6,975 research outputs found
Field-assisted doublon manipulation in the Hubbard model. A quantum doublon ratchet
For the fermionic Hubbard model at strong coupling, we demonstrate that
directional transport of localized doublons (repulsively bound pairs of two
particles occupying the same site of the crystal lattice) can be achieved by
applying an unbiased ac field of time-asymmetric (sawtooth-like) shape. The
mechanism involves a transition to intermediate states of virtually zero double
occupation which are reached by splitting the doublon by fields of the order of
the Hubbard interaction. The process is discussed on the basis of numerically
exact calculations for small clusters, and we apply it to more complex states
to manipulate the charge order pattern of one-dimensional systems.Comment: 6 pages, 6 figure
Nocturnal Changes in Knee Cartilage Thickness in Young Healthy Adults
Magnetic resonance imaging (MRI) allows one to analyze cartilage physiology in vivo. Cartilage deforms during loading, but little is known about its recovery after deformation. Here we study `nocturnal' changes in knee cartilage thickness and whether postexercise deformation differs between morning and evening. Axial magnetic resonance (MR) images were acquired in the right knees of 17 healthy volunteers (age 23.5 +/- 3.0 years) after a normal day, and then after 30 deep knee bends. Coronal images were additionally acquired in 8 of these volunteers after a normal day and then after 2 min of static loading of the leg with 150% body weight. The volunteers then remained unloaded overnight and the same protocol was repeated in the morning. A significant increase (p < 0.01) in cartilage thickness was observed between evening (preexercise) and morning (preexercise): +2.4% in the patella, +8.4% in the medial tibia and +6.2% in the lateral tibia. Deformation in the morning (-6.8/-4.6/-5.1%) was generally greater than that in the evening (-5.4/-3.2/-3.7%), but this difference did not reach statistical significance. No significant difference in the nocturnal thickness increase (or postexercise deformation) was observed between men and women. We conclude that knee cartilage (thickness) recovers overnight by approximately 2-8%, independent of sex. Given the lack of `predeformation' after nocturnal periods of unloading, morning postexercise deformation of the cartilage may have a greater magnitude than evening postexercise deformation. Copyright (C) 2012 S. Karger AG, Base
Ultrafast and reversible control of the exchange interaction in Mott insulators
The strongest interaction between microscopic spins in magnetic materials is
the exchange interaction . Therefore, ultrafast control of
holds the promise to control spins on ultimately fast timescales.
We demonstrate that time-periodic modulation of the electronic structure by
electric fields can be used to reversibly control on ultrafast
timescales in extended antiferromagnetic Mott insulators. In the regime of weak
driving strength, we find that can be enhanced and reduced for
frequencies below and above the Mott gap, respectively. Moreover, for strong
driving strength, even the sign of can be reversed and we show
that this causes time reversal of the associated quantum spin dynamics. These
results suggest wide applications, not only to control magnetism in condensed
matter systems, for example, via the excitation of spin resonances, but also to
assess fundamental questions concerning the reversibility of the quantum
many-body dynamics in cold atom systems.Comment: 9 pages, 4 figure
Präzision MRT-basierter Gelenkflächen- und Knorpeldickenanalysen im Kniegelenk bei Verwendung einer schnellen Wasseranregungs-Sequenz und eines semiautomatischen Segmentierungs-Algorithmus
The aim of this study was to analyse the precision of three-dimensional joint surface and cartilage thickness measurements in the knee, using a fast, high-resolution water-excitation sequence and a semiautomated segmentation algorithm. The knee joint of 8 healthy volunteers, aged 22 to 29 years, were examined at a resolution of 1.5 mm x 0.31 mm x 0.31 mm, with four sagittal data sets being acquired after repositioning the joint. After semiautomated segmentation with a B-spline Snake algorithm and 3D reconstruction of the patellar, femoral and tibial cartilages, the joint surface areas (triangulation), cartilage volume, and mean and maximum thickness (Euclidean distance transformation) were analysed, independently of the orientation of the sections. The precision (CV%) for the surface areas was 2.1 to 6.6%. The mean cartilage thickness and cartilage volume showed coefficients of 1.9 to 3.5% (except for the femoral condyles), the value for the medial femoral condyle being 9.1%, and for the lateral condyle 6.5%. For maximum thickness, coefficients of between 2.6 and 5.9% were found. In the present study we investigate for the first time the precision of MRI-based joint surface area measurements in the knee, and of cartilage thickness analyses in the femur. Using a selective water-excitation sequence, the acquisition time can be reduced by more than 50%. The poorer precision in the femoral condyles can be attributed to partial Volume effects that occur at the edges of the joint surfaces with a sagittal image protocol. Since MRI is non-invasive, it is highly suitable for examination of healthy subjects (generation of individual finite element models, analysis of functional adaptation to mechanical stimulation, measurement of cartilage deformation in vivo) and as a diagnostic tool for follow-up, indication for therapy, and objective evaluation of new therapeutic agents in osteoarthritis
From quantum pulse gate to quantum pulse shaper -- enigneered frequency conversion in nonlinear optical waveguides
Full control over the spatio-temporal structure of quantum states of light is
an important goal in quantum optics, to generate for instance single-mode
quantum pulses or to encode information on multiple modes, enhancing channel
capacities. Quantum light pulses feature an inherent, rich spectral
broadband-mode structure. In recent years, exploring the use of integrated
optics as well as source-engineering has led to a deep understanding of the
pulse-mode structure of guided quantum states of light. In addition, several
groups have started to investigate the manipulation of quantum states by means
of single-photon frequency conversion. In this paper we explore new routes
towards complete control of the inherent pulse-modes of ultrafast pulsed
quantum states by employing specifically designed nonlinear waveguides with
adapted dispersion properties. Starting from our recently proposed quantum
pulse gate (QPG) we further generalize the concept of spatio-spectral
engineering for arbitrary \chitwo-based quantum processes. We analyse the
sum-frequency generation based QPG and introduce the difference-frequency
generation based quantum pulse shaper (QPS). Together, these versatile and
robust integrated optics devices allow for arbitrary manipulations of the
pulse-mode structure of ultrafast pulsed quantum states. The QPG can be
utilized to select an arbitrary pulse mode from a multimode input state,
whereas the QPS enables the generation of specific pulse modes from an input
wavepacket with Gaussian-shaped spectrum.Comment: 21 pages, 9 figure
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