Identifying Dark Matter Burners in the Galactic center

If the supermassive black hole (SMBH) at the center of our Galaxy grew adiabatically, then a dense "spike" of dark matter is expected to have formed around it. Assuming that dark matter is composed primarily of weakly interacting massive particles (WIMPs), a star orbiting close enough to the SMBH can capture WIMPs at an extremely high rate. The stellar luminosity due to annihilation of captured WIMPs in the stellar core may be comparable to or even exceed the luminosity of the star due to thermonuclear burning. The model thus predicts the existence of unusual stars, i.e. "WIMP burners", in the vicinity of an adiabatically grown SMBH. We find that the most efficient WIMP burners are stars with degenerate electron cores, e.g. white dwarfs (WD) or degenerate cores with envelopes. If found, such stars would provide evidence for the existence of particle dark matter and could possibly be used to establish its density profile. In our previous paper we computed the luminosity from WIMP burning for a range of dark matter spike density profiles, degenerate core masses, and distances from the SMBH. Here we compare our results with the observed stars closest to the Galactic center and find that they could be consistent with WIMP burners in the form of degenerate cores with envelopes. We also cross-check the WIMP burner hypothesis with the EGRET observed flux of gamma-rays from the Galactic center, which imposes a constraint on the dark matter spike density profile and annihilation cross-section. We find that the EGRET data is consistent with the WIMP burner hypothesis. New high precision measurements by GLAST will confirm or set stringent limits on a dark matter spike at the Galactic center, which will in turn support or set stringent limits on the existence of WIMP burners at the Galactic center.Comment: 2 pages, 2 figures; to appear in the Proc. of the First Int. GLAST Symp. (Stanford, Feb. 5-8, 2007), eds. S.Ritz, P.F.Michelson, and C.Meegan, AIP Conf. Pro

Is the term "type-1.5 superconductivity" warranted by Ginzburg-Landau theory?

It is shown that within the Ginzburg-Landau (GL) approximation the order parameters Delta1(r, T) and Delta2(r, T) in two-band superconductors vary on the same length scale, the difference in the zero-T coherence lengths xi0_i ~vF_i/Delta_i(0), i = 1, 2 notwithstanding. This amounts to a single physical GL parameter kappa and the classic GL dichotomy: kappa < 1/sqrt(2) for type-I and kappa > 1/sqrt(2) for type-II.Comment: 5 pages, revised and extended version; previous title "Two-band superconductors near Tc" change

Detecting Bose-Einstein condensation of exciton-polaritons via electron transport

We examine the Bose-Einstein condensation of exciton-polaritons in a semiconductor microcavity via an electrical current. We propose that by embedding a quantum dot p-i-n junction inside the cavity, the tunneling current through the device can reveal features of condensation due to a one-to-one correspondence of the photons to the condensate polaritons. Such a device can also be used to observe the phase interference of the order parameters from two condensates.Comment: 5 Pages, 3 Figure

Diagrammatic theory for Periodic Anderson Model: Stationary property of the thermodynamic potential

Diagrammatic theory for Periodic Anderson Model has been developed, supposing the Coulomb repulsion of $f-$ localized electrons as a main parameter of the theory. $f-$ electrons are strongly correlated and $c-$ conduction electrons are uncorrelated. Correlation function for $f-$ and mass operator for $c-$ electrons are determined. The Dyson equation for $c-$ and Dyson-type equation for $f-$ electrons are formulated for their propagators. The skeleton diagrams are defined for correlation function and thermodynamic functional. The stationary property of renormalized thermodynamic potential about the variation of the mass operator is established. The result is appropriate as for normal and as for superconducting state of the system.Comment: 12 pages, 10 figure

Time-reversal symmetry breaking state near the surface of s±s_{\pm}-superconductor

The structure of superconducting order parameter near the surface of a two-band superconductor with $s_{\pm}$ order parameter in the bulk is theoretically investigated. The main parameter of the surface, which determines the appropriate physics is the coefficient of the interband scattering $R_{12}$. For small $R_{12}$ the superconducting order parameter is only suppressed to some extent near the surface for the both bands. For intermediate and strong interband scattering there are two possible non-trivial surface states of the order parameter: (i) purely real solution, where the symmetry of the superconducting state near the surface is changed from $s_{\pm}$ to conventional $s_{++}$ and (ii) time-reversal symmetry breaking (TRB) state. In this state the order parameters in the two bands acquire phases $\phi_{1,2}(x) \neq (0,\pi)$ upon approaching the surface. We argue that at low temperatures the TRB surface state can be more energetically favorable than the $s_{\pm} \to s_{++}$ time reversal symmetry conserving state (TR). For higher temperatures up to $T_c$ only the TR state can exist. The transition between the two temperature regions is rather sharp. Signatures of the transition between the TRB and the TR surface states can be detected by the measurements of the local density of states and the angle-resolved density of states.Comment: 9 pages, 9 figures. arXiv admin note: substantial text overlap with arXiv:1003.277

Excitonic BCS-BEC crossover at finite temperature: Effects of repulsion and electron-hole mass difference

The BCS to Bose-Einstein condensation (BEC) crossover of electron-hole (e-h) pairs in optically excited semiconductors is studied using the two-band Hubbard model with both repulsive and attractive interactions. Applying the self-consistent t-matrix approximation combined with a local approximation, we examine the properties of a normal phase and an excitonic instability. The transition temperature from the normal phase to an e-h pair condensed one is studied to clarify the crossover from an e-h BCS-like state to an excitonic Bose-Einstein condensation, which takes place on increasing the e-h attraction strength. To investigate effects of the repulsive interaction and the e-h mass difference, we calculate the transition temperature for various parameters of the interaction strengths, the e-h particle density, and the mass difference. While the transition temperature in the e-h BCS regime is sufficiently suppressed by the repulsive interaction, that of the excitonic BEC is largely insensitive to it. We also show quantitatively that in the whole regime the mass difference leads to large suppression of the transition temperature.Comment: 8 pages, 7 figures, to be published in Phys. Rev.

Two-band superconductors: Hidden criticality deep in the superconducting state

We show that two-band superconductors harbor hidden criticality deep in the superconducting state, stemming from the critical temperature of the weaker band taken as an independent system. For sufficiently small interband coupling $\gamma$ the coherence length of the weaker band exhibits a remarkable deviation from the conventional monotonic increase with temperature, namely, a pronounced peak close to the hidden critical point. The magnitude of the peak scales proportionally to \gamma^(-\mu), with the Landau critical exponent \mu = 1/3, the same as found for the mean-field critical behavior with respect to the source field in ferromagnets and ferroelectrics. Here reported hidden criticality of multi-band superconductors can be experimentally observed by, e.g., imaging of the variations of the vortex core in a broader temperature range. Similar effects are expected for the superconducting multilayers.Comment: 6 pages, 2 figures, Supplementary material included. Accepted for publication in PR