719 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
The Contractile Fine Structure of Vertebrate Smooth Muscle
About 30 years ago, Ernst Fischer introduced a new approach to muscle research by comparing the fine structure, and the function of the contractile mechanism of smooth and striated muscle. At that time (Fischer, 1936a and b; 1938) he systematically and successfully investigated the total, the intrinsic, and the form birefringence of smooth muscles and compared his results with analogous data concerning the contractile structure (Noll and Weber, 1935) and the oriented actomyosin threads (Weber, 1935) of skeletal muscle. These investigations were especially important because the birefringence of all muscles is based on its contractile structure and functional state, and because birefringence was better understood in micellar and molecular terms since Wiener\u27s theory
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
Neutron scattering study of the field-dependent ground state and the spin dynamics in S=1/2 NH4CuCl3
Elastic and inelastic neutron scattering experiments have been performed on the dimer spin system NH4CuCl3, which shows plateaus in the magnetization curve at m=1/4 and m=3/4 of the saturation value. Two structural phase transitions at T1≈156  K and at T2=70  K lead to a doubling of the crystallographic unit cell along the b direction and as a consequence a segregation into different dimer subsystems. Long-range magnetic ordering is reported below TN=1.3  K. The magnetic field dependence of the excitation spectrum identifies successive quantum phase transitions of the dimer subsystems as the driving mechanism for the unconventional magnetization process in agreement with a recent theoretical model
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
Role of multiple subband renormalization in the electronic transport of correlated oxide superlattices
Metallic behavior of band-insulator/ Mott-insulator interfaces was observed
in artificial perovskite superlattices such as in nanoscale SrTiO3/LaTiO3
multilayers. Applying a semiclassical perspective to the parallel electronic
transport we identify two major ingredients relevant for such systems: i) the
quantum confinement of the conduction electrons (superlattice modulation) leads
to a complex, quasi-two dimensional subband structure with both hole- and
electron-like Fermi surfaces. ii) strong electron-electron interaction requires
a substantial renormalization of the quasi-particle dispersion. We characterize
this renormalization by two sets of parameters, namely, the quasi-particle
weight and the induced particle-hole asymmetry of each partially filled
subband. In our study, the quasi-particle dispersion is calculated
self-consistently as function of microscopic parameters using the slave-boson
mean-field approximation introduced by Kotliar and Ruckenstein. We discuss the
consequences of strong local correlations on the normal-state free-carrier
response in the optical conductivity and on the thermoelectric effects.Comment: 11 pages, 4 figure
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