We report the magnetization, ac susceptibility, and specific heat of optically float-zoned single crystals of Fe$_{1-x}$Co$_{x}$Si, $0.20 \leq x \leq 0.50$. We determine the magnetic phase diagrams for all major crystallographic directions and cooling histories. After zero-field cooling, the phase diagrams resemble that of the archetypal stoichiometric cubic chiral magnet MnSi. Besides the helical and conical state, we observe a pocket of skyrmion lattice phase just below the helimagnetic ordering temperature. At the phase boundaries between these states evidence for slow dynamics is observed. When the sample is cooled in small magnetic fields, the phase pocket of skyrmion lattice may persist metastably down to lowest temperatures. Taken together with the large variation of the transition temperatures, transition fields, and the helix wavelength as a function of composition, this hysteresis identifies Fe$_{1-x}$Co$_{x}$Si as an ideal material for future experiments exploring, for instance, the topological unwinding of the skyrmion lattice.
We discuss the history dependence and memory effects which are observed in the out-of-equilibrium conductivity of electron glasses. The experiments can be understood by assuming that the local density of states retains a memory of the sample history. We provide analytical arguments for the consistency of this assumption, and discuss the saturation of the memory effect with increasing gate voltage change. This picture is bolstered by numerical simulations at zero temperature, which moreover demonstrate the incompressibility of the Coulomb glass on short timescales.
We use a calorimetric technique operating in sweeping magnetic field to study the thermomagnetic history- dependence of the magnetocaloric effect (MCE) in Mn0.985Fe0.015As. We study the magnetization history for which a "colossal" MCE has been reported when inferred indirectly via a Maxwell relation. We observe no colossal effect in the direct calorimetric measurement. We further examine the impact of mixed-phase state on the MCE and show that the first order contribution scales linearly with the phase fraction. This validates various phase-fraction based methods developed to remove the coloss…
We report a study on the temperature dependence of the charge-neutral crystal field (dd) excitations in cupric oxide, using nonresonant inelastic x-ray scattering (IXS) spectroscopy. Thanks to a very high energy resolution (60 meV), we observe thermal effects on the dd excitation spectrum fine structure between temperatures of 10-320 K. With an increasing temperature, the spectra broaden considerably. We assign the temperature dependence of the dd excitations to the relatively large electron-phonon coupling.
We show that nanostructure and resistivity of Pr0.5Ca0.5Mn0.98Cr0.02O3 are sensitive to whether the sample is zero-field-cooled (ZFC) of field-cooled (FC) either in the 'self magnetic field (H = 2 T)' of the electron microscope or under the external magnetic field of 2 T. FC resistivity at H = 2 T is lower than ZFC values below 140 K. The average value of the chare-orbital modulation vector (q = 0.44) of the FC crystallites is lower than that of the ZFC cystallites (q = 0.48) and the FC crystallites exhibit numerous defects like discommensuration, dislocations and regios with loss of superstru…
We present the design and construction of an apparatus that measures the Seebeck coefficient of single crystals under in-situ tunable strain at cryogenic temperatures. A home-built three piezostack apparatus applies uni-axial stress to a single crystalline sample and modulates anisotropic strain up to 0.7%. An alternating heater system and cernox sensor thermometry measures the Seebeck coefficient along the uniaxial stress direction. To demonstrate the efficacy of this apparatus, we applied uniaxial stress to detwin single crystals of BaFe2As2 in the orthorhombic phase. The obtained Seebeck co…
The idea of breaking time-translation symmetry has fascinated humanity at least since ancient proposals of the perpetuum mobile. Unlike the breaking of other symmetries, such as spatial translation in a crystal or spin rotation in a magnet, time translation symmetry breaking (TTSB) has been tantalisingly elusive. We review this history up to recent developments which have shown that discrete TTSB does takes place in periodically driven (Floquet) systems in the presence of many-body localization. Such Floquet time-crystals represent a new paradigm in quantum statistical mechanics --- that of an…
The temperature dependence of electronic and magnetic properties of the organic charge-transfer salt (DOEO)$_4$[HgBr$_4$]TCE was investigated using magnetometry. Electronic transport properties revealed three distinct phases which are related to different magnetic coupling phenomena. In the low-temperature insulating phase (T<70 K) the antiferromagnetic coupling between two distinct sites of magnetic moments causes antiferromagnetic order below the Néel temperature T$_N$=40 K. In the temperature region 70-120 K the (DOEO)$_4$[HgBr$_4$]TCE shows metallic-like behavior and with further increasin…
We present a high-resolution photoemission study on the strongly correlated Ce-compounds CeCu_6, CeCu_2Si_2, CeRu_2Si_2, CeNi_2Ge_2, and CeSi_2. Using a normalization procedure based on a division by the Fermi-Dirac distribution we get access to the spectral density of states up to an energy of 5 k_BT above the Fermi energy E_F. Thus we can resolve the Kondo resonance and the crystal field (CF) fine-structure for different temperatures above and around the Kondo temperature T_K. The CF peaks are identified with multiple Kondo resonances within the multiorbital Anderson impurity model. Our theo…
The mixed-valence (MV) compound YbInCu4 was investigated by electrical resistivity and ac specific heat at low temperatures and high pressures. At atmospheric pressure, its Kadowaki-Woods (KW) ratio, A/γ^2, is 16 times smaller than the universal value R_{KW}(=1.0 x 10^-5 μΩcm mol^2 K^2 mJ^-2), but sharply increases to 16.5R_{KW} at 27 kbar. The pressure-induced change in the KW ratio and deviation from R_{KW} are analyzed in terms of the change in f-orbital degeneracy N and carrier density n. This analysis is further supported by a dramatic change in residual resistivity ρ_0 near 25 kbar, wher…
We report plane-polarized Raman scattering spectra of iron oxypnictide superconductor NdFeAsO$_{1-x}$F$_x$ single crystals with varying fluorine $x$ content. The spectra exhibit sharp and symmetrical phonon lines with a weak dependence on fluorine doping $x$. The temperature dependence does not show any phonon anomaly at the superconducting transition. The Fe related phonon intensity shows a strong resonant enhancement below 2 eV. We associate the resonant enhancement to the presence of an interband transition around 2 eV observed in optical conductivity. Our results point to a rather weak cou…
The lineshape of crystal-field transitions in single crystals of Mn12ac molecular magnets is determined by the magnetic history. The absorption lines are symmetric and Gaussian for the non-magnetized state obtained by zero-field cooling (zfc). In the magnetized state which is reached when the sample is cooled in a magnetic field (fc), however, they are asymmetric even in the absence of an external magnetic field. These observations are quantitatively explained by inhomogeneous symmetrical (Gaussian) broadening of the crystal-field transitions combined with a contribution of off-diagonal compon…
We measured electrical resistivity, specific heat and magnetic susceptibility of single crystals of highly conductive oxide Ag_5Pb_2O_6, which has a layered structure containing a Kagome lattice. Both the out-of-plane and in-plane resistivity show T^2 dependence in an unusually wide range of temperatures up to room temperature. This behavior cannot be accounted for either by electron correlation or by electron-phonon scattering with high frequency optic phonons. In addition, a phase transition with a large diamagnetic signal was found in the ac susceptibility, which strongly suggests the exist…
The temperature dependence of the in-plane, λ_{\parallel}, and interplane, λ_{\perp}, London penetration depth was measured in the metal-free all-organic superconductor β-ET (see title) ($T_c \approx$ 5.2 K). λ_{\parallel} ~T^3 up to 0.5 Tc, a power law previously observed only in materials thought to be p-wave superconductors. λ_{\perp} is larger than the sample dimensions down to the lowest temperatures (0.35 K), implying an anisotropy of λ_{\perp}/λ_{\parallel} ~ 400-800.
A Wigner crystal structure of the electronic ground state is induced by strong Coulomb interactions at low temperature in clean or disordered two-dimensional (2d) samples. For fermions on a mesoscopic disordered 2d lattice, being closed to a torus, we study the persistent current in the regime of strong interaction at zero temperature. We perform a perturbation expansion starting from the Wigner crystal limit which yields power laws for the dependence of the persistent current on the interaction strength. The sign of the persistent current in the strong interaction limit is independent of the …
In the tetragonal heavy fermion system CeCoIn5 the unconventional superconducting state is probed by means of muon spin rotation. The pressure dependence (0-1 GPa) of the basal-plane magnetic penetration depth (λ_a), the penetration depth anisotropy (γ=λ_c/λ_a) and the temperature dependence of 1/λ_i^2 (i=a,c) were studied in single crystals. A strong decrease of λ_a with pressure was observed, while γand λ_i^2(0)/λ_i^2(T) are pressure independent. A linear relationship between 1/λ_a^2(270 mK) and Tc was also found. The large decrease of λ_a with pressure is the signature of an increase of the…
We report a novel feature of the temperature induced magnetization jump observed along the a-axis of the GdVO3 single crystal at temperature TM = 0.8 K. Below TM, the compound shows no coercivity and remanent magnetization indicating a homogenous antiferromagnetic structure. However, we will demonstrate that the magnetic state below TM is indeed history dependent and it shows up in different jumps in the magnetization only when warming the sample through TM. Such a magnetic memory effect is highly unusual and suggesting different domain arrangements in the supposedly homogenous antiferromagnet…
Magnetic relaxation measurements on a series of Y$_{1-x}$Pr$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-δ}$ ($x = 0.13, 0.34, 0.47$) single crystals were performed over a large field and temperature range in order to investigate the characteristics of the vortex matter across the second magnetization peak (SMP). The magnitude of the SMP varies non-monotonically with the Pr concentration; i.e., the irreversible magnetization normalized by its value at the onset field $H_{on}$ displays a maximum for the $x = 0.34$ single crystal. The two characteristic fields, $H_{on}$ and $H_{sp}$, follow different temperature …
The results of a computer analysis of a simple 2D quantum mechanical tunnelling model are reported. These suggest that the spatial dependence of the superconducting energy gap observed by Scanning Tunnelling Spectroscopy (STS) studies of single crystals of the high Tc superconductor Bi:2212 is not necessarily caused by nanoscale inhomogeneity. Instead the spatial dependence of the STS data could arise from the momentum (k) dependence of the energy gap, which is a defining feature of a d-wave superconductor. It is possible that this viewpoint could be exploited to obtain k-dependent information…
Magnetoresistance (MR) in the a-axis resistivity of untwinned YBa_{2}Cu_{3}O_{y} single crystals is measured for a wide range of doping (y = 6.45 - 7.0). The y-dependence of the in-plane coherence length ξ_{ab} estimated from the fluctuation magnetoconductance indicates that the superconductivity is anomalously weakened in the 60-K phase; this gives evidence, together with the Hall coefficient and the a-axis thermopower data that suggest the hole doping to be 12% for y = 6.65, that the origin of the 60-K plateau is the 1/8 anomaly. At high temperatures, the normal-state MR data show signatures…
In terms of a semi-phenomenological exchange charge model, we have obtained estimates of parameters of the crystal field and parameters of the electron-deformation interaction in terbium titanate Tb2Ti2O7 with a pyrochlore structure. The obtained set of parameters has been refined based on the analysis of spectra of neutron inelastic scattering and Raman light scattering, field dependences of the forced magnetostriction, and temperature dependences of elastic constants.
The temperature and magnetic-field (H) dependences of thermal conductivity (κ) of Bi_2Sr_2CaCu_2O_{8+δ} (Bi2212) are systematically measured for a broad doping range by using both pure Bi2212 single crystals with tuned oxygen contents and Bi_2Sr_2Ca_{1-x}Dy_xCu_2O_{8+δ} (Dy-Bi2212) single crystals with different Dy contents x. In the underdoped samples, the quasiparticle (QP) peak below T_c is strongly suppressed, indicating strong QP scattering by impurities or oxygen defects, whereas the phonon conductivity is enhanced in moderately Dy-doped samples and a phonon peak at 10 K is observed for …
Mn1.4PtSn is the first material in which antiskyrmions have been observed in ultra-thin single crystalline specimens. While bulk crystals exhibit fractal patterns of purely ferromagnetic domain ordering at room temperature, ultra-thin Mn1.4PtSn lamellae clearly show antiskyrmion lattices with lattice spacings up to several $μ$m. In the work presented here, we systematically investigate the thickness region from 400 nm to 10 $μ$m using 100 $\times$ 100 $μ$m$^2$ -wide Mn1.4PtSn plates, and identify the critical thickness-to-width aspect ratio $α_0 = 0.044$ for the ferromagnetic fractal domain to…
