Metastability of (d+n)-dimensional elastic manifolds

We investigate the depinning of a massive elastic manifold with $d$ internal dimensions, embedded in a $(d+n)$-dimensional space, and subject to an isotropic pinning potential $V({\bf u})=V(|{\bf u}|).$ The tunneling process is driven by a small external force ${\bf F}.$ We find the zero temperature and high temperature instantons and show that for the case $1\le d\le 6$ the problem exhibits a sharp transition from quantum to classical behavior: At low temperatures $T<T_{c}$ the Euclidean action is constant up to exponentially small corrections, while for $T> T_{c},$ ${S_{\rm Eucl}(d,T)}/{\hbar} = {U(d)}/{T}.$ The results are universal and do not depend on the detailed shape of the trapping potential $V({\bf u})$. Possible applications of the problem to the depinning of vortices in high-$T_{c}$ superconductors and nucleation in $d$-dimensional phase transitions are discussed. In addition, we determine the high-temperature asymptotics of the preexponential factor for the $(1+1)$-dimensional problem.Comment: RevTeX, 10 pages, 3 figures inserte

Thermally activated Hall creep of flux lines from a columnar defect

We analyse the thermally activated depinning of an elastic string (line tension $\epsilon$) governed by Hall dynamics from a columnar defect modelled as a cylindrical potential well of depth $V_{0}$ for the case of a small external force $F.$ An effective 1D field Hamiltonian is derived in order to describe the 2D string motion. At high temperatures the decay rate is proportional to $F^{{5}/{2}}T^{-{1}/{2}} \exp{\left [{F_{0}}/{F}-{U(F)}/{T}\right ]},$ with $F_{0}$ a constant of order of the critical force and U(F) \sim{\left ({\epsilon V_{0}})}^{{1}/{2}}{V_{0}/{F}} the activation energy. The results are applied to vortices pinned by columnar defects in superclean superconductors.Comment: 12 pages, RevTeX, 2 figures inserte

Free-energy distribution functions for the randomly forced directed polymer

We study the $1+1$-dimensional random directed polymer problem, i.e., an elastic string $\phi(x)$ subject to a Gaussian random potential $V(\phi,x)$ and confined within a plane. We mainly concentrate on the short-scale and finite-temperature behavior of this problem described by a short- but finite-ranged disorder correlator $U(\phi)$ and introduce two types of approximations amenable to exact solutions. Expanding the disorder potential $V(\phi,x) \approx V_0(x) + f(x) \phi(x)$ at short distances, we study the random force (or Larkin) problem with $V_0(x) = 0$ as well as the shifted random force problem including the random offset $V_0(x)$; as such, these models remain well defined at all scales. Alternatively, we analyze the harmonic approximation to the correlator $U(\phi)$ in a consistent manner. Using direct averaging as well as the replica technique, we derive the distribution functions ${\cal P}_{L,y}(F)$ and ${\cal P}_L(F)$ of free energies $F$ of a polymer of length $L$ for both fixed ($\phi(L) = y$) and free boundary conditions on the displacement field $\phi(x)$ and determine the mean displacement correlators on the distance $L$. The inconsistencies encountered in the analysis of the harmonic approximation to the correlator are traced back to its non-spectral correlator; we discuss how to implement this approximation in a proper way and present a general criterion for physically admissible disorder correlators $U(\phi)$.Comment: 16 pages, 5 figure

Quantum depinning of a pancake-vortex from a columnar defect

We consider the problem of the depinning of a weakly driven ($F\ll F_{c}$) pancake vortex from a columnar defect in a Josephson-coupled superconductor, where $F$ denotes the force acting on the vortex ($F_{c}$ is the critical force). The dynamics of the vortex is supposed to be of the Hall type. The Euclidean action $S_{Eucl}(T)$ is calculated in the entire temperature range; the result is universal and does not depend on the detailed form of the pinning potential. We show that the transition from quantum to classical behavior is second-order like with the temperature $T_{c}$ of the transition scaling like $F^{{4}/{3}}.$ Special attention is paid to the regime of applicability of our results, in particular, the influence of the large vortex mass appearing in the superclean limit is discussed.Comment: 11 pages, RevTeX, 4 figures inserte

Chemostratigraphy of Neoproterozoic carbonates: implications for 'blind dating'

The delta C-13(carb) and Sr-87/Sr-86 secular variations in Neoproteozoic seawater have been used for the purpose of 'isotope stratigraphy' but there are a number of problems that can preclude its routine use. In particular, it cannot be used with confidence for 'blind dating'. The compilation of isotopic data on carbonate rocks reveals a high level of inconsistency between various carbon isotope age curves constructed for Neoproteozoic seawater, caused by a relatively high frequency of both global and local delta C-13(carb) fluctuations combined with few reliable age determinations. Further complication is caused by the unresolved problem as to whether two or four glaciations, and associated negative delta C-13(carb) excursions, can be reliably documented. Carbon isotope stratigraphy cannot be used alone for geological correlation and 'blind dating'. Strontium isotope stratigraphy is a more reliable and precise tool for stratigraphic correlations and indirect age determinations. Combining strontium and carbon isotope stratigraphy, several discrete ages within the 590-544 Myr interval, and two age-groups at 660-610 and 740-690 Myr can be resolved

Exact free energy distribution function of a randomly forced directed polymer

We study the elastic (1+1)-dimensional string subject to a random gaussian potential on scales smaller than the correlation radius of the disorder potential (Larkin problem). We present an exact calculation of the probability function ${\cal P} [F(u,L)]$ for the free energy $F$ of a string starting at $(0,0)$ and ending at $(u,L)$. The function ${\cal P}(F)$ is strongly asymmetric, with the left tail decaying exponentially ($\ln {\cal P}(F\to-\infty)\propto F$) and the right tail vanishing as $\ln {\cal P}(F\to +\infty)\propto -F^{3}$. Our analysis defines a strategy for future attacks on this class of problems.Comment: RevTeX, 4 pages, 1 figure inserte

EFFECTIVENESS OF RADIOFREQUENCY DENERVATION FOR PAIN RELIEF IN HIP DEGENERATIVE DISEASES

Objective - to compare the effectiveness of radiofrequency denervation (RFD) and conservative treatment of patients with hip degenerative diseases. Material and methods. 66 patients with various stages of hip degeneration were followed up for 12 months, including 36 patients who underwent RFD and 30 patients of control group with conservative treatment. Treatment results were assessed using Visual Analogue Scale (VAS) and Harris Hip Score before treatment, during the first day, 2 days later, as well as at 1, 6 and 12 months. Results: RFD patients demonstrated significantly better results compared with conservative group. Patients with coxarthrosis stage I and II, besides higher efficiency had prolonged period of remission, while patients with coxarthrosis stage III returned to the near-baseline level of pain only in 3-6 months. Conclusions: RFD is an effective treatment modality for patients with initial coxarthrosis stages, which helps to reduce significantly the disability period compared with conservative therapy. RFD is the only method of effective hip pain relieve in patients with severe comorbidities allowing to reduce analgesic consumption. This method is minimally invasive, has low cost, permits repeated procedures, making it attractive for both physicians and patients

Metastability in Josephson transmission lines

Thermal activation and macroscopic quantum tunneling in current-biased discrete Josephson transmission lines are studied theoretically. The degrees of freedom under consideration are the phases across the junctions which are coupled to each other via the inductances of the system. The resistively shunted junctions that we investigate constitute a system of N interacting degrees of freedom with an overdamped dynamics. We calculate the decay rate within exponential accuracy as a function of temperature and current. Slightly below the critical current, the decay from the metastable state occurs via a unique ("rigid") saddlepoint solution of the Euclidean action describing the simultaneous decay of the phases in all the junctions. When the current is reduced, a crossover to a regime takes place, where the decay occurs via an "elastic" saddlepoint solution and the phases across the junctions leave the metastable state one after another. This leads to an increased decay rate compared with the rigid case both in the thermal and the quantum regime. The rigid-to-elastic crossover can be sharp or smooth analogous to first- or second- order phase transitions, respectively. The various regimes are summarized in a current-temperature decay diagram.Comment: 11 pages, RevTeX, 3 PS-figures, revised versio

Crossover from thermal hopping to quantum tunneling in Mn_{12}Ac

The crossover from thermal hopping to quantum tunneling is studied. We show that the decay rate $\Gamma$ with dissipation can accurately be determined near the crossover temperature. Besides considering the Wentzel-Kramers-Brillouin (WKB) exponent, we also calculate contribution of the fluctuation modes around the saddle point and give an extended account of a previous study of crossover region. We deal with two dangerous fluctuation modes whose contribution can't be calculated by the steepest descent method and show that higher order couplings between the two dangerous modes need to be taken into considerations. At last the crossover from thermal hopping to quantum tunneling in the molecular magnet Mn_{12}Ac is studied.Comment: 10 pages, 3 figure

Quantum-Classical Phase Transition of Escape rate in Biaxial Spin Particles

The escape rates of the biaxial single domain spin particles with and without an applied magnetic field are investigated. Using the strict potential field description of spin systems developed by Ulyanov and Zaslavskii we obtain new effective Hamiltonians which are considered to be in exact spin-coordinate correspondence unlike the well studied effective Hamiltonians with the approximate correspondence. The sharp first-order transition is found in both cases. The phase diagram of the transitions depending on the anisotropy constant and the external field is also given.Comment: 15 pages, 8 figure