download report - Sapienza

More documents

Recommendations

Info

Scientific Report 2007-2009 Condensed matter physics and biophysics control and manipulation of individual quantum degrees of freedom. On this perspective completely new schemes of information transfer and processing, enabling new forms of communication and enhancing the computational power, are under development (C43). Quantum optics, beside providing the basis for quantum computation, is also becoming a powerful tool for experiments on foundation of quantum mechanics. Experiments on multiple qubit (i.e., multiple quantum two-level systems) generated via nonlinear optical process, performed in Rome, are contributing to demonstrate genuine entanglement and persistency of entanglement against the loss of qubits, deeply testing the Bell inequality (C45). The experimental research developed by members of the Physics Department is carried out not only in the laboratories located in Rome, but also in several international facilities (Grenoble, Trieste, Frascati). Very often, our scientists have been members of the groups which have designed and realized the beam lines of these facilities. For example, we have recently investigated the possibility to produce coherent THz radiation from our beamline SISSI (Synchrotron Source for Spectroscopy and Imaging) at the third generaton machine ELETTRA (Trieste) (C46). This range of the electromagnetic spectrum, which is roughly located between the infrared and the microwave region (0.1-20 THz), has been indeed scarcely investigated so far mainly because of the lack of intense and stable THz sources. THz radiation will disclose the ps and sub-ps scale dynamics of collective modes in superconductors and in exotic electronic materials, the ps-scale rearrangement dynamics in the secondary structure of proteins and biological macromolecules, early cancer diagnosis, and security applications. Francesco Sciortino Sapienza Università di Roma 53 Dipartimento di Fisica
Scientific Report 2007-2009 Condensed matter physics and biophysics C1. Superconductivity in low-dimensional materials Large part of the experimental and theoretical research on new superconducting (SC) materials focuses nowadays on systems that are effectively low dimensional. To this category belong layered materials with weakly-coupled planes, like the high-temperature cuprate superconductors, as well as confined 2D structures, like thin films or conducting layers at the interface of artificial heterostructures. Besides the low dimensionality, a common characteristic of many of these systems is the low superfluid density, i.e. the energy scale which controls the phase fluctuations of the SC order parameter. Under these conditions, the SC transition could share a Berezinsky-Kosterlitz-Thouless (BKT) character, where vortex excitations play a crucial role. The BKT transition has in principle very specific signatures, both below and above the SC transition temperature T BKT . For example the superfluid density J s vanishes with an “universal” jump approaching T BKT from below, while the SC correlation length ξ(T ), that is probed by several quantities like paraconductivity, diamagnetism or Nerst effect, should diverge exponentially as T → T BKT from above. This form would be in marked contrast with the typical power-law behavior of ξ(T ) due to Ginzburg-Landau (GL) SC fluctuations, where modulus and phase fluctuate simultaneously. The observation of clear signatures of BKT physics in these new systems is still debated. In cuprates contradicting conclusions emerge from different probes: while Nerst effect and diamagnetism point towards a relevance of BKT physics, no clear signature of the would be universal jump of the superfluid density has been reported. Moreover paraconductivity in underdoped compounds provides evidence of a more traditional Aslamazov-Larkin GL-behavior [1]. This scenario calls for a deepr theoretical investigation of the additional effects that can affect the “universal” character of the BKT transition, mainly related to the quasi-2D structure and/or disorder. We have investigated these issues [2-3] using the sine-Gordon description of the BKT transition, which allows us to go beyond standard results derived in the XY model. First we discussed the role played by the energy µ of the vortex core in a weaklycoupled layered system. Here two ratios are relevant: µ/J s , which fixes the temperature where vortices would like to unbind, driving the J s to zero, and J ⊥ /J s , where J ⊥ is the interlayer Josephson coupling, which tends to keep J s finite. While in the XY model µ/J s has a fixed value, we showed that in the more general case the behavior of the system strongly depend on the ratio µ/J s , the increasing of which effectively enhances J ⊥ /J s [2]. As a consequence, even though the transition can ultimatively have a BKT character (i.e. vortex excitations are relevant) the jump of the superfluid-density J s is reduced and smoothed, and does not occur any more at the “universal” temperature. A similar ’non-universal’ µ/J s dependence of the BKT transition was found in the analysis of the magnetic-field effect [3]. Indeed, we showed that the standard linear scaling of the field-induced diamagnetism M ≃ −ξ 2 (T )H above T c is restricted to a range of fields that decreases as µ/J s increases, accounting thus for the persistent non-linear effects reported in experiments in high-T c superconductors. R/R N 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 T(K) Data Hom Inhom 0.15 0.2 0.25 0.3 0.35 0.4 Figure 1: Resistivity in the presence of GL+BKT fluctuations, in the absence (Hom) and in the presence (Inhom) of inhomoegenity, compared to experimental data in SC heterostructures. [4] Interestingly, a revised approach to the BKT transition can be necessary also in the case of low-temperature superconductivity, as it is shown by our recent analysis of 2D superconducting heterostructures [4]. Our work shows that the contribution of SC BKT fluctuations to the conductivity cannot be computed neglecting the prominent role of the intrinsic sample inhomogeneity (see Fig. 1). This result could give new insight also on the nature of the superconductor-insulator transition, that in these systems can be induced in field-effect devices, which represent a promising candidate for future technological applications. References 1. S. Caprara et al., Phys. Rev. B 79, 024506 (2009). 2. L. Benfatto et al., Phys. Rev. Lett. 98, 117008 (2007). 3. L. Benfatto et al., Phys. Rev. Lett. 99, 207002 (2007). 4. L. Benfatto et al., Phys. Rev. B 80, 214506 (2009). Authors L. Benfatto 3 , S. Caprara, C. Castellani, M. Grilli http://theprestige.phys.uniroma1.it/clc/ Sapienza Università di Roma 54 Dipartimento di Fisica
Page 2 and 3:
Sapienza Università di Roma Dipart
Page 4 and 5: Scientific Report 2007-2009 Introdu
Page 10 and 11: Scientific Report 2007-2009 Organiz
Page 20 and 21: Scientific Report 2007-2009 Theoret
Page 48 and 49: Scientific Report 2007-2009 Condens
Page 102 and 103: Scientific Report 2007-2009 Particl
Page 104 and 105:
Scientific Report 2007-2009 Particl
Page 106 and 107:
Page 108 and 109:
Page 110 and 111:
Page 112 and 113:
Page 114 and 115:
Page 116 and 117:
Page 118 and 119:
Page 120 and 121:
Page 122 and 123:
Page 124 and 125:
Page 126 and 127:
Page 128 and 129:
Page 130 and 131:
2 2 2 Scientific Report 2007-2009 P
Page 132 and 133:
Page 134 and 135:
Page 136 and 137:
Page 138 and 139:
Page 140 and 141:
Page 142 and 143:
Page 144 and 145:
Scientific Report 2007-2009 Astrono
Page 146 and 147:
Page 148 and 149:
Page 150 and 151:
Page 152 and 153:
Page 154 and 155:
Page 156 and 157:
Page 158 and 159:
Page 160 and 161:
Page 162 and 163:
Page 164 and 165:
Page 166 and 167:
Scientific Report 2007-2009 Geophys
Page 168 and 169:
Page 170 and 171:
Page 172 and 173:
Scientific Report 2007-2009 History
Page 174 and 175:
Scientific Report 2007-2009 History
Page 176 and 177:
Scientific Report 2007-2009 Laborat
Page 178 and 179:
Page 180 and 181:
Page 182 and 183:
Page 184 and 185:
Page 186 and 187:
Page 188 and 189:
Page 190 and 191:
Page 192 and 193:
Page 194 and 195:
Page 196 and 197:
Page 198 and 199:
Page 200 and 201:
Page 202 and 203:
Scientific Report 2007-2009 Grants
Page 204 and 205:
Page 206 and 207:
Page 208 and 209:
Page 210 and 211:
Scientific Report 2007-2009 Dissemi
Page 212 and 213:
Page 214 and 215:
Page 216 and 217:
Page 218 and 219:
Page 220 and 221:
Page 222 and 223:
Page 224 and 225:
Page 226 and 227:
Page 228 and 229:
Page 230 and 231:
Page 232 and 233:
Page 234 and 235:
Page 236 and 237:
Page 238 and 239:
Page 240 and 241:
Page 242 and 243:
Page 244 and 245:
Page 246 and 247:
Page 248 and 249:
Page 250 and 251:
Page 252 and 253:
Page 254 and 255:
Page 256 and 257:
Page 258 and 259:
Page 260:
show all

download report - Sapienza

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?