Thomas Neuhaus

Multicanonical ensemble: A New approach to simulate first order phase transitions

Relying on the recent proposed multicanonical algorithm, we present a numerical simulation of the first-order phase transition in the 2D 10-state Potts model on lattices up to sizes 100\ifmmode\times\else\texttimes\fi{}100. It is demonstrated that the new algorithm lacks an exponentially fast increase of the tunneling time between metastable states as a function of the linear size L of the system. Instead, the tunneling time diverges approximately proportional to

Multicanonical algorithms for first order phase transitions

{\mathit{L}}^{2.65}

The triviality bound of the four-component Φ4 model☆

. On our largest lattice we gain more than 2 orders of magnitude as compared to a standard heat-bath algorithm. As a first physical application we report a high-precision computation of the interfacial free energy per unit area.

U(1) lattice gauge theory near the phase transition

Monte Carlo simulations are discussed for systems of volume V = Ld which undergo a first order phase transition in the finite volume limit. Conventional canonical, local Monte Carlo algorithms suffer from exponentially fast slowing down ≈V2 exp (cLd−1). Here we present a class of multicanonical Monte Carlo algorithms which can reduce the slowing down to a quadratic power law ≈V2.

Renal Tubular Dysgenesis, a Not Uncommon Autosomal Recessive Disorder Leading to Oligohydramnios: Role of the Renin-Angiotensin System

Abstract The four-dimensional O(4) Φ4 scalar theory is investigated in the broken phase at different values of the quartic coupling λ. The scalar mass, the field expectation value and the wave function renormalization constant are calculated. We demonstrate the λ dependence of the ratio R s = m s 〈ϕ s ren 〉 and estimate its upper bound to be around 2.7(1).

Phase diagram of a lattice SU(2) ⊗ SU(2) scalar-fermion model with naive and Wilson fermions

Abstract The U(1) lattice gauge theory with the Wilson action is investigated in a high statistics Monte Carlo calculation in the close proximity of the deconfinement phase transition. On a 16 4 lattice, we obtain the renormalized charge e r in the Coulomb phase with a maximal value e 2 r /4 π = 0.195 ± 0.01 at the phase transition point β T = 1.0106 ± 0.0018. The mean plaquette develops a small hysteresis. On a 6 4 lattice, two metastable states can clearly be seen. This suggests that the phase transition could be of first order.

Screening for NPHS2 Mutations May Help Predict FSGS Recurrence after Transplantation

Renal tubular dysgenesis is a clinical disorder that is observed in fetuses and characterized by the absence or poor development of proximal tubules, early onset and persistent oligohydramnios that leads to the Potter sequence, and skull ossification defects. It may be acquired during fetal development or inherited as an autosomal recessive disease. It was shown recently that autosomal recessive renal tubular dysgenesis is genetically heterogeneous and linked to mutations in the genes that encode components of the renin-angiotensin system. This study analyzed the clinical expression of the disease in 29 fetus/neonates from 18 unrelated families and evaluated changes in renal morphology and expression of the renin-angiotensin system. The disease was uniformly severe, with perinatal death in all cases as a result of persistent anuria and hypoxia related to pulmonary hypoplasia. Severe defects in proximal tubules were observed in all fetuses from 18 gestational weeks onward, and lesions also involved other tubular segments. They were associated with thickening of the renal arterial vasculature, from the arcuate to the afferent arteries. Renal renin expression was strikingly increased in 19 of 24 patients studied, from 13 families, whereas no renal renin was detected in four patients from three families. Angiotensinogen and angiotensin-converting enzyme were absent or present in only small amounts in the proximal tubule, in correlation with the severity of tubular abnormalities. No specific changes were detected in angiotensin II receptor expression. The severity and the early onset of the clinical and pathologic expression of the disease underline the major importance of this system in fetal kidney function and development in humans. The identification of the disease on the basis of precise histologic analysis and the research of the genetic defect now allow genetic counseling and early prenatal diagnosis.

Properties of interfaces in the two and three dimensional Ising model

Abstract We present the phase structure of the chiral SU(2) L ⊗ SU(2) R scalar-fermion model on the lattice with on-site Yukawa coupling y and Wilson-Yukawa coupling w for positive y and w . The hopping parameter κ of the four-component scalar field of fixed length is both positive and negative. From the different behaviour of several observables ferromagnetic, paramagnetic and antiferromagnetic phases can be distinguished. They split into different regions or phases with small and large y + 4 w . A similar structure is also found in the quenched approximation. In addition, in the unquenched case a ferrimagnetic phase is found at negative κ around y + 4 w ≈ √2. We discuss fermion masses in various regions and point out the possibilities of decoupling the unwanted fermion doublers in the continuum limit in analogy to the Wilson mechanism.

Non-perturbative study of the strongly coupled scalar-fermion model

Steroid-resistant focal segmental glomerulosclerosis (FSGS) often recurs after renal transplantation. In this international survey, we sought to identify genotype-phenotype correlations of recurrent FSGS. We surveyed 83 patients with childhood-onset primary FSGS who received at least one renal allograft and analyzed 53 of these patients for NPHS2 mutations. The mean age at diagnosis was 6.7 years, and the mean age at first renal transplantation was 13 years. FSGS recurred in 30 patients (36%) after a median of 13 days (range, 1.5 to 152 days). Twenty-three patients received a second kidney transplant, and FSGS recurred in 11 (48%) after a median of 16 days (range, 2.7 to 66 days). None of the 11 patients with homozygous or compound heterozygous NPHS2 mutations developed recurrent FSGS compared with 45% of patients without mutations. These data suggest that genetic testing for pathogenic mutations may be important for prognosis and treatment of FSGS both before and after transplantation.

Decoupling Doublers of Chiral Lattice Fermions in a Quenched Fermion Higgs Model

To investigate order-order interfaces, we perform multimagnetical Monte Carlo simulations of the