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We report an angle-resolved photoemission spectroscopy study of the iron-based superconductor family, Ba_{1-x}Na_{x}Fe_{2}As_{2}. This system harbors the recently discovered double-Q magnetic order appearing in a reentrant C_{4} phase deep within the underdoped regime of the phase diagram that is otherwise dominated by the coupled nematic phase and collinear antiferromagnetic order. From a detailed temperature-dependence study, we identify the electronic response to the nematic phase in an orbital-dependent band shift that strictly follows the rotational symmetry of the lattice and disappears when the system restores C_{4} symmetry in the low temperature phase. In addition, we report the observation of a distinct electronic reconstruction that cannot be explained by the known electronic orders in the system.

Using resistivity, heat-capacity, thermal-expansion, and susceptibility measurements we study the normal-state behavior of ${\mathrm{KFe}}_{2}{\mathrm{As}}_{2}$. Both the Sommerfeld coefficient ($\ensuremath{\gamma}\ensuremath{\approx}103\text{ }\text{ }\mathrm{mJ}\text{ }{\mathrm{mol}}^{\ensuremath{-}1}\text{ }{\mathrm{K}}^{\ensuremath{-}2}$) and the Pauli susceptibility ($\ensuremath{\chi}\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$) are strongly enhanced, which confirm the existence of heavy quasiparticles inferred from previous de Haas--van Alphen and angle-resolved photoemission spectroscopy experiments. We discuss this large enhancement using a Gutzwiller slave-boson mean-field calculation, which shows the proximity of ${\mathrm{KFe}}_{2}{\mathrm{As}}_{2}$ to an orbital-selective Mott transition. The temperature dependence of the magnetic susceptibility and the thermal expansion provide strong experimental evidence for the existence of a coherence-incoherence crossover, similar to what is found in heavy fermion and ruthenate compounds, due to Hund's coupling between orbitals.

Using resistivity, heat-capacity, thermal-expansion, and susceptibility measurements we study the normal-state behavior of KFe2As2. Both the Sommerfeld coefficient (γ≈103 mJ mol(-1) K(-2)) and the Pauli susceptibility (χ≈4×10(-4)) are strongly enhanced, which confirm the existence of heavy quasiparticles inferred from previous de Haas-van Alphen and angle-resolved photoemission spectroscopy experiments. We discuss this large enhancement using a Gutzwiller slave-boson mean-field calculation, which shows the proximity of KFe2As2 to an orbital-selective Mott transition. The temperature dependence of the magnetic susceptibility and the thermal expansion provide strong experimental evidence for the existence of a coherence-incoherence crossover, similar to what is found in heavy fermion and ruthenate compounds, due to Hund's coupling between orbitals.

This paper introduces an experimental probe of the sterile neutrino with a novel, high-intensity source of electron antineutrinos from the production and subsequent decay of $^{8}\mathrm{Li}$. When paired with an existing $\ensuremath{\sim}1$ kton scintillator-based detector, this $⟨{E}_{\ensuremath{\nu}}⟩=6.4$ MeV source opens a wide range of possible searches for beyond standard model physics via studies of the inverse beta decay interaction ${\overline{\ensuremath{\nu}}}_{e}+p\ensuremath{\rightarrow}{e}^{+}+n$. In particular, the experimental design described here has unprecedented sensitivity to ${\overline{\ensuremath{\nu}}}_{e}$ disappearance at $\ensuremath{\Delta}{m}^{2}\ensuremath{\sim}1\text{ }\text{ }{\mathrm{eV}}^{2}$ and features the ability to distinguish between the existence of zero, one, and two sterile neutrinos.

This paper introduces an experimental probe of the sterile neutrino with a novel, high-intensity source of electron antineutrinos from the production and subsequent decay of 8Li. When paired with an existing ∼1 kton scintillator-based detector, thisE(ν)= 6.4 MeV source opens a wide range of possible searches for beyond standard model physics via studies of the inverse beta decay interaction ν(e) + p → e+ + n. In particular, the experimental design described here has unprecedented sensitivity to ν(e) disappearance at Δm2 ∼ 1 eV2 and features the ability to distinguish between the existence of zero, one, and two sterile neutrinos.

Thermal expansion data are used to study the uniaxial pressure dependence of the electronic-magnetic entropy of Ba(Fe(1-x)Co(x))2As2. Uniaxial pressure is found to be proportional to doping and, thus, also an appropriate tuning parameter in this system. Many of the features predicted to occur for a pressure-tuned quantum critical system, in which superconductivity is an emergent phase hiding the critical point, are observed. The electronic-magnetic Gruneisen parameters associated with the spin-density wave and superconducting transitions further demonstrate an intimate connection between both ordering phenomena.

Thermal expansion data are used to study the uniaxial pressure dependence of the electronic-magnetic entropy of Ba(Fe(1-x)Co(x))2As2. Uniaxial pressure is found to be proportional to doping and, thus, also an appropriate tuning parameter in this system. Many of the features predicted to occur for a pressure-tuned quantum critical system, in which superconductivity is an emergent phase hiding the critical point, are observed. The electronic-magnetic Grüneisen parameters associated with the spin-density wave and superconducting transitions further demonstrate an intimate connection between both ordering phenomena.

The specific heat of ${\mathrm{Nd}}_{2\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Ce}}_{\mathit{y}}$${\mathrm{CuO}}_{4}$, 0\ensuremath{\le}y\ensuremath{\le}0.2, was measured between 35 mK and 10 K. A Schottky anomaly in ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$, due to exchange interactions between Nd and Cu moments, shifts upon Ce doping to lower temperatures. In the metallic state (y\ensuremath{\ge}0.15), a linear term with coefficient \ensuremath{\gamma}\ensuremath{\simeq}4J/${\mathrm{K}}^{2}$ per mole Nd is found below 300 mK. For y=0.2, a constant value of the magnetic susceptibility in the same temperature range and changes of ${\mathit{C}}_{\mathit{p}}$ in applied magnetic fields suggest an interpretation in analogy to a heavy-fermion system resulting from the coupling between Nd moments and conduction electrons.

Using high-resolution dilatometry, we study the thermodynamic response of the lattice parameters to superconducting order in a self-flux grown Ba(Fe 0.92 Co 0.08 ) 2 As 2 single crystal. The uniaxial-pressure dependencies of the critical temperature T c calculated using our thermal-expansion and specific-heat data via the Ehrenfest relation, are found to be quite large and very anisotropic (dT c /dp a = 3.1(1) K/GPa and dT c/ dp c = ―7.0(2) K/GPa). Our results show that there is a strong coupling of the c/a ratio to superconducting order, which demonstrates that T c is far from the optimal value.

Using high-resolution dilatometry, we study the thermodynamic response of the lattice parameters to superconducting order in a self-flux grown Ba(Fe0.92Co0.08)2As2 single crystal. The uniaxial-pressure dependencies of the critical temperature Tc calculated using our thermal-expansion and specific-heat data via the Ehrenfest relation, are found to be quite large and very anisotropic (dTc/dpa=3.1(1) K/GPa and dTc/dpc=-7.0(2) K/GPa). Our results show that there is a strong coupling of the c/a ratio to superconducting order, which demonstrates that Tc is far from the optimal value.

The phase diagram of the quasi-two-dimensional antiferromagnet BaNi(2)V(2)O(8) is studied by specific heat, thermal expansion, magnetostriction, and magnetization for magnetic fields applied perpendicular to c. At micro(o)H* approximately 1.5 T, a crossover to a high-field state, where T(N)(H) increases linearly, arises from a competition of intrinsic and field-induced in-plane anisotropies. The pressure dependences of T(N) and H* are interpreted using the picture of a pressure-induced in-plane anisotropy. Even at zero field and ambient pressure, in-plane anisotropy cannot be neglected, which implies deviations from pure Berezinskii-Kosterlitz-Thouless behavior.

The phase diagram of the quasi-two-dimensional antiferromagnet BaNi(2)V(2)O(8) is studied by specific heat, thermal expansion, magnetostriction, and magnetization for magnetic fields applied perpendicular to c. At micro(o)H* approximately 1.5 T, a crossover to a high-field state, where T(N)(H) increases linearly, arises from a competition of intrinsic and field-induced in-plane anisotropies. The pressure dependences of T(N) and H* are interpreted using the picture of a pressure-induced in-plane anisotropy. Even at zero field and ambient pressure, in-plane anisotropy cannot be neglected, which implies deviations from pure Berezinskii-Kosterlitz-Thouless behavior.

We show that the accumulation of spin-polarized electrons at a forward-biased Schottky tunnel barrier between Fe and n-GaAs can be detected electrically. The spin accumulation leads to an additional voltage drop across the barrier that is suppressed by a small transverse magnetic field, which depolarizes the spins in the semiconductor. The dependence of the electrical accumulation signal on magnetic field, bias current, and temperature is in good agreement with the predictions of a drift-diffusion model for spin-polarized transport., Comment: Submitted to Phys. Rev. Lett

The phonon spectrum of ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}{\mathrm{B}}_{2}$ shows a strong dependence on the aluminum content $x$. This is experimentally demonstrated by both Raman and inelastic neutron scattering and theoretically predicted by first-principles calculations. The observed changes in the phonon spectrum are put into perspective with respect to the superconducting properties within this family of materials.

The phonon spectrum of Mg(1-x)Al(x)B2 shows a strong dependence on the aluminum content x. This is experimentally demonstrated by both Raman and inelastic neutron scattering and theoretically predicted by first-principles calculations. The observed changes in the phonon spectrum are put into perspective with respect to the superconducting properties within this family of materials.