered phases, and thus of the transitions separating these. Both
pressure- and field-induced transitions may be considered as the Bose-Einstein
condensation of triplet magnon excitations, and the respective phases of
staggered magnetic order as linear combinations of dimer singlet and triplet
modes. We focus on the evolution with applied pressure and field of the
magnetic excitations in each phase, and in particular on the gapless
(Goldstone) modes in the ordered regimes which correspond to phase fluctuations
of the ordered moment. The bond-operator description yields a good account of
the magnetization curves and of magnon dispersion relations observed by
inelastic neutron scattering under applied fields, and a variety of
experimental predictions for pressure-dependent measurements.Comment: 20 pages, 17 figure
Quantum Magnets under Pressure: Controlling Elementary Excitations in TlCuCl3
We follow the evolution of the elementary excitations of the quantum
antiferromagnet TlCuCl3 through the pressure-induced quantum critical point,
which separates a dimer-based quantum disordered phase from a phase of
long-ranged magnetic order. We demonstrate by neutron spectroscopy the
continuous emergence in the weakly ordered state of a low-lying but massive
excitation corresponding to longitudinal fluctuations of the magnetic moment.
This mode is not present in a classical description of ordered magnets, but is
a direct consequence of the quantum critical point.Comment: 4 pages, 4 figure
Irradiation of benzene molecules by ion-induced and light-induced intense fields
Benzene, with its sea of delocalized -electrons in the valence orbitals,
is identified as an example of a class of molecules that enable establishment
of the correspondence between intense ion-induced and laser-light-induced
fields in experiments that probe ionization dynamics in temporal regimes
spanning the attosecond and picosecond ranges.Comment: 4 ps figure
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