http://www.physics.umn.edu/rss/ en-us http://www.physics.umn.edu/images/BrokenbarIT3.jpg http://www.physics.umn.edu http://www.physics.umn.edu/about/news/615/ by LiDong Pan, Shun Wang, C. S. Hsu, and C. C. Huang in PRL 103, 187802 (2009). Highlighted with a Synopsis on the Physics website by the editor.<br><br><div class="wikitext"><p>Free standing films of a liquid crystal compound with simple surface enhanced order were studied. The resultant phase diagram demonstrates that (1) the short helical pitch smectic-Ca phase disappears below a film thickness of 10 layers, and (2) the temperature window of a distorted 4 layer smectic-CFI2 phase increases dramatically upon decreasing film thickness. The experimental findings were attributed to the reduced dimensionality and enhanced surface effects in thin films. The results of the smectic-Ca phase are consistent with what have been reported for helically ordered magnetic thin films, with a noticeable difference due to the opposite effect of the surface on ordering in the two systems.</p> </div><br> http://www.physics.umn.edu/about/news/615/ Fri, 06 Nov 2009 00:00:00 CST http://www.physics.umn.edu/about/news/616/ by Yu Chen, S. D. Snyder, and A. M. Goldman in Physical Review Letters 103, 127002 (2009).<br><br><div class="wikitext"><p>Four-terminal resistance measurements have been carried out on Zn nanowires formed using electronbeam lithography. When driven resistive by current, these wires reenter the superconducting state upon application of small magnetic fields. The data are qualitatively different from those of previous experiments<br /> on superconducting nanowires, which revealed either negative magnetoresistance near Tc or highmagnetic-field-enhanced critical currents.</p> </div><br><br>More information: <a href="http://www.vjnano.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PRLTAO000103000012127002000001&idtype=cvips&src=vj&subid=VIRT01">http://www.vjnano.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PRLTAO000103000012127002000001&idtype=cvips&src=vj&subid=VIRT01</a> http://www.physics.umn.edu/about/news/616/ Fri, 06 Nov 2009 00:00:00 CST http://www.physics.umn.edu/about/news/624/ by Ahmad Alkhatib, Yoichi Watanabe and John H. Broadhurst in Med. Phys. 36 (8) August 2009.<br><br><div class="wikitext"><p>With the advent of therapeutic radiation treatment machines with photon end point energies of several MeV, a new channel is available to transfer the photon energy to biological material, namely, pair production. This process has a photon threshold energy of 1.02 MeV. The probability of pair production, which depends on the square of the atomic number (Z) of the interacting material, increases markedly as the photon energy is further increased. As the goal of treatment planning in radiation therapy is to locally maximize the absorbed dose in abnormal cells and minimize the dose in surrounding normal cells, in this study the authors measured the dose enhancement which could be expected if a high-Z material such as gold was present adjacent to tumor sites during irradiation. The authors used photon beams produced by electron accelerators with energies ranging from 6 to 25 MV. They chose either gold or lead foils as high-Z materials, the measurements being repeated using the same geometry but replacing the high-Z materials with a low-Z material (aluminum). The comparison of the experimental results using low- and high-Z materials verified the theoretical prediction of the expected dose enhancement. The effect of finite range of the electron-positron pairs was also studied by varying the spacing between two foils placed parallel or orthogonal to the incident photon beam. Using an 18 MV photon beam, the authors observed a maximum dose enhancement of 44%. They intend therefore to proceed from these phantom studies to animal measurements.</p> </div><br><br>More information: <a href="http://www.ncbi.nlm.nih.gov/pubmed/19746788">http://www.ncbi.nlm.nih.gov/pubmed/19746788</a> http://www.physics.umn.edu/about/news/624/ Fri, 06 Nov 2009 00:00:00 CST http://www.physics.umn.edu/about/news/614/ by G. Yu1, Y. Li1, E. M. Motoyama1 & M. Greven2,3 Nature Physics 18 October 2009<br><br><div class="wikitext"><p>Superconductivity involves the formation of electron pairs (Cooper pairs) and their condensation into a macroscopic quantum state. In conventional superconductors, such as Nb3Ge and elemental Hg, weakly interacting electrons pair through the electron–phonon interaction. In contrast, unconventional superconductivity occurs in correlated-electron materials in which electronic interactions are significant and the pairing mechanism may not be phononic. In the cuprates, the superconductivity arises on doping charge carriers into the copper–oxygen layers of antiferromagnetic Mott insulators1. Other examples of unconventional superconductors are the heavy-fermion compounds, which are metals with coupled conduction and localized f-shell electrons2, and the recently discovered iron–arsenide superconductors3. These unconventional superconductors show a magnetic resonance, a prominent collective spin-1 excitation mode in the superconducting state4, 5, 6, 7, 8. Here we demonstrate the existence of a universal linear relation, Erproportional to2Delta, between the magnetic resonance energy (Er) and the superconducting pairing gap (Delta), which spans two orders of magnitude in energy. This relationship is valid for the three different classes of unconventional superconductors, which range from being close to the Mott-insulating limit to being on the border of itinerant magnetism. As the common excitonic picture of the resonance has not led to such universality, our observation suggests a much deeper connection between antiferromagnetic fluctuations and unconventional superconductivity.</p> </div><br><br>More information: <a href="http://www.nature.com/nphys/journal/vaop/ncurrent/abs/nphys1426.html">http://www.nature.com/nphys/journal/vaop/ncurrent/abs/nphys1426.html</a> http://www.physics.umn.edu/about/news/614/ Fri, 06 Nov 2009 00:00:00 CST http://www.physics.umn.edu/about/news/618/ by K. Halterman and O.T. Valls in Phys. Rev. B 80, 104502 (2009).<br><br><div class="wikitext"><p>We study triplet pairing correlations induced in an SFS trilayer (where F is a ferromagnet and S an ordinary <i>s</i> -wave superconductor) by spin flip scattering at the interfaces, via the derivation and self consistent solution of the appropriate Bogoliubov-de Gennes equations in the clean limit. We find that the spin flip scattering generates <i>m</i> = &plusmn;1 triplet correlations, odd in time and study the general spatial behavior of these and of <i>m</i> = 0 correlations as a function of position and of spin-flip strength, <i>H</i>_{<i>s</i><i>p</i><i>i</i><i>n</i>} , concentrating on the case where the ferromagnet is half-metallic. For certain values of <i>H</i>_{<i>s</i><i>p</i><i>i</i><i>n</i>} , the triplet correlations pervade the magnetic layer and can penetrate deeply into the superconductor. The behavior we find depends very strongly on whether the singlet order parameter is in the 0 or &pi; state, which must in turn be determined self-consistently. We also present results for the density of states (DOS) and for the local magnetization, which, due to spin-flip processes, is not in general aligned with the magnetization of the half metal, and near the interfaces, rotates as a function of position and <i>H</i>_{<i>s</i><i>p</i><i>i</i><i>n</i>} . The average DOS in both F and S is shown to exhibit various subgap bound states positioned at energies that depend strongly on the particular junction state and the spin flip scattering strength.</p> </div><br><br>More information: <a href="http://arxiv.org/abs/0907.2688">http://arxiv.org/abs/0907.2688</a> http://www.physics.umn.edu/about/news/618/ Fri, 06 Nov 2009 00:00:00 CST http://www.physics.umn.edu/about/news/621/ by Chandan Dasgupta and Oriol T. Valls in Phys. Rev. E79, 016303 (2009).<br><br><div class="wikitext"><p>Motivated by many recent experimental studies of non-classical rotational inertia (NCRI) in superfluid and supersolid samples, we present a study of the hydrodynamics of a superfluid confined in the two-dimensional region (equivalent to a long cylinder) between two concentric arcs of radii <i>b</i> and <i>a</i> (<i>b</i> &lt; <i>a</i> ) subtending an angle &beta; , with 0&le;&beta;&le;2&pi; . The case &beta; = 2&pi; corresponds to a blocked ring. We discuss the methodology to compute the NCRI effects, and calculate these effects both for small angular velocities, when no vortices are present, and in the presence of a vortex. We find that, for a blocked ring, the NCRI effect is small, and that therefore there will be a large discontinuity in the moment of inertia associated with blocking or unblocking circular paths. For blocked wedges (<i>b</i> = 0 ) with &beta; &gt; &pi; , we find an unexpected divergence of the velocity at the origin, which implies the presence of either a region of normal fluid or a vortex for {\it any} nonzero value of the angular velocity. Implications of our results for experiments on "supersolid" behavior in solid ^4{\rm He} are discussed. A number of mathematical issues are pointed out and resolved.</p> </div><br><br>More information: <a href="http://arxiv.org/abs/0812.3112">http://arxiv.org/abs/0812.3112</a> http://www.physics.umn.edu/about/news/621/ Fri, 06 Nov 2009 00:00:00 CST http://www.physics.umn.edu/about/news/619/ by C. Dasgupta and O.T. Valls in Phys. Rev B 80, 094517 (2009).<br><br><div class="wikitext"><p>We study the vortex matter phase diagram of a layered superconductor in the presence of columnar pinning defects, {\it tilted} with respect to the normal to the layers. We use numerical minimization of the free energy written as a functional of the time averaged vortex density of the Ramakrishnan-Yussouff form, supplemented by the appropriate pinning potential. We study the case where the pin density is smaller than the areal vortex density</p> </div><br><br>More information: <a href="http://arxiv.org/abs/0909.0215">http://arxiv.org/abs/0909.0215</a> http://www.physics.umn.edu/about/news/619/ Fri, 06 Nov 2009 00:00:00 CST http://www.physics.umn.edu/about/news/620/ by Paul H. Barsic and Oriol T. Valls in Phys. Rev. B79, 014502 (2009)<br><br><div class="wikitext"><p>We evaluate the tunneling conductance of clean Ferromomagnet/Superconductor junctions via a fully self-consistent numerical solution of the microscopic Bogoliubov-DeGennes equations. We present results for a relevant range of values of the Fermi wavevector mismatch (FWM), the spin polarization, and the interfacial scattering strength. For nonzero spin polarization, the conductance curves vary nonmonotonically with FWM. The FWM dependence of the self-consistent results is stronger than that previously found in non-self-consistent calculations, since, in the self-consistent case, the effective scattering potential near the interface depends on the FWM. The dependence on interfacial scattering is monotonic. These results confirm that it is impossible to characterize both the the FWM and the interfacial scattering by a single effective parameter and that analysis of experimental data via the use of such one-parameter models is unreliable.</p> </div><br><br>More information: <a href="http://arxiv.org/abs/0806.3416">http://arxiv.org/abs/0806.3416</a> http://www.physics.umn.edu/about/news/620/ Fri, 06 Nov 2009 00:00:00 CST http://www.physics.umn.edu/about/news/622/ by Lei Dai in Physical Review Letters, vol. 102, Issue 24, 2009<br><br><div class="wikitext"><p>We propose an analytic approach to the problem of collisionless magnetic reconnection formulated as a process of Alfven eigenmodes’ generation and dissipation. Alfven eigenmodes are confined by the current sheet in the same way that quantum mechanical waves are confined by the tanh2 potential. The dynamical time scale of reconnection is the system scale divided by the eigenvalue propagation velocity of the n=1 mode. The prediction of the n=1 mode shows good agreement with the in situ measurement of the reconnection-associated Hall fields.</p> </div><br><br>More information: <a href="http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000102000024245003000001&idtype=cvips&gifs=yes">http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000102000024245003000001&idtype=cvips&gifs=yes</a> http://www.physics.umn.edu/about/news/622/ Fri, 06 Nov 2009 00:00:00 CST http://www.physics.umn.edu/about/news/623/ by Z. Ahmed,1 D. S. Akerib,2 S. Arrenberg,17 C. N. Bailey,2 D. Balakishiyeva,15 L. Baudis,17 D. A. Bauer,3 J. Beaty,16 P. L. Brink,9 T. Bruch,17 R. Bunker,13 B. Cabrera,9 D. O. Caldwell,13 J. Cooley,9 P. Cushman,16 F. DeJongh,3 M. R. Dragowsky,2 L. Duong,16 E. Figueroa-Feliciano,5 J. Filippini,12,1 M. Fritts,16 S. R. Golwala,1 D. R. Grant,2 J. Hall,3 R. Hennings-Yeomans,2 S. Hertel,5 D. Holmgren,3 L. Hsu,3 M. E. Huber,14 O. Kamaev,16 M. Kiveni,10 M. Kos,10 S. W. Leman,5 R. Mahapatra,11 V. Mandic,16 D. Moore<br><br><div class="wikitext"><p>We report on the first axion search results from the Cryogenic Dark Matter Search (CDMS) experiment at the Soudan Underground Laboratory. An energy threshold of 2 keV for electron-recoil events allows a search for possible solar axion conversion into photons or local galactic axion conversion into electrons in the germanium crystal detectors. The solar axion search sets an upper limit on the Primakov coupling gagammagamma of 2.4×10-9 GeV-1 at the 95% confidence level for an axion mass less than 0.1 keV/c2. This limit benefits from the first precise measurement of the absolute crystal plane orientations in this type of experiment. The galactic axion search analysis sets a world-leading experimental upper limit on the axioelectric coupling g_{a<a href="overline%20e">overline e</a>e} of 1.4×10-12 at the 90% confidence level for an axion mass of 2.5 keV/c2.</p> </div><br><br>More information: <a href="http://link.aps.org/abstract/PRL/v103/e141802">http://link.aps.org/abstract/PRL/v103/e141802</a> http://www.physics.umn.edu/about/news/623/ Fri, 06 Nov 2009 00:00:00 CST http://www.physics.umn.edu/about/news/625/ by L. B. L. B., III, C. A. Cattell, P. J. Kellogg, K. Goetz, K. Kersten, J. C. Kasper, A. Szabo, and K. Meziane (2009), , J. Geophys. Res., 114, A10106, doi:10.1029/2009JA014376.<br><br><div class="wikitext"><p>We present observations of low-frequency waves (0.25 Hz < f < 10 Hz) at five quasi-perpendicular interplanetary (IP) shocks observed by the Wind spacecraft. Four of the five IP shocks had oblique precursor whistler waves propagating at angles with respect to the magnetic field of 20°–50° and large propagation angles with respect to the shock normal; thus they do not appear to be phase standing. One event, the strongest in our study and likely supercritical, had low-frequency waves consistent with steepened magnetosonic waves called shocklets. The shocklets are seen in association with diffuse ion distributions. Both the shocklets and precursor whistlers are often seen simultaneously with anisotropic electron distributions unstable to the whistler heat flux instability. The IP shock with upstream shocklets showed much stronger electron heating across the shock ramp than the four events without upstream shocklets. These results may offer new insights into collisionless shock dissipation<br /> and wave-particle interactions in the solar wind.</p> </div><br> http://www.physics.umn.edu/about/news/625/ Fri, 06 Nov 2009 00:00:00 CST http://www.physics.umn.edu/about/news/605/ By the LIGO Scientific Collaboration and the Virgo Collaboration.<br><br><div class="wikitext"><p>A stochastic background of gravitational waves is expected to arise from a superposition of a large number of unresolved gravitational-wave sources of astrophysical and cosmological origin. It should carry unique signatures from the earliest epochs in the evolution of the Universe, inaccessible to standard astrophysical observations1. Direct measurements of the amplitude of this background are therefore of fundamental importance for understanding the evolution of the Universe when it was younger than one minute. Here we report limits on the amplitude of the stochastic gravitational-wave background using the data from a two-year science run of the Laser Interferometer Gravitational-wave Observatory2 (LIGO). Our result constrains the energy density of the stochastic gravitational-wave background normalized by the critical energy density of the Universe, in the frequency band around 100 Hz, to be <6.9 times 10-6 at 95% confidence. The data rule out models of early Universe evolution with relatively large equation-of-state parameter3, as well as cosmic (super)string models with relatively small string tension4 that are favoured in some string theory models5. This search for the stochastic background improves on the indirect limits from Big Bang nucleosynthesis1, 6 and cosmic microwave background7 at 100 Hz.</p> </div><br><br>More information: <a href="http://www.nature.com/nature/journal/v460/n7258/full/nature08278.html">http://www.nature.com/nature/journal/v460/n7258/full/nature08278.html</a> http://www.physics.umn.edu/about/news/605/ Thu, 20 Aug 2009 00:00:00 CST http://www.physics.umn.edu/about/news/491/ by P. Guazzoni, L. Zetta, A. Covello, A. Gargano, B.F. Bayman, T. Faestermann, G. Graw, R. Hertenberger, H.-F. Wirth and M. Jaskola in Physical Review C, 78, No 6. 064608<br><br><div class="wikitext"><p>Cross-section angular distributions of 38 (p, t) transitions to final states of 118Sn up to an excitation energy of 3.597 MeV have been measured in a high-resolution experiment at an incident proton energy of 21 MeV. A distorted-wave Born approximation (DWBA) analysis of the 38 experimental differential cross sections, carried out by using conventional Woods-Saxon potentials, allowed us either 18 confirmations of previous spin and parity values or new assignments of spin and parity to 14 states of 118Sn. A shell-model calculation has been performed by using a realistic two-body effective interaction derived from the CD-Bonn nucleon-nucleon potential. The doubly-magic nucleus 132Sn is assumed as a closed core, with 14 valence neutron holes occupying the five levels of the 50–82 shell. Within this model space the calculations are performed by employing the seniority scheme including states with seniority up to 4. The energy spectrum of 118Sn has been calculated and compared with the experimental one. The theoretical two-neutron spectroscopic amplitudes are used in the microscopic DWBA calculations of some cross-section angular distributions.</p> </div><br><br>More information: <a href="http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRVCAN000078000006064608000001&idtype=cvips&gifs=yes">http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRVCAN000078000006064608000001&idtype=cvips&gifs=yes</a> http://www.physics.umn.edu/about/news/491/ Mon, 29 Jun 2009 00:00:00 CST http://www.physics.umn.edu/about/news/484/ by E. S. Bowman and J. I. Kapusta in Physical Review C 79, 015202 (2009).<br><br><div class="wikitext"><p>We employ a simple effective model to study the chiral dynamics of two flavors of quarks at finite temperature and density. In particular, we determine the phase diagram in the plane of temperature and baryon chemical potential as a function of the pion mass. An interesting phase structure occurs that results in zero, one, or two critical points depending on the value of the vacuum pion mass.</p> </div><br><br>More information: <a href="http://link.aps.org/doi/10.1103/PhysRevC.79.015202">http://link.aps.org/doi/10.1103/PhysRevC.79.015202</a> http://www.physics.umn.edu/about/news/484/ Tue, 26 May 2009 00:00:00 CST http://www.physics.umn.edu/about/news/485/ by Tony Gherghetta, Joseph I. Kapusta, and Thomas M. Kelley in Physical Review D (in press.<br><br><div class="wikitext"><p>We show how to incorporate chiral symmetry breaking in the soft-wall version of the AdS/QCD model by using a modified dilaton profile and a quartic term in the bulk scalar potential. This allows one to separate the dependence on spontaneous and explicit chiral symmetry breaking. Moreover our 5D model automatically incorporates linear trajectories and non chiral-symmetry restoration for highly excited radial states. We compare our resulting mass spectra in the scalar, vector and axial-vector sectors with the respective QCD resonances and find reasonable agreement using the known values for the pion mass and decay constant.</p> </div><br><br>More information: <a href="http://arxiv.org/abs/0902.1998">http://arxiv.org/abs/0902.1998</a> http://www.physics.umn.edu/about/news/485/ Tue, 26 May 2009 00:00:00 CST