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We report the development of a technique to measure heat capacity at large uniaxial pressure using a piezoelectric-driven device generating compressive and tensile strain in the sample. Our setup is optimized for temperatures ranging from 8 K down to millikelvin. Using an AC heat-capacity technique we are able to achieve an extremely high resolution and to probe a homogeneously strained part of the sample. We demonstrate the capabilities of our setup on the unconventional superconductor Sr$_2$RuO$_4$. By replacing thermometer and adjusting the remaining setup accordingly the temperature regime of the experiment can be adapted to other temperature ranges of interest., Comment: This manuscript is an extended version of the experimental methods part of arXiv:1906.07597, which is being split into two separate papers

A key question regarding the unconventional superconductivity of Sr$_2$RuO$_4$ remains whether the order parameter is single- or two-component. Under a hypothesis of two-component superconductivity, uniaxial pressure is expected to lift their degeneracy, resulting in a split transition. The most direct and fundamental probe of a split transition is heat capacity. Here, we report development of new high-frequency methodology for measurement of heat capacity of samples subject to large and highly homogeneous uniaxial pressure. We place an upper limit on the heat capacity signature of any second transition of a few per cent of the primary superconducting transition. The normalized jump in heat capacity, $\Delta C/C$, grows smoothly as a function of uniaxial pressure, but we find no qualitative evidence of a pressure-induced order parameter transition. Thanks to the high precision of our measurements, these findings place stringent constraints on theories of the superconductivity of Sr$_2$RuO$_4$., Comment: 9 pages, 5 figures, supplementary material included as separate PDF file

We present the ultra-low-temperature specific heat and thermal conductivity measurements on the single crystals of YbMgGaO$_4$, which was recently argued to be a promising candidate for quantum spin liquid (QSL). In the zero magnetic field, a large magnetic contribution of specific heat is observed, and exhibits a power-law temperature dependence ($C_m \sim T^{0.74}$). On the contrary, we do not observed any significant contribution of thermal conductivity from magnetic excitations. In magnetic fields $H \ge$ 6 T, the exponential $T$-dependence of $C_m$ and the enhanced thermal conductivity indicate a magnon gap of the fully-polarized state. The absence of magnetic thermal conductivity at the zero field in this QSL candidate puts a strong constraint on the theories of its ground state., Comment: 6 pages, 3 figures, accepted for publication in Physical Review Letters

We used STM to study the tunneling spectra of FeSe0.3Te0.7 films with two orientations of ab-planes and the connection ramp between them. We have discovered that, using pulse laser deposition (PLD) method, the a- and b-axis of the FeSe0.3Te0.7 film deposited on Ar-ion-milled Magnesium Oxide (MgO) substrate are rotated 45 degree with respect to those of MgO, while the a- and b-axis of the film grown on pristine MgO substrate are parallel to those of MgO. With photolithography and this technique, we can prepare FeSe0.3Te0.7 film with two kinds of orientations on the same MgO substrate, and the connection between them forms a ramp at an angle about 25 degree to the substrate plane. In the planar region with either 0 degree or 45 degree orientation, we have observed tunneling spectra with a superconducting gap about 5 meV and 1.78 meV respectively. However, a much larger gap about 18 meV has been observed in the ramp region. Furthermore, we have also observed a small zero-bias conductance peak (ZBCP) inside the large gap at T = 4.3K. The ZBCP becomes smaller and disappear with increasing temperature to 7 K. The possible causes of such large gap and the ZBCP will be discussed., Comment: 13 pages, 4 figures

The electric transport properties of superconducting FeSe0.3Te0.7/Au c-axis (S/N) junctions, fabricated by using Pulsed Laser Deposition technique, have been investigated in the temperature range of 2 K to the superconducting transition temperature TC ~ 13.5 K, and in the presence of applied magnetic fields from 0 to 9 T. A large zero bias conductance peak has always been observed in the conductance spectra of several batches of junctions. In addition, we have also found several gap-like features. Using the extended BTK theory with the currently favored nodeless S+- wave symmetry, our conductance spectrum can be reproduced quite well in the low bias range with {\Delta}1 = 4 meV, and {\Delta}2 = 6 meV at 2 K. However the experimental conductance spectrum is substantially below the calculated one in the high bias range. Furthermore, there is a conductance minimum at about 20 meV, which may be the reason for the discrepancy., Comment: 21 pages, 10 figures

One of the strongest predictions of the LambdaCDM cosmological model is the presence of dark satellites orbiting all types of galaxies. We focus here on the dynamical effects of such satellites on disky dwarf galaxies, and demonstrate that these encounters can be dramatic. Although mergers with M_sat > M_d are not very common, because of the lower baryonic content they occur much more frequently on the dwarf scale than for L_*-galaxies. As an example, we present a numerical simulation of a 20% (virial) mass ratio merger between a dark satellite and a disky dwarf (akin to the Fornax dwarf galaxy in luminosity) that shows that the merger remnant has a spheroidal morphology. We conclude that perturbations by dark satellites provide a plausible path for the formation of dSph systems and also could trigger starbursts in gas rich dwarf galaxies. Therefore the transition from disky to the often amorphous, irregular, or spheroidal morphologies of dwarfs could be a natural consequence of the dynamical heating of hitherto unobservable dark satellites., Comment: Submitted to ApJ Letters. 5 pages, 4 figures. A movie showing an encounter between a disky dwarf galaxy and a dark satellite can be found at http://www.astro.rug.nl/~ahelmi/dwarfs-morphologies.html

Based upon the currently predominant theory of Fe-based superconductors, the order parameter symmetry is the nodaless s\pm-wave. Andreev conductance involving normal metals and such superconductors does not have a zero bias conductance peak (ZBCP) except maybe for a very narrow range of a junction-dependent fitting parameter. We have measured differential conductance of the in-plane boundary junctions between Au and superconducting FeSe0.3Te0.7 films. The Au/FeSe0.3Te0.7 ramp-type junctions are fabricated with the bias current direction along the <110> axis of the epitaxial FeSe0.3Te0.7 film. We have always observed a pronounced ZBCP as well as some gap-like features in the conductance spectra of all samples studied, thus our experimental results suggest not the s\pm-wave theory but the possible existence of a nodal point around 45{\deg} in at least one of the gaps in FeSe0.3Te0.7 superconductor., Comment: 17 pages, 7 figures, submitted

We have performed Raman-scattering measurements on high-quality single crystals of A$_{0.8}$Fe$_{1.6}$Se$_2$ superconductors of several compositions. We find a broad, asymmetric peak around 1600 cm$^{-1}$ (200 meV), which we identify as a two-magnon process involving optical magnons. The intensity of the two-magnon peak falls sharply on entering the superconducting phase. This effect, which is entirely absent in the non-superconducting system KFe$_{1.5}$Se$_2$, requires a strong mutual exclusion between antiferromagnetism and superconductivity arising from proximity effects within regions of microscale phase separation., Comment: 8 pages, 5 figures

A generalized quantum search algorithm, where phase inversions for the marked state and the prepared state are replaced by $\pi/2$ phase rotations, is realized in a 2-qubit NMR heteronuclear system. The quantum algorithm searches a marked state with a smaller step compared to standard Grover algorithm. Phase matching requirement in quantum searching is demonstrated by comparing it with another generalized algorithm where the two phase rotations are $\pi/2$ and $3\pi/2$ respectively. Pulse sequences which include non 90 degree pulses are given., Comment: 12 pages, 2 figures, accepted for publication in Plysics Letters A

The quantum nature of bulk ensemble NMR quantum computing --the center of recent heated debate, is addressed. Concepts of the mixed state and entanglement are examined, and the data in a 2 qubit liquid NMR quantum computation are analyzed. It is pointed out that the key problem in the current debate is the understanding of entanglement in a mixed state system. The following points are concluded in this Letter: 1)Density matrix describes the "state" of an average particle in an ensemble. It can not describe the state of an individual particle in an ensemble in detail; 2) Entanglement is a property of the wave function of a quantum particle(such as an molecule in a liquid NMR sample). Separability of the density matrix can not be used to measure the entanglement of mixed ensemble; 3)The evolution of states in bulk-ensemble NMR quantum computation is quantum mechanical; 4) The coefficient before the effective pure state density matrix, $\epsilon$, is an measure of the simultaneity of the molecules in an ensemble. It reflects the intensity of the NMR signal and has no significance in quantifying the entanglement in the bulk ensemble NMR system. We conclude that the liquid NMR quantum computation is genuine, not just classical simulations., Comment: 4 pages