Quantum effects in a superconducting glass model

We study disordered Josephson junctions arrays with long-range interaction and charging effects. The model consists of two orthogonal sets of positionally disordered $N$ parallel filaments (or wires) Josephson coupled at each crossing and in the presence of a homogeneous and transverse magnetic field. The large charging energy (resulting from small self-capacitance of the ultrathin wires) introduces important quantum fluctuations of the superconducting phase within each filament. Positional disorder and magnetic field frustration induce spin-glass like ground state, characterized by not having long-range order of the phases. The stability of this phase is destroyed for sufficiently large charging energy. We have evaluated the temperature vs charging energy phase diagram by extending the methods developed in the theory of infinite-range spin glasses, in the limit of large magnetic field. The phase diagram in the different temperature regimes is evaluated by using variety of methods, to wit: semiclassical WKB and variational methods, Rayleigh-Schr\"{o}dinger perturbation theory and pseudospin effective Hamiltonians. Possible experimental consequences of these results are briefly discussed.Comment: 17 pages REVTEX. Two Postscript figures can be obtained from the authors. To appear in PR

Reentrant phase transitions involving glassy and superfluid orders in the random hopping Bose-Hubbard model

We study a system of strongly correlated bosons with off-diagonal disorder, i.e., randomness in the kinetic energy, and find a family of reentrant phase transitions that occur as a function of the on-site interaction. We model the system using the paradigmatic Bose-Hubbard Hamiltonian with a random hopping term and solve it employing the replica trick and Trotter-Suzuki expansion known from quantum spin-glasses. From subsequent numerical calculations, we find three distinct phase boundaries at which the reentrant transitions occur: between glass and disordered phase, between superglass and superfluid ones, and between superfluid and disordered phases. All three happen at temperatures slightly above critical temperatures of corresponding non-interacting systems. When the emerging and disappearing order is glassy, this corresponds to the interplay of the thermal energy and the spread of hoppings. When superfluidity is involved, thermal fluctuations must slightly overcome the mean hopping in turn for the reentrance to occur.Comment: 9 pages, 4 figure

42P Zastosowanie ultrasonografii w diagnostyce układu chłonnego szyi u chorych z nowotworami głowy i szyi

Układ chłonny szyi jest najważniejszą droga szerzenia się komórek nowotworowych raków głowy i szyi a przerzuty do regionalnych węzłów chłonnych najczęstszym żródłem niepowodzeń leczenia onkologicznego. Stąd niezwykle ważna jest dokładna ocena układu chłonnego szyi zarówno w okresie przedoperacyjnym jak i podczas kontroli ambulatoryjnych po zakończeniu leczenia. Ultrasonografia jest powszechnie znaną i szeroko stosowaną metodą badawczą we wszystkich niemal dziedzinach medycyny.Celem badań było wykazanie wyższości ultrasonografii nad badaniem palpacyjnym w diagnostyce przedoperacyjnej i w monitorowaniu układu chłonnego chorych operowanych i napromienianych z powodu nowotworów głowy i szyi. US przedoperacyjna pozwala na wykrycie palpacyjne niewyczuwalnych węzłów, ocenę rozległości i topografii guzów wyczuwalnych oraz ich stosunku do ważnych struktur szyi, pozwalające tym samym na uściślenie cechy N i planowania zabiegu operacyjnego lub radioterapii. kolejnym niezwykle ważnym zadaniem jest monitorowanie i stała kontrola układu chłonnego szyi u chorych po zakończonym leczeniu chirurgicznym lub skojarzonym. Dotychczas chorzy z póżnymi przerzutami lub wznowa na szyi zgłaszali się do kontroli z bardzo zaawansowanymi zmianami, dlatego u niewielkiego odsetka spośród nich (30%) można było opanować proces nowotworowy ponownym zabiegiem chirurgicznym.Określano wartość kliniczną wczesnego wykrycia zmian przerzutowych, co w praktyce oznacz możliwość zastosowania bardziej oszczędnych typów reoperacji przy równoczesnym wzroście odsetka zabiegów radykalnych bądź zastosowaniu radioterapii.MateriałU 1479 chorych leczonych w Klinice Otolaryngologii AM w Poznaniu w latach 1990–1997 przed zabiegiem operacyjnym wykonywano badania ultrasonograficzne szyi. Przeprowadzono następnie konfrontację sonograficzno-śródoperacyjną i sonograficzno-histologiczną wykrytych węzłów chłonnych. Korelacje były rzędu odpowiednio 92% i 85%. Wszystkich chorych poddawano badaniom kontrolnym. U chorych zgłaszających się do kontroli w odstępach 1–2 miesięcznych przeprowadzono badanie laryngologiczne, badanie palpacyjne oraz ultrasonograficzne szyi. Przeprowadzono blisko 7000 kontrolnych badań USG. U 177 chorych stwierdzono wznowę w węzłach chłonnych i poddano ich reoperacji. U 79 chorych zmiany na szyi były palpacyjnie nie wyczuwalne, u 48 bardzo dyskretne, trudne do zróżnicowania klinicznego z bliznami lub obrzękiem popromiennym. W tej grupie 127 chorych ze zmianami subkliniczmymi u 100 potwierdzono przerzuty badaniem histologicznym. Przeprowadzone reoperacje szyi były radykalne u 142 chorych (80%), w tym u 99% chorych z grupy subklinicznej, wykrytej sonograficznie

Nexus between quantum criticality and the chemical potential pinning in high-TcT_c cuprates

For strongly correlated electrons the relation between total number of charge carriers $n_e$ and the chemical potential $\mu$ reveals for large Coulomb energy the apparently paradoxical pinning of $\mu$ within the Mott gap, as observed in high-$T_c$ cuprates. By unravelling consequences of the non-trivial topology of the charge gauge U(1) group and the associated ground state degeneracy we found a close kinship between the pinning of $\mu$ and the zero-temperature divergence of the charge compressibility $\kappa\sim\partial n_e/\partial\mu$, which marks a novel quantum criticality governed by topological charges rather than Landau principle of the symmetry breaking.Comment: 4+ pages, 2 figures, typos corrected, version as publishe

A generalized spherical version of the Blume-Emery-Griffits model with ferromagnetic and antiferromagnetic interactions

We have investigated analitycally the phase diagram of a generalized spherical version of the Blume-Emery-Griffiths model that includes ferromagnetic or antiferromagnetic spin interactions as well as quadrupole interactions in zero and nonzero magnetic field. We show that in three dimensions and zero magnetic field a regular paramagnetic-ferromagnetic (PM-FM) or a paramagnetic-antiferromagnetic (PM-AFM) phase transition occurs whenever the magnetic spin interactions dominate over the quadrupole interactions. However, when spin and quadrupole interactions are important, there appears a reentrant FM-PM or AFM-PM phase transition at low temperatures, in addition to the regular PM-FM or PM-AFM phase transitions. On the other hand, in a nonzero homogeneous external magnetic field $H$, we find no evidence of a transition to the state with spontaneous magnetization for FM interactions in three dimensions. Nonethelesss, for AFM interactions we do get a scenario similar to that described above for zero external magnetic field, except that the critical temperatures are now functions of $H$. We also find two critical field values, $H_{c1}$, at which the reentrance phenomenon dissapears and $H_{c2}$ ($H_{c1}\approx 0.5H_{c2}$), above which the PM-AFM transition temperature vanishes.Comment: 21 pages, 6 figs. Title changed, abstract and introduction as well as section IV were rewritten relaxing the emphasis on spin S=1 and Figs. 5 an 6 were improved in presentation. However, all the results remain valid. Accepted for publication in Phys. Rev.

Local dissipation effects in two-dimensional quantum Josephson junction arrays with magnetic field

We study the quantum phase transitions in two-dimensional arrays of Josephson-couples junctions with short range Josephson couplings (given by the Josephson energy) and the charging energy. We map the problem onto the solvable quantum generalization of the spherical model that improves over the mean-field theory method. The arrays are placed on the top of a two-dimensional electron gas separated by an insulator. We include effects of the local dissipation in the presence of an external magnetic flux f in square lattice for several rational fluxes f=0,1/2,1/3,1/4 and 1/6. We also have examined the T=0 superconducting-insulator phase boundary as function of a dissipation alpha for two different geometry of the lattice: square and triangular. We have found critical value of the dissipation parameter independent on geometry of the lattice and presence magnetic field.Comment: accepted to PR

Three-dimensional Josephson-junction arrays in the quantum regime

We study the quantum phase transition properties of a three-dimensional periodic array of Josephson junctions with charging energy that includes both the self and mutual junction capacitances. We use the phase fluctuation algebra between number and phase operators, given by the Euclidean group E_2, and we effectively map the problem onto a solvable quantum generalization of the spherical model. We obtain a phase diagram as a function of temperature, Josephson coupling and charging energy. We also analyze the corresponding fluctuation conductivity and its universal scaling form in the vicinity of the zero-temperature quantum critical point.Comment: 9 pages, LATEX, three PostScript figures. Submitted to Phys. Rev. Let

Finite-temperature effects on the superfluid Bose-Einstein condensation of confined ultracold atoms in three-dimensional optical lattices

We discuss the finite-temperature phase diagram in the three-dimensional Bose-Hubbard (BH) model in the strong correlation regime, relevant for Bose-Einstein condensates in optical lattices, by employing a quantum rotor approach. In systems with strong on site repulsive interactions, the rotor U(1) phase variable dual to the local boson density emerges as an important collective field. After establishing the connection between the rotor construction and the the on--site interaction in the BH model the robust effective action formalism is developed which allows us to study the superfluid phase transition in various temperature--interaction regimes

Neel Order and Electron Spectral Functions in the Two-Dimensional Hubbard Model: a Spin-Charge Rotating Frame Approach

Using recently developed quantum SU(2)xU(1) rotor approach, that provides a self-consistent treatment of the antiferromagnetic state we have performed electronic spectral function calculations for the Hubbard model on the square lattice. The collective variables for charge and spin are isolated in the form of the space-time fluctuating U(1) phase field and rotating spin quantization axis governed by the SU(2) symmetry, respectively. As a result interacting electrons appear as composite objects consisting of bare fermions with attached U(1) and SU(2) gauge fields. This allows us to write the fermion Green's function in the space-time domain as the product CP^1 propagator resulting from the SU(2) gauge fields, U(1) phase propagator and the pseudo-fermion correlation function. As a result the problem of calculating the spectral line shapes now becomes one of performing the convolution of spin, charge and pseudo-fermion Green's functions. The collective spin and charge fluctuations are governed by the effective actions that are derived from the Hubbard model for any value of the Coulomb interaction. The emergence of a sharp peak in the electron spectral function in the antiferromagnetic state indicates the decay of the electron into separate spin and charge carrying particle excitations.Comment: 16 pages, 5 figures, submitted to Phys. Rev.