Dwarf Novae in the Shortest Orbital Period Regime: II. WZ Sge Stars as the Missing Population near the Period Minimum

WZ Sge-type dwarf novae are characterized by long recurrence times of outbursts (~10 yr) and short orbital periods (<~ 85 min). A significant part of WZ Sge stars may remain undiscovered because of low outburst activity. Recently, the observed orbital period distribution of cataclysmic variables (CVs) has changed partly because outbursts of new WZ Sge stars have been discovered routinely. Hence, the estimation of the intrinsic population of WZ Sge stars is important for the study of the population and evolution of CVs. In this paper, we present a Bayesian approach to estimate the intrinsic period distribution of dwarf novae from observed samples. In this Bayesian model, we assumed a simple relationship between the recurrence time and the orbital period which is consistent with observations of WZ Sge stars and other dwarf novae. As a result, the minimum orbital period was estimated to be ~70 min. The population of WZ Sge stars exhibited a spike-like feature at the shortest period regime in the orbital period distribution. These features are consistent with the orbital period distribution previously predicted by population synthesis studies. We propose that WZ Sge stars and CVs with a low mass-transfer rate are excellent candidates for the missing population predicted by the evolution theory of CVs.Comment: 9 pages, 5 figures, accepted for publication in PAS

Study of the in-plane magnetic penetration depth in the cuprate superconductor Ca_2-xNa_xCuO_2Cl_2: role of the apical sites

A study of the in-plane magnetic penetration depth \lambda_ab in a series of the cuprate superconductors Ca_2-xNa_xCuO_2Cl_2 (Na-CCOC) with Na content x=0.11, 0.12, 0.15, 0.18, and 0.19 is reported. The zero temperature values of \lambda_ab(0) were obtained by means of the muon-spin rotation technique, as well as from measurements of the intrinsic susceptibility \chi^int(0) by using the procedure developed by Kanigel et al. [Phys.Rev.B 71, 224511 (2005)]. \lambda_ab at T=0K was found to increase with decreasing doping from \lambda_ab(0)=316(19)nm for the x=0.19 sample to \lambda_ab(0)=430(26)nm for the x=0.11 one. From a comparison of the present Na-CCOC data with those of Bi2201 and La214 cuprate superconductors it is concluded that substitution of the apical oxygen by chlorine decreases the coupling between the superconducting CuO_2 planes, leading to an enhancement of the two-dimensional properties of Na-CCOC.Comment: 8 pages, 7 figure

Magnetic Phase Diagram of the Hole-doped Ca2−x_{2-x}Nax_{x}CuO2_{2}Cl2_{2} Cuprate Superconductor

We report on the magnetic phase diagram of a hole-doped cuprate Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_{2}$, which is free from buckling of CuO$_2$ planes, determined by muon spin rotation and relaxation. It is characterized by a quasi-static spin glass-like phase over a range of sodium concentration ($0.05\leq x\leq 0.12$), which is held between long range antiferromagnetic (AF) phase ($x\leq 0.02$) and superconducting phase where the system is non-magnetic for $x\geq 0.15$. The obtained phase diagram qualitatively agrees well with that commonly found for hole-doped high-\tc cuprates, strongly suggesting that the incomplete suppression of the AF order for $x>0.02$ is an essential feature of the hole-doped cuprates.Comment: 5 pages, submitted to Phys. Rev. Let

Muon spin relaxation studies of incommensurate magnetism and superconductivity in stage-4 La2_{2}CuO4.11_{4.11} and La1.88_{1.88}Sr0.12_{0.12}CuO4_{4}

This paper reports muon spin relaxation (MuSR) measurements of two single crystals of the title high-Tc cuprate systems where static incommensurate magnetism and superconductivity coexist. By zero-field MuSR measurements and subsequent analyses with simulations, we show that (1) the maximum ordered Cu moment size (0.36 Bohr magneton) and local spin structure are identical to those in prototypical stripe spin systems with the 1/8 hole concentration; (2) the static magnetism is confined to less than a half of the volume of the sample, and (3) regions with static magnetism form nano-scale islands with the size comparable to the in-plane superconducting coherence length. By transverse-field MuSR measurements, we show that Tc of these systems is related to the superfluid density, in the same way as observed in cuprate systems without static magnetism. We discuss a heuristic model involving percolation of these nanoscale islands with static magnetism as a possible picture to reconcile heterogeneity found by the present MuSR study and long-range spin correlations found by neutron scattering.Comment: 19 pages, 15 figures, submitted to Phys. Rev. B. E-mail: [email protected]

Photometry of VS0329+1250: A New, Short-Period SU Ursae Majoris Star

Time-resolved CCD photometry is presented of the recently-discovered (V~15 at maximum light) eruptive variable star in Taurus, which we dub VS0329+1250. A total of ~20 hr of data obtained over six nights reveals superhumps in the light curves, confirming the star as a member of the SU UMa class of dwarf novae. The superhumps recur with a mean period of 0.053394(7) days (76.89 min), which represents the shortest superhump period known in a classical SU UMa star. A quadratic fit to the timings of superhump maxima reveals that the superhump period was increasing at a rate given by dP/dt ~ (2.1 +/- 0.8) x 10^{-5} over the course of our observations. An empirical relation between orbital period and the absolute visual magnitude of dwarf novae at maximum light, suggests that VS0329+1250 lies at a distance of ~1.2 +/- 0.2 kpc.Comment: V2 - The paper has been modified to incorporate the referee's comments, and has now been accepted for publication in the PASP. The most significant change is that we are now able to confirm that the superhump period was increasing during the course of our observation

Emergent states in heavy electron materials

We obtain the conditions necessary for the emergence of various low temperature ordered states (local moment antiferromagnetism, unconventional superconductivity, quantum criticality, and Landau Fermi liquid behavior) in Kondo lattice materials by extending the two-fluid phenomenological theory of heavy electron behavior to incorporate the concept of hybridization effectiveness. We use this expanded framework to present a new phase digram and consistent physical explanation and quantitative description of measured emergent behaviors such as the pressure variation of the onset of local moment antiferromagnetic ordering at T_N, the magnitude of the ordered moment, the growth of superconductivity within that ordered state, the location of a quantum critical point, and of a delocalization line in the pressure/temperature phase diagram at which local moments have disappeared and the heavy electron Fermi surface has grown to its maximum size. We apply our model to CeRhIn_5 and a number of other heavy electron materials and find good agreement with experiment.Comment: 20 pages, 8 figures, 1 tabl

Application of Total Variation Minimization to Doppler Tomography

We have developed a new model of the Doppler tomography using total variation minimization (DTTVM). We demonstrated that this method can reconstruct localized and non-axisymmetric profiles possibly having sharp edges in the Doppler map. We apply this model to the real data of the dwarf nova, WZ Sge in superoutburst. DTTVM can reproduce the observed spectra with a high precision, while the previous models fail to reproduce localized sources

Quantum interference between charge excitation paths in a solid state Mott insulator

The competition between electron localization and de-localization in Mott insulators underpins the physics of strongly-correlated electron systems. Photo-excitation, which re-distributes charge between sites, can control this many-body process on the ultrafast timescale. To date, time-resolved studies have been performed in solids in which other degrees of freedom, such as lattice, spin, or orbital excitations come into play. However, the underlying quantum dynamics of bare electronic excitations has remained out of reach. Quantum many-body dynamics have only been detected in the controlled environment of optical lattices where the dynamics are slower and lattice excitations are absent. By using nearly-single-cycle near-IR pulses, we have measured coherent electronic excitations in the organic salt ET-F2TCNQ, a prototypical one-dimensional Mott Insulator. After photo-excitation, a new resonance appears on the low-energy side of the Mott gap, which oscillates at 25 THz. Time-dependent simulations of the Mott-Hubbard Hamiltonian reproduce the oscillations, showing that electronic delocalization occurs through quantum interference between bound and ionized holon-doublon pairs.Comment: 4 figures and supplementary informatio

Photometric Observations of an SU UMa-type Dwarf Nova VW Coronae Borealis during Outbursts

We report the photometric observations of an SU UMa-type dwarf nova VW CrB during two superoutbursts in 2001 and 2003 and a normal outburst in 2003. Superhumps with a period of 0.07287(1) d were observed during the 2003 superoutburst. The change rate of the superhump period was positive. During the normal outburst, there are some hint of modulation up to a 0.2-mag amplitude. However, any periodicity was not found. The recurrence cycles of the normal outburst and the superoutburst, and the distance were estimated to be > ~50 d, 270-500 d, and 690(+230, -170) pc, respectively. These recurrence cycles are usual values for an SU UMa-type dwarf nova having this superhump period. The superhump period of VW CrB was the longest among those of the SU UMa stars with positive derivatives of the superhump period. The coverage of our observations was, however, not enough, and the variation of the Psh change rate of VW CrB is still unknown. A superhump regrowth and a brightening were seen near the end of the plateau phase. Measuring the deviation of the start timings of the brightening and the superhump regrowth (>2 days in VW CrB) will be a key to reveal the mechanism of these phenomena.Comment: 7 pages, 8 figures, to appear in PAS