251 research outputs found
N\'eel-Dimer Transition in Antiferromagnetic Heisenberg Model and Deconfinement of Spinons at the Critical Point
Quantum phase transition from the N\'eel to the dimer states in an
antiferromagnetic(AF) Heisenberg model on square lattice is studied. We
introduce a control parameter for the exchange coupling which connects
the N\'eel () and the dimer () states. We employ the
(the Schwinger boson) representation of the spin operator and
integrate out the half of the variables at odd sites and we obtain a
nonlinear model. The effective coupling constant is a function
of and at the model is in the ordered phase which
corresponds to the N\'eel state of the AF Heisenberg model. A phase transition
to the dimer state occurs at a certain critical value of as
increases. In the N\'eel state, the dynamical composite U(1) gauge field in the
model is in a Higgs phase and low-energy excitations are gapless spin
wave. In the dimer phase, a confinement phase of the gauge theory is realized
and low-energy excitations are magnons. For the critical point, we argue
that a deconfinement phase, which is similar to the Coulomb phase in 3 spatial
dimensions, is realized and spinons appear as low-energy
excitations
Deconfinement Phase Transition in a 3D Nonlocal U(1) Lattice Gauge Theory
We introduce a 3D compact U(1) lattice gauge theory having nonlocal
interactions in the temporal direction, and study its phase structure. The
model is relevant for the compact QED and strongly correlated electron
systems like the t-J model of cuprates. For a power-law decaying long-range
interaction, which simulates the effect of gapless matter fields, a
second-order phase transition takes place separating the confinement and
deconfinement phases. For an exponentially decaying interaction simulating
matter fields with gaps, the system exhibits no signals of a second-order
transition.Comment: 4 pages, 5 figures, typos correcte
Phase Structure of the Random-Plaquette Z_2 Gauge Model: Accuracy Threshold for a Toric Quantum Memory
We study the phase structure of the random-plaquette Z_2 lattice gauge model
in three dimensions. In this model, the "gauge coupling" for each plaquette is
a quenched random variable that takes the value \beta with the probability 1-p
and -\beta with the probability p. This model is relevant for the recently
proposed quantum memory of toric code. The parameter p is the concentration of
the plaquettes with "wrong-sign" couplings -\beta, and interpreted as the error
probability per qubit in quantum code. In the gauge system with p=0, i.e., with
the uniform gauge couplings \beta, it is known that there exists a second-order
phase transition at a certain critical "temperature", T(\equiv \beta^{-1}) =
T_c =1.31, which separates an ordered(Higgs) phase at T<T_c and a
disordered(confinement) phase at T>T_c. As p increases, the critical
temperature T_c(p) decreases. In the p-T plane, the curve T_c(p) intersects
with the Nishimori line T_{N}(p) at the certain point (p_c, T_{N}(p_c)). The
value p_c is just the accuracy threshold for a fault-tolerant quantum memory
and associated quantum computations. By the Monte-Carlo simulations, we
calculate the specific heat and the expectation values of the Wilson loop to
obtain the phase-transition line T_c(p) numerically. The accuracy threshold is
estimated as p_c \simeq 0.033.Comment: 24 pages, 14 figures, some clarification
Mass Spectrometric Study of the Negative Ion Formation of Nitroethylene at High Electron Energies
The formation of the molecular and the dimer negative ions of nitroethylene at high electron energies was studied. The intensity of the molecular negative ion showed a second order dependence on the pressure below 10⁻⁴ torr and a higher than second order dependence above 10⁻⁴ torr. The intensity of the dimer negative ion showed a second order dependence greater than that of the molecular negative ion. The intensity of the molecular negative ion was found to be dependent on the pressure of additional rare gases. This suggests that the collisional stabilization of excited ions is very important at high electron energies
Self-Duality and Phase Structure of the 4D Random-Plaquette Z_2 Gauge Model
In the present paper, we shall study the 4-dimensional Z_2 lattice gauge
model with a random gauge coupling; the random-plaquette gauge model(RPGM). The
random gauge coupling at each plaquette takes the value J with the probability
1-p and -J with p. This model exhibits a confinement-Higgs phase transition. We
numerically obtain a phase boundary curve in the (p-T)-plane where T is the
"temperature" measured in unit of J/k_B. This model plays an important role in
estimating the accuracy threshold of a quantum memory of a toric code. In this
paper, we are mainly interested in its "self-duality" aspect, and the
relationship with the random-bond Ising model(RBIM) in 2-dimensions. The
"self-duality" argument can be applied both for RPGM and RBIM, giving the same
duality equations, hence predicting the same phase boundary. The phase boundary
curve obtained by our numerical simulation almost coincides with this predicted
phase boundary at the high-temperature region. The phase transition is of first
order for relatively small values of p < 0.08, but becomes of second order for
larger p. The value of p at the intersection of the phase boundary curve and
the Nishimori line is regarded as the accuracy threshold of errors in a toric
quantum memory. It is estimated as p=0.110\pm0.002, which is very close to the
value conjectured by Takeda and Nishimori through the "self-duality" argument.Comment: 14 pages, 7 figure
Fine and Large Coulomb Diamonds in a Silicon Quantum Dot
We experimentally study the transport properties of silicon quantum dots
(QDs) fabricated from a highly doped n-type silicon-on-insulator wafer. Low
noise electrical measurements using a low temperature complementary
metal-oxide-semiconductor (LTCMOS) amplifier are performed at 4.2 K in liquid
helium. Two series of Coulomb peaks are observed: long-period oscillations and
fine structures, and both of them show clear source drain voltage dependence.
We also observe two series of Coulomb diamonds having different periodicity.
The obtained experimental results are well reproduced by a master equation
analysis using a model of double QDs coupled in parallel.Comment: 5 pages, 4 figures, to appear in Jpn. J. Appl. Phy
Gemcitabine sensitivity-related mRNA expression in endoscopic ultrasound-guided fine-needle aspiration biopsy of unresectable pancreatic cancer
<p>Abstract</p> <p>Background</p> <p>The aim of this study was to determine a predictive indicator of gemcitabine (GEM) efficacy in unresectable pancreatic cancer using tissue obtained by endoscopic ultrasound-guided fine-needle aspiration biopsy (EUS-FNA).</p> <p>Methods</p> <p>mRNAs extracted from 35 pancreatic tubular adenocarcinoma tissues obtained by EUS-FNA before GEM-treatment were studied. mRNAs were amplified and applied to a Focused DNA Array, which was restricted to well-known genes, including GEM sensitivity-related genes, deoxycytidine kinase (dCK), human equilibrative nucleoside transporter 1 (hENT1), hENT2, dCMP deaminase, cytidine deaminase, 5'-nucleotidase, ribonucleotide reductase 1 (RRM1) and RRM2. mRNA levels were classified into high and low expression based on a cut-off value defined as the average expression of 35 samples. These 35 patients were divided into the following two groups. Patients with partial response and those with stable disease whose tumor markers decreased by 50% or more were classified as the effective group. The rest of patients were classified as the non-effective group. The relationship between GEM efficacy and mRNA expression was then examined by chi-squared test.</p> <p>Results</p> <p>Among these GEM sensitivity-related genes, dCK alone showed a significant correlation with GEM efficacy. Eight of 12 patients in the effective group had high dCK expression, whereas 16 of 23 patients in non-effective group had low dCK expressions (<it>P </it>= 0.0398).</p> <p>Conclusion</p> <p>dCK mRNA expression is a candidate indicator for GEM efficacy in unresectable pancreatic cancer. Quantitative mRNA measurements of dCK using EUS-FNA samples are necessary for definitive conclusions.</p
Phase Structure of a 3D Nonlocal U(1) Gauge Theory: Deconfinement by Gapless Matter Fields
In this paper, we study a 3D compact U(1) lattice gauge theory with a variety
of nonlocal interactions that simulates the effects of gapless/gapful matter
fields. This theory is quite important to investigate the phase structures of
QED and strongly-correlated electron systems like the 2D quantum spin
models, the fractional quantum Hall effect, the t-J model of high-temperature
superconductivity. We restrict the nonlocal interactions among gauge variables
only to those along the temporal direction and adjust their coupling constants
optimally to simulate the isotropic nonlocal couplings of the original models.
We perform numerical studies of the model to find that, for a certain class of
power-decaying couplings, there appears a second-order phase transition to the
deconfinement phase as the gauge coupling constant is decreased. On the other
hand, for the exponentially-decaying coupling, there are no signals for
second-order phase transition. These results indicate the possibility that
introduction of sufficient number of massless matter fields destabilizes the
permanent confinement in the 3D compact U(1) pure gauge theory due to
instantons.Comment: The version to be published in Nucl.Phys.
Bile Acids Induce Cdx2 Expression Through the Farnesoid X Receptor in Gastric Epithelial Cells
Clinical and experimental studies showed that the reflux of bile into the stomach contributes to the induction of intestinal metaplasia of the stomach and gastric carcinogenesis. Caudal-type homeobox 2 (Cdx2) plays a key role in the exhibition of intestinal phenotypes by regulating the expression of intestine-specific genes such as goblet-specific gene mucin 2 (MUC2). We investigated the involvement of the farnesoid X receptor (FXR), a nuclear receptor for bile acids, in the chenodeoxycholic acid (CDCA)-induced expression of Cdx2 and MUC2 in normal rat gastric epithelial cells (RGM-1 cells). RGM-1 cells were treated with CDCA or GW4064, an FXR agonist, in the presence or absence of guggulsterone, an FXR antagonist. CDCA induced dose-dependent expression of Cdx2 and MUC2 at both the mRNA and protein levels. The maximum stimulation of Cdx2 and MUC2 mRNA induced by CDCA was observed at 3 h and by 6 h, respectively. GW4064 also induced expression of these molecules. The effects of CDCA and GW4064 on expression of Cdx2 and MUC2 were abolished by guggulsterone. These findings suggest that bile acids may induce gastric intestinal metaplasia and carcinogenesis through the FXR
- …