Do GRE scores help predict getting a physics Ph.D.? A comment on a paper by Miller et al.

A recent paper in Science Advances by Miller et al. concludes that Graduate Record Examinations (GREs) do not help predict whether physics graduate students will get Ph.D.'s. Here, I argue that the presented analyses reflect collider-like stratification bias, variance inflation by collinearity and range restriction, omission of parts of a needed correlation matrix, a peculiar choice of null hypothesis on subsamples, blurring the distinction between failure to reject a null and accepting a null, and an unusual procedure that inflates the confidence intervals in a figure. Release of results of a model that leaves out stratification by the rank of the graduate program would fix many of the problems.

Anomalous noise in the pseudogap regime of YBa2Cu3O(7-delta).

An unusual noise component is found near and below about 250 K in the normal state of underdoped YBCO and Ca-YBCO films. This noise regime, unlike the more typical noise above 250 K, has features expected for a symmetry-breaking collective electronic state. These include large individual fluctuators, a magnetic sensitivity, and aging effects. A possible interpretation in terms of fluctuating charge nematic order is presented.

Electronic transport in underdoped YBa2Cu3O7-delta nanowires: evidence for fluctuating domain structures.

We have measured the transport properties of a series of underdoped YBa(2)Cu(3)O(7-delta) nanowires fabricated with widths of 100-250 nm. We observe large telegraphlike fluctuations in the resistance between the pseudogap temperature T* and the superconducting transition temperature T(c), consistent with the formation and dynamics of a domain structure. We also find anomalous hysteretic steps in the current-voltage characteristics well below T(c).

Artificial dielectric superlattices with broken inversion symmetry.

A variety of three-constituent superlattice patterns were made in atomic layer-by-layer films, with patterns breaking inversion symmetry giving effective permanent bias fields ranging up to about 200 kV/cm. Dielectric constants at room temperature were nearly 10(3), with loss tangents under 0.01. Most of the response came from discrete dipoles comprising multiple unit cells, but without any ferroelectric phase transition.

Barkhausen noise in a relaxor ferroelectric.

Barkhausen noise, including both periodic and aperiodic components, is found in and near the relaxor regime of a familiar relaxor ferroelectric, PbMg(1/3)Nb(2/3)O3, driven by a periodic electric field. The temperature dependences of both the amplitude and spectral form show that the size of the coherent changes in the dipole moment shrink as the relaxor regime is entered, contrary to expectations based on some simple models.

Density-noise power fluctuations in vibrated granular media.

The noise power spectra of the fluctuations in density of a vibrated column of granular material are found to be time dependent. Spectral analysis of these noise power fluctuations shows nontrivial frequency dependences. The noise powers at different frequencies are also found to fluctuate in a partially correlated way. In most instances, the slow variations of the noise are strongly correlated over a broad range of frequencies. These results indicate that highly cooperative interactions exist between fluctuators. In contrast, effects of such strongly coupled fluctuators are absent in the one-dimensional parking-lot-model, one of the simplest systems used to provide a model for recent granular compaction experiments.

Aging in a relaxor ferroelectric: scaling and memory effects.

A crossover as a function of temperature is found in the zero-field aging properties of the relaxor ferroelectric Pb(Mg(1/3)Nb(2/3))O3. Below a temperature near which nonlinear susceptibility has indicated a suspected phase transition, the time-frequency dependence shows simple scale-independent behavior resembling that for spin glasses. As in spin glasses, high temperature aging is stable as further aging occurs at lower temperature, but not vice versa, indicating hierarchical state arrangement. A more general interpretation of such effects is briefly discussed.

Comparison of the magnetic properties of deoxy- and photodissociated myoglobin.

The magnetic susceptibility of photodissociated carbon monoxy myoglobin has been measured over the temperature range from 1.7 to 25 K at 10 and 50 kG with a superconducting susceptometer. The spin and the crystal field parameters of the iron ion were extracted by a spin Hamiltonian approach. Under equivalent conditions the magnetic susceptibility of deoxy myoglobin was measured. In both experiments the CO-bound protein was used as a diamagnetic reference. Above about 5 K the metastable photolysed state and the equilibrium deoxy form of myoglobin are magnetically indistinguishable and can be fitted with S = 2 and g = 2. The transition from spin 0 to spin 2 and the conformational changes known to accompany the electronic change thus also occur after photolysis at low temperature. At temperatures below 5 K, differences become apparent, indicating a somewhat smaller zero-field splitting in the photoproduct as compared to the ligand-free state at equilibrium. In qualitative agreement with observations made by other techniques, the data imply that even at 1.7 K substantial structural relaxation occurs in the heme region of myoglobin after photodissociation. The results are important for the interpretation of the ligand binding kinetics after flash photolysis at low temperature and contribute to the understanding of the relationship between electronic structure and function in heme proteins.

The molecular weight and packaging of dsRNAs in the mycovirus from Ustilago maydis killer strains.

The mycoviruses of Ustilago maydis killer strains are isometric, 43 nm in diameter, and contain several dsRNA segments designated heavy (H), medium (M), and light (L) according to their relative size. To determine the number of dsRNA segments per virion, major sedimenting and density components were isolated, their molecular weights determined from hydrodynamic properties, and their dsRNA contents determined by electron microscopy and/or polyacrylamide gel electrophoresis. The H dsRNA segments of 2.9, 3.1, and 4.2 x 10(6) daltons are separately encapsidated in isometric capsids that band in CsCI at 1.383, 1.394, and 1.418 g/cm8, respectively. The P1 strain contains the 3.1 and 4.2 x 10(6)-dalton segments, and the 3103 strain contains the 2.9 and 4.2 x 106-dalton segments. The T-4 strain contains the 3.1 x 106-dalton H segment and two M segments of 0.67 and 0.60 x 10(6) daltons. The H segments are separately encapsidated in virions which banded at 1.394 g/cm8, whereas the M segments are encapsidated in sets of one, two, or three in virions which banded at 1.314, 1.341, and 1.370 g/cm8. Molecular weights of 9.8 and 13.0 x 106 daltons were calculated for empty capsids (pCsCl = 1.278 g/cm8) and capsids containing the 3.1 x 10(6)-dalton dsRNA segments (pCsCl = 1.394 g/cm8). Analysis of components that banded at other densities in CsCl were consistent with the hypothesis that the banding pattern is the result of the encapsidation of a finite number of dsRNA segments in a capsid of 9.8 x 106 daltons. Although one to three M dsRNA segments were encapsidated in a single virion, no particles were detected with more than one H dsRNA segment per virion.

A mechanism for 1/f noise in diffusing membrane channels.

The diffusion polarization effect is shown to produce 1/f (omega-1) noise in the conductance of membranes containing diffusing ion channels. The magnitude and frequency range of the effect are calculated.

Models for 1/f noise in nerve membranes.

A recently proposed model for 1/f(w-1) noise in nerve membrane (Clay and Schlesinger, 1976; Lundström and McQueen, 1974) is shown to be mathematically inconsistent in several respects. A self-consistent model based on similar membranes lipid orientation fluctuation effects is proposed.