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We report an angle-resolved photoemission study of a series of hole and electron doped iron-based superconductors, their parent compound BaFe2As2, and their cousins BaCr2As2 and BaCo2As2. We focus on the energy (E) dependent scattering rate Gamma(E) as a function of the 3d count and on the renormalization function Z(E) of the inner hole pocket, which is the hot spot in these compounds. We obtain a non-Fermi-liquid-like linear in energy scattering rate Gamma(E>> kBT), independent of the dopant concentration. The main result is that the slope beta=Gamma(E >> kBT)/E, reaches its maxima near optimal doping and scales with the superconducting transition temperature. This supports the spin fluctuation model for superconductivity for these materials. In the optimally hole-doped compound, the slope of the scattering rate of the inner hole pocket is about three times bigger than the Planckian limit Gamma(E)/E~1. This result together with the energy dependence of the renormalization function Z(E) signals very incoherent charge carriers in the normal state which transform at low temperatures to a coherent unconventional superconducting state., 5 pages, 2 figures

Detailed experimental investigations of thermal and magnetic properties are presented for Cu(NH3)4SO4.H2O, an ideal uniform Heisenberg spin half chain compound. A comparison of these properties with relevant spin models is also presented. The temperature dependent magnetic susceptibility and specific heat data has been compared with the exact solution for uniform Heisenberg chain model derived by means of Bethe ansatz technique. Field dependent isothermal magnetization curves are simulated by Quantum Monte Carlo technique and compared with the corresponding experimental ones. Specific heat as a function of magnetic field (up to 7T) and temperature (down to 2K) is reported. Subsequently, the data are compared with the corresponding theoretical curves for the infinite Heisenberg spin half chain model with J=6K. Moreover, internal energy and entropy are calculated by analyzing the experimental specific heat data. Magnetic field and temperature dependent behavior of entropy and internal energy are in good agreement with the theoretical predictions.

The paper studies the effect of charge carrier doping on quantum criticality in the heavy-fermion metal YbRh2Si2. It focuses in particular on the critical temperature crossover-scale, which previously has been assigned as signature of the disintegration of heavy quasiparticles at the quantum critical point. The data are incompatible with the presumed Kondo breakdown and instead point at a Zeeman driven magnetic polarization underlying critical temperature.

Nuclear magnetic resonance measurements were performed on CeCu_{2}Si_{2} in the presence of a magnetic field close to the upper critical field μ_{0}H_{c2} in order to investigate its superconducting (SC) properties near pair-breaking fields. In lower fields, the Knight shift and nuclear spin-lattice relaxation rate divided by temperature 1/T_{1}T abruptly decreased below the SC transition temperature T_{c}(H), a phenomenon understood within the framework of conventional spin-singlet superconductivity. In contrast, 1/T_{1}T was enhanced just below T_{c}(H) and exhibited a broad maximum when magnetic fields close to μ_{0}H_{c2}(0) were applied parallel or perpendicular to the c axis; although the Knight shift decreased just below T_{c}(H). This enhancement of 1/T_{1}T, which was recently observed in the organic superconductor κ-(BEDT-TTF)_{2}Cu(NCS)_{2}, suggests the presence of high-density Andreev bound states in the inhomogeneous SC region, a hallmark of the Fulde-Ferrell-Larkin-Ovchinnikov phase.

The nature of the pairing symmetry of the first heavy fermion superconductor CeCu

The nature of the pairing symmetry of the first heavy fermion superconductor CeCu$_2$Si$_2$ has recently become the subject of controversy. While CeCu$_2$Si$_2$ was generally believed to be a $d$-wave superconductor, recent low temperature specific heat measurements showed evidence for fully gapped superconductivity, contrary to the nodal behavior inferred from earlier results. Here we report London penetration depth measurements, which also reveal fully gapped behavior at very low temperatures. To explain these seemingly conflicting results, we propose a fully gapped $d+d$ band-mixing pairing state for CeCu$_2$Si$_2$, which yields very good fits to both the superfluid density and specific heat, as well as accounting for a sign change of the superconducting order parameter, as previously concluded from inelastic neutron scattering results., Comment: 9 pages, 6 figures including Supplemental material. Published version in PNAS available online ahead of print at http://www.pnas.org/content/early/2018/05/07/1720291115

We have performed $^{63}\mathrm{Cu}$ nuclear-magnetic-resonance/nuclear-quadrupole-resonance measurements to investigate the magnetic and superconducting (SC) properties on a ``superconductivity dominant'' ($S$-type) single crystal of ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$. Although the development of antiferromagnetic (AFM) fluctuations down to 1 K indicated that the AFM criticality was close, Korringa behavior was observed below 0.8 K, and no magnetic anomaly was observed above ${T}_{\mathrm{c}}\ensuremath{\sim}0.6$ K. These behaviors were expected in $S$-type ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$. The temperature dependence of the nuclear spin-lattice relaxation rate $1/{T}_{1}$ at zero field was almost identical to that in the previous polycrystalline samples down to 130 mK, but the temperature dependence deviated downward below 120 mK. In fact, $1/{T}_{1}$ in the SC state could be fitted with the two-gap ${s}_{\ifmmode\pm\else\textpm\fi{}}$-wave model rather than the two-gap ${s}_{++}$-wave model down to 90 mK. Under magnetic fields, the spin susceptibility in both directions clearly decreased below ${T}_{\mathrm{c}}$, which is indicative of the formation of spin-singlet pairing. The residual part of the spin susceptibility was understood by the field-induced residual density of states evaluated from $1/{T}_{1}T$, which was ascribed to the effect of the vortex cores. No magnetic anomaly was observed above the upper critical field ${H}_{c2}$, but the development of AFM fluctuations was observed, indicating that superconductivity was realized in strong AFM fluctuations.

In exotic superconductors including high-$T_c$ copper-oxides, the interactions mediating electron Cooper-pairing are widely considered to have a magnetic rather than the conventional electron-phonon origin. Interest in such exotic pairing was initiated by the 1979 discovery of heavy-fermion superconductivity in CeCu$_2$Si$_2$, which exhibits strong antiferromagnetic fluctuations. A hallmark of unconventional pairing by anisotropic repulsive interactions is that the superconducting energy gap changes sign as a function of the electron momentum, often leading to nodes where the gap goes to zero. Here, we report low-temperature specific heat, thermal conductivity and magnetic penetration depth measurements in CeCu$_2$Si$_2$, demonstrating the absence of gap nodes at any point on the Fermi surface. Moreover, electron-irradiation experiments reveal that the superconductivity survives even when the electron mean free path becomes substantially shorter than the superconducting coherence length. This indicates that superconductivity is robust against impurities, implying that there is no sign change in the gap function. These results show that, contrary to long-standing belief, heavy electrons with extremely strong Coulomb repulsions can condense into a fully-gapped s-wave superconducting state, which has an on-site attractive pairing interaction., Comment: 8 pages, 5 figures + Supplement (3 pages, 5 figures)

We present an experimental study of the local and collective magnetism of ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ that is isostructural with the high-temperature superconductor parent compound ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$. In contrast to ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$, where only Fe spins order, ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ has an additional magnetic transition below 20 K due to the ordering of the ${\mathrm{Eu}}^{2+}$ spins $(J=7/2$, with $L=0$ and $S=7/2)$ in an $A$-type antiferromagnetic texture (ferromagnetic layers stacked antiferromagnetically). This may potentially affect the FeAs layer and its local and correlated magnetism. $\mathrm{Fe}\phantom{\rule{0.16em}{0ex}}K\ensuremath{\beta}$ x-ray emission experiments on ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ single crystals reveal a local magnetic moment of $1.3\ifmmode\pm\else\textpm\fi{}0.15{\ensuremath{\mu}}_{B}$ at 15 K that increases slightly to $1.45\ifmmode\pm\else\textpm\fi{}0.15{\ensuremath{\mu}}_{B}$ at 300 K. Resonant inelastic x-ray scattering experiments performed on the same crystals show dispersive broad (in energy) magnetic excitations along $(0,0)\ensuremath{\rightarrow}(1,0)$ and $(0,0)\ensuremath{\rightarrow}(1,1)$ with a bandwidth on the order of 170--180 meV. These results on local and collective magnetism are in line with other parent compounds of the ${\mathrm{AFe}}_{2}{\mathrm{As}}_{2}$ series $(A=\mathrm{Ba}$, Ca, and Sr), especially the well-characterized ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$. Thus, our experiments lead us to the conclusion that the effect of the high magnetic moment of Eu on the magnitude of both the Fe local magnetic moment and spin excitations is small and confined to low-energy excitations.

Angle-resolved photoemission spectroscopy (ARPES) is used to study the scattering rates of charge carriers from the hole pockets near Gamma in the iron-based high-Tc hole doped superconductors KxBa1-xFe2As2 x=0.4 and KxEu1-xFe2As2 x=0.55$ and the electron doped compound Ba(Fe1-xCox)2As2 x=0.075. The scattering rate for any given band is found to depend linearly on energy, indicating a non-Fermi liquid regime. The scattering rates in the hole-doped compound are considerably larger than those in the electron-doped compounds. In the hole-doped systems the scattering rate of the charge carriers of the inner hole pocket is about three times bigger than the binding energy indicating that the spectral weight is heavily incoherent. The strength of the scattering rates and the difference between electron and hole doped compounds signals the importance of Hund's exchange coupling for correlation effects in these iron-based high-Tc superconductors. The experimental results are in qualitative agreement with theoretical calculations in the framework of combined density functional dynamical mean-field theory., 5 pages; 3 figures

Metallic super-heavy fermion compounds can be used for adiabatic demagnetization cooling to temperatures well below 0.1 K., Low-temperature refrigeration is of crucial importance in fundamental research of condensed matter physics, because the investigations of fascinating quantum phenomena, such as superconductivity, superfluidity, and quantum criticality, often require refrigeration down to very low temperatures. Currently, cryogenic refrigerators with 3He gas are widely used for cooling below 1 K. However, usage of the gas has been increasingly difficult because of the current worldwide shortage. Therefore, it is important to consider alternative methods of refrigeration. We show that a new type of refrigerant, the super-heavy electron metal YbCo2Zn20, can be used for adiabatic demagnetization refrigeration, which does not require 3He gas. This method has a number of advantages, including much better metallic thermal conductivity compared to the conventional insulating refrigerants. We also demonstrate that the cooling performance is optimized in Yb1−xScxCo2Zn20 by partial Sc substitution, with x ~ 0.19. The substitution induces chemical pressure that drives the materials to a zero-field quantum critical point. This leads to an additional enhancement of the magnetocaloric effect in low fields and low temperatures, enabling final temperatures well below 100 mK. This performance has, up to now, been restricted to insulators. For nearly a century, the same principle of using local magnetic moments has been applied for adiabatic demagnetization cooling. This study opens new possibilities of using itinerant magnetic moments for cryogen-free refrigeration.

In a comprehensive study, we investigate the electronic scattering effects in EuFe 2 ( As 1 − x P x ) 2 by using Fourier-transform infrared spectroscopy. In spite of the fact that Eu 2 + local moments order at around T Eu ∼ 20 K , the overall optical response is strikingly similar to the one of the well-known Ba-122 pnictides. The main difference lies within the suppression of the lower spin-density-wave gap feature. By analyzing our spectra with a multi-component model, we find that the high-energy feature around 0.7 eV – often associated with Hund's rule coupling – is highly sensitive to the spin-density-wave ordering; this further confirms its direct relationship to the dynamics of itinerant carriers. The same model is also used to investigate the in-plane anisotropy of magnetically detwinned EuFe 2 As 2 in the antiferromagnetically ordered state, yielding a higher Drude weight and lower scattering rate along the crystallographic a-axis. Finally, we analyze the development of the room-temperature spectra with isovalent phosphor substitution and highlight changes in the scattering rate of hole-like carriers induced by a Lifshitz transition.

The origin of unconventional superconductivity, including high-temperature and heavy-fermion superconductivity, is still a matter of controversy. Spin excitations instead of phonons are thought to be responsible for the formation of Cooper pairs. Using inelastic neutron scattering, we present the first in-depth study of the magnetic excitation spectrum in momentum and energy space in the superconducting and the normal states of CeCu2Si2. A clear spin excitation gap is observed in the superconducting state. We determine a lowering of the magnetic exchange energy in the superconducting state, in an amount considerably larger than the superconducting condensation energy. Our findings identify the antiferromagnetic excitations as the major driving force for superconducting pairing in this prototypical heavy-fermion compound located near an antiferromagnetic quantum critical point., Comment: 30 pages, including supplementary material

At any pressure sensitive quantum critical point (QCP) the thermal expansion is more singular than the specific heat leading to a divergence of the Gruneisen parameter. For a magnetic field sensitive QCP, the complementary property is the magnetic Gruneisen ratio which equals the magnetocaloric effect. Here we use both properties to investigate magnetic QCPs in different heavy fermion (HF) metals starting from CeNi2Ge2. The influence of dimensionality on quantum criticality is addressed by the comparison of cubic CeIn3−xSnx with layered CeMIn5−xSnx (M = Co, Rh) systems, in which Sn doping both acts as tuning parameter and introduces slight disorder. Near the field-tuned QCP in undoped CeCoIn5 a crossover scale T⋆ is discovered which separates 2D (at T>T⋆) from 3D (at T

We report on inelastic neutron scattering experiments on the heavy-fermion superconductor CeCu2Si2 to study the magnetic excitation spectrum in single crystals. Elastic short-range magnetic correlations have been observed in the superconducting state. While the low-energy magnetic response seems to be quasielastic in the normal state above T c ≈ 600 mK , a truly inelastic response is detected in the superconducting state well below T c . This inelastic response is discussed in connection to the properties of the superconducting state.

Angle-resolved photoemission spectroscopy (ARPES) is used to study the band dispersion and the quasiparticle scattering rates in two ferropnictides systems. Our ARPES results show linear-in-energy dependent scattering rates which are constant in a wide range of control parameter and which depend on the orbital character of the bands. We demonstrate that the linear energy dependence gives rise to weakly dispersing band with a strong mass enhancement when the band maximum crosses the chemical potential. In the superconducting phase the related small effective Fermi energy favors a Bardeen-Cooper-Schrieffer (BCS)\,\cite{Bardeen1957}-Bose-Einstein (BE)\,\cite{Bose1924} crossover state., Comment: 5 pages, 4 figures Supplement 4 pages, 6 figures

We report on a detailed investigation of the magnetism in A-type CeCu2Si2 by neutron scattering, thermodynamic and transport measurements. The linewidths of the magnetic peaks broaden well below the Neel temperature TN ≈ 800mK pointing to a finite domain size/correlation length. In contrast, within the resolution of the experiments the magnetic order is static. Complementary measurements of the electrical resistivity observed a superconducting transition at T< 450mK well below TN. However, no anomaly has been detected in heat capacity measurements. All results indicate that A-type single crystals exhibit parasitic superconductivity limiting the size of the antiferromagnetically ordered regions. The observed behavior can be understood by the slight off-stoichiometry of A-type CeCu2Si2.

We have synthesized polycrystalline Sr2IrO4 and measured its magnetic susceptibility, electrical resistivity, specific heat, Seebeck coefficient, and thermal conductivity. The magnetic susceptibility χ(T) shows a ferromagnetic transition at 250 K while the behaviour above the transition temperature is well described by a Curie–Weiss fit with a small effective moment μeff = 0.33 μB and a paramagnetic Curie–Weiss temperature, θCW = +251 K, consistent with previous studies on this compound. However, specific heat, Seebeck coefficient, and thermal conductivity are all dominated by the phonon contribution and show no anomalies at the ferromagnetic transition. Electrical resistivity, unlike the single crystal, shows a huge increase, three orders of magnitude, with decreasing temperature. The temperature dependence of resistivity is logarithmic at high temperatures (210 K

Zero-field μ SR measurements have been performed on CeCu2Si2 single crystals. Antiferromagnetic order shows in an initial rapid damping of the μ + polarization. In a single crystal exhibiting antiferromagnetic order and superconductivity, there exists clear evidence of phase separation of both phenomena with a first order transition from antiferromagnetism at higher temperature to superconductivity at low temperature.

For the prototypical heavy-fermion (HF) metal CeCu2Si2 superconductivity extends to surprisingly high pressures (PO10 GPa), quite different from other Ce-based HF superconductors, like CePd2Si2, where superconductivity is confined to a narrow P-range centered around an antiferromagnetic quantum critical point (QCP). By deliberately reducing the HF mean free path, i.e. by moderate Ge-substitution for Si, superconductivity could be weakened and two distinct superconducting regimes could be established at low and high P, respectively. The low-P regime is centered at a magnetic QCP. The nature of the magnetic phase could recently be unraveled with the aid of neutron diffractometry and appears to be an incommensurate low-moment spin-density-wave phase. q 2005 Published by Elsevier Ltd.

Our comprehensive study on ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ reveals a dramatic reduction of magnetic detwinning fields compared to other $A{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ ($A=\mathrm{Ba}$, Sr, Ca) iron pnictides by indirect magnetoelastic coupling of the ${\mathrm{Eu}}^{2+}$ ions. We find that only $\ensuremath{\sim}0.1\text{ }\text{ }\mathrm{T}$ are sufficient for persistent detwinning below the local ${\mathrm{Eu}}^{2+}$ ordering; above ${T}_{\mathrm{Eu}}=19\text{ }\text{ }\mathrm{K}$, higher fields are necessary. Even after the field is switched off, a significant imbalance of twin domains remains constant up to the structural and electronic phase transition (190 K). This persistent detwinning provides the unique possibility to study the low temperature electronic in-plane anisotropy of iron pnictides without applying any symmetry-breaking external force.

We investigated the magnetic properties of the two compounds YbCo4Ge2 and YbNi5Ge3 by means of magnetic susceptibility χ, specific heat Cp and electrical resistivity ρ measurements down to 0.5 K. Our results show that both compounds present a rather well localized, trivalent Yb state which undergoes a transition into an antiferromagnetic state at T N = 2.7 K (YbNi5Ge3) and T N = 2.5 K (YbCo4Ge2), respectively.

We report on extensive neutron diffraction on single crystalline CeCu 2 ( Si 1 - x Ge x ) 2 to determine the antiferromagnetic order in the regime with low Ge content, initially called A phase due to the unknown origin of the magnetic order. Starting from the x = 0.45 alloy we followed the magnetic order to lower Ge concentrations and to pure CeCu 2 Si 2 . For all Ge concentrations x ⩽ 1 an incommensurate magnetic order with a propagation vector close to τ = ( 0.25 0.25 0.5 ) is observed. The magnetic order is determined by the nesting properties of the Fermi surface.

The electron spin resonance (ESR) signal of the heavy-fermion metal YbRh2Si2 is investigated in terms of its dependence on the partial substitution of Rh by Fe. This non-isovalent substitution gives rise to a chemical pressure and also leads to a reduction of the conduction electron density of states (hole doping). The values and anisotropy of the ESR parameters are comparable with those of Yb(Rh1−x Cox)2Si2 for Fe-contents up to at least 10%. This result provides additional evidence that the Kondo interaction allows narrow ESR spectra in a heavy fermion compound. The absence of an ESR signal for 30% Fe content points towards a distribution of different crystal electric field symmetries at the Yb-sites, leading to a dominating disorder-related broadening.

The magnetic structure of the Eu2+ moments in the superconducting EuFe2(As1-xPx)2 sample with x = 0.19 has been determined using neutron scattering. We conclude that the Eu2+ moments are aligned along the c direction below T_C = 19.0(1) K with an ordered moment of 6.6(2) mu_B in the superconducting state. An impurity phase similar to the underdoped phase exists within the bulk sample which orders antiferromagnetically below T_N = 17.0(2) K. We found no indication of iron magnetic order, nor any incommensurate magnetic order of the Eu2+ moments in the sample.

We present our results of a local probe study on EuFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ single crystals with $x$=0.13, 0.19 and 0.28 by means of muon spin rotation and ${}^{57}$Fe M\"ossbauer spectroscopy. We focus our discussion on the sample with $x$=0.19 viz. at the optimal substitution level, where bulk superconductivity ($T_{\text{SC}}=28$ K) sets in above static europium order ($T^{\text{Eu}}=20$K) but well below the onset of the iron antiferromagnetic (AFM) transition ($\sim$100 K). We find enhanced spin dynamics in the Fe sublattice closely above $T_{\text{SC}}$ and propose that these are related to enhanced Eu fluctuations due to the evident coupling of both sublattices observed in our experiments., Comment: Contribution to the 13th International Conference on Muon Spin Rotation, Relaxation and Resonance (MuSR2014)

The magnetic structure of the Eu2+ moments in the superconducting EuFe2(As1-xPx)2 sample with x = 0.15 has been determined using element specific x-ray resonant magnetic scattering. Combining magnetic, thermodynamic and scattering measurements, we conclude that the long range ferromagnetic order of the Eu2+ moments aligned primarily along the c axis coexists with the bulk superconductivity at zero field. At an applied magnetic field >= 0.6 T, superconductivity still coexists with the ferromagnetic Eu2+ moments which are polarized along the field direction. We propose a spontaneous vortex state for the coexistence of superconductivity and ferromagnetism in EuFe2(As0.85P0.15)2., Accepted, Phys. Rev. B

In this article, we review our angle- and time-resolved photoemission studies (ARPES and trARPES) on various ferropnictides. In the ARPES studies, we focus first on the band structure as a function of control parameters. We find near optimally doped compounds a Lifshitz transition of hole/electron pocket vanishing type. Second, we investigated the inelastic scattering rates as a function of the control parameter. In contrast to the heavily discussed quantum critical scenario, we find no enhancement of the scattering rate near optimally doping. Correlation effects which show up by the non-Fermi-liquid behavior of the scattering rates, together with the Lifshitz transition offer a new explanation for the strange normal state properties and suggests an interpolating superconducting state between BCS and BE condensation. Adding femtosecond time resolution to ARPES provides complementary information on electron and lattice dynamics. We report on the response of the chemical potential by a collective periodic variation coupled to coherent optical phonons in combination with incoherent electron and phonon dynamics described by a three temperature heat bath model. We quantify electron phonon coupling in terms of 2 and show that the analysis of the electron excess energy relaxation is a robust approach. The spin density wave ordering leads to a pronounced momentum dependent relaxation dynamics. In the vicinity of kF, hot electrons dissipate their energy by electron-phonon coupling with a characteristic time constant of 200fs. Electrons at the center of the hole pocket exhibit a four time slower relaxation which is explained by spin-dependent dynamics with its smaller relaxation phase space. This finding has implications beyond the material class of Fe-pnictides because it establishes experimental access to spin-dependent dynamics in materials with spin density waves. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The angular dependence of the upper critical field B c2 for A/S-type CeCu 2 Si 2 samples is determined. Both, the anisotropies within and outside the basal plane are measured using an angle-resolved resistivity technique, where an external magnetic field serves as a directional probe. Two high-quality A/S-type single crystals are studied (T N ~ 0.8 K, T c ~ 0.6 K). For the case where the magnetic field rotates within the basal plane, small variations of ~0.5% are observed and ascribed to sample misalignment. When the magnetic field rotates from the basal plane towards the [001] direction, a strong anisotropy of B c2 (~20%) is observed. By comparing the expected orbital- and Pauli-limiting fields, the strong effect of Pauli paramagnetism is realized. The latter is taken into account when using the anisotropic mass model to study the field anisotropy. Deviations are observed from such a conventional model.

The low-temperature properties of CeCu 2 Si 2 are extremely sensitive to deviations from stoichiometry and defects, leading to a strong dependence of the observed properties on sample preparation conditions. In the past few years we have developed a flux method which allows the growth of large single crystals with controlled physical properties. Here, we demonstrate that the Ce content in the starting melt, one of the most important parameters in our growth process, mainly influences the Cu/Si ratio in the single crystals but not their Ce content. A higher Cu/ Si ratio yields a smaller c/a ratio, while the unit cell volume remains roughly constant. We observe a clear correlation between the Cu/Si ratio and the physical properties, showing that electronic effects are more important than volume effects.

In order to investigate the low-temperature electronic properties of the heavy-fermion metal CeNi 2 Ge 2 , single crystals were grown using the floating zone technique. Previous investigations on polycrystals of CeNi 2 Ge 2 have revealed that the low-temperature properties, i.e., non-Fermi liquid (NFL) behavior and incipient superconductivity, are strongly sample dependent, possibly due to a slight variation of the Ni and Ge content. We prepared polycrystalline feed rods of slightly off-stoichiometric composition by induction or arc-melting methods and grew single crystals using a four-mirror optical furnace under high-purity argon atmosphere. As a result, large single crystals of few centimeter size have been obtained. Annealed pieces were investigated by measurements of the temperature dependence of the electrical resistivity ρ(T). A clear correlation between the starting composition and the obtained resistivity ratio (RR 2K ) was found with the highest ratio for a nominal starting composition of Ce 0.98 Ni 2.025 Ge 1.975 (RR 2K = 29). The temperature dependence of the electrical resistivity is similar as reported previously and displays NFL behavior below 1 K.

We investigated the non-monotonous evolution of superconductivity with pressure ( P ) in a single crystal of CeCu 2 ( Si 0.9 Ge 0.1 ) 2 ( P = 0 : antiferromagnetic order at T N ≈ 1.2 K , superconductivity at T c ≈ 0.35 K ). Electrical resistivity measurements were performed at low temperatures T ⩾ 50 mK under nearly hydrostatic pressure conditions ( 0 P 4.5 GPa ) . The existence of two distinct superconducting domes is observed in the T – P phase diagram, one at 0 P 2.2 GPa and the other at 2.2 P 4.5 GPa . Surprisingly, our preliminary data suggest that the T – P phase diagram is more rich in structure than reported previously.

By systematic investigations of the magnetic, transport, and thermodynamic properties of single crystals of ${\mathrm{EuFe}}_{2}({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}{)}_{2}$ ($0\ensuremath{\le}x\ensuremath{\le}1$), we explore the complex interplay of superconductivity and ${\mathrm{Eu}}^{2+}$ magnetism. Below 30 K, two magnetic transitions are observed for all P substituted crystals, suggesting a revision of the phase diagram. In addition to the canted $A$-type antiferromagnetic order of ${\mathrm{Eu}}^{2+}$ at $\ensuremath{\sim}20\text{ }\text{ }\mathrm{K}$, a spin glass transition is discovered at lower temperatures. Most remarkably, the reentrant spin glass state of ${\mathrm{EuFe}}_{2}({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}{)}_{2}$ coexists with superconductivity around $x\ensuremath{\approx}0.2$.

The $H\ensuremath{-}T$ phase diagrams of single crystalline electron-doped ${\text{K}}_{0.83}{\text{Fe}}_{1.83}{\text{Se}}_{2}$ (KFS1), ${\text{K}}_{0.8}{\text{Fe}}_{2}{\text{Se}}_{2}$ (KFS2) and hole-doped ${\text{Eu}}_{0.5}{\text{K}}_{0.5}{\text{Fe}}_{2}{\text{As}}_{2}$ (EKFA) have been deduced from tunnel diode oscillator-based contactless measurements in pulsed magnetic fields up to $57\phantom{\rule{4.pt}{0ex}}\text{T}$ for the interplane ($H\ensuremath{\parallel}c$) and in-plane ($H\ensuremath{\parallel}ab$) directions. The temperature dependence of the upper critical magnetic field ${H}_{c2}(T)$ relevant to EFKA is accounted for by Pauli model including an anisotropic Pauli paramagnetic contribution (${\ensuremath{\mu}}_{B}{H}_{p}=114\phantom{\rule{4.pt}{0ex}}\text{T}$ for $H\ensuremath{\parallel}ab$ and $86\phantom{\rule{4.pt}{0ex}}\text{T}$ for $H\ensuremath{\parallel}c$). This is also the case of KFS1 and KFS2 for $H\ensuremath{\parallel}ab$ whereas a significant upward curvature, accounted for by a two-gap model, is observed for $H\ensuremath{\parallel}c$. Despite the presence of antiferromagnetic lattice order within the superconducting state of the studied compounds, no influence of magnetic ordering on the temperature dependence of ${H}_{c2}(T)$ is observed.

Journal ref.: J. Phys.: Condens. Matter 25, 065602 (2013): We investigated the optical reflectivity of the heavy-fermion metal CeCu2Si2 in the energy range 3 meV - 30 eV for temperatures between 4K - 300K. The results for the charge dynamics indicate a behavior that is expected for the formation of a coherent heavy quasiparticle state: Upon cooling the spectra of the optical conductivity indicate a narrowing of the coherent response. Below temperatures of 30 K a considerable suppression of conductivity evolves below a peak structure at 13 meV. We assign this gap-like feature to strong electron correlations due to the 4f-conduction electron hybridization., Comment: 7 pages, 3 figures

Based on results from femtosecond time-resolved photoemission, we compare three different methods for determination of the electron-phonon coupling constant {\lambda} in Eu and Ba-based 122 FeAs compounds. We find good agreement between all three methods, which reveal a small {\lambda} < 0.2. This makes simple electron-phonon mediated superconductivity unlikely in these compounds., Comment: 11 pages, 3 figures

The interplay between superconductivity and Eu${}^{2+}$ magnetic moments in EuFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$ is studied with electrical resistivity measurements under hydrostatic pressure on $x=0.13$ and $x=0.18$ single crystals. We can map hydrostatic pressure to chemical pressure $x$ and show that superconductivity is confined to a very narrow range $0.18\ensuremath{\le}x\ensuremath{\le}0.23$ in the phase diagram, beyond which ferromagnetic (FM) Eu ordering suppresses superconductivity. The change from antiferro- to FM Eu ordering at the latter concentration coincides with a Lifshitz transition and the complete depression of iron magnetic order.

authoren We present a broadband optical-conductivity study of superconducting single-crystalline EuFe(AsP) with three different substitutional levels. We analyze the normal-state electrodynamics by decomposing the conductivity spectra using a Drude–Lorentz model with two Drude terms representing two groups of carriers with different scattering rates. The analysis reveals that the scattering rate of at least one of the Drude components develops linearly with temperature for each doping level. This points toward strong electron–electron correlations and a non-Fermi-liquid behavior in the P-substituted superconducting Eu-122 pnictides. We also detect a transfer of the spectral weight from mid-infrared to higher frequencies and assign it to the Hund's-rule coupling between itinerant and localized carriers. The conductivity spectra below the superconducting transition show no sharp features to be associated with the dirty-limit superconducting s-wave gaps. We interpret these results in terms of clean-limit superconductivity in EuFe(AsP). The best parametrization fit can be achieved using a two-gap model. We find that the larger gap at the hole pockets of the Fermi surface is likely to be isotropic, while the smaller gap at the electron pockets is anisotropic or even nodal. Schematic phase diagram of EuFe(AsP). The indicated phases are: spin density wave (SDW), superconductivity (SC), magnetic ordering of Eu spins (, brown), and a spin-glass state of the Eu subsystem (, green). Above the SC dome, a non-Fermi-liquid behavior (NFL) points toward a possible quantum critical point (QCP). The black arrows indicate the approximate positions of the compositions investigated in this study.

Employing the momentum sensitivity of time- and angle-resolved photoemission spectroscopy we demonstrate the analysis of ultrafast single- and many-particle dynamics in antiferromagnetic ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$. Their separation is based on a temperature-dependent difference of photoexcited hole and electron relaxation times probing the single-particle band and the spin density wave gap, respectively. Reformation of the magnetic order occurs at 800 fs, which is 4 times slower compared to electron-phonon equilibration due to a smaller spin-dependent relaxation phase space.

We study the effect of hole-doping and chemical pressure (isovalent doping) in single crystals of K$_x$Eu$_{1-x}$Fe$_2$As$_2$ and EuFe$_2$(As$_{1-y}$P$_y$)$_2$, respectively, by measurements of the thermopower, $S(T)$, and electrical resistivity, $\rho(T)$. The evolution of $S(T)$ upon doping indicates drastic changes of the electronic configuration at critical values $x_{\mathrm{cr}}=0.3$ and $y_{\mathrm{cr}}=0.21$, respectively, as the spin-density-wave transition is completely suppressed and superconductivity (SC) emerges. For the case of chemical pressure, the comparison with published ARPES measurements indicates a Lifshitz transition at $y_{cr}$. The temperature dependences $S(T)/T\propto \log T$ and $\Delta\rho\propto T$ observed in the normal state above the SC transition suggest quantum criticality in both systems.

We employed femtosecond time- and angle-resolved photoelectron spectroscopy to analyze the response of the electronic structure of the 122 Fe-pnictide parent compounds Ba/EuFe_2As_2 and optimally doped BaFe_{1.85}Co_{0.15}As_2 near the \Gamma point to femtosecond optical excitation. We identify pronounced changes of the electron population within several 100 meV above and below the Fermi level, which we explain as combination of (i) coherent lattice vibrations, (ii) a hot electron and hole distribution, and (iii) transient modifications of the chemical potential. The response of the three different materials is very similar. In the Fourier transformation of the time-dependent photoemission intensity we identify three modes at 5.6, 3.3, and 2.6 THz. While the highest frequency mode is safely assigned to the A_{1g} mode, the other two modes require a discussion in comparison to literature. The time-dependent evolution of the hot electron distribution follows a simplified description of a transient three temperature model which considers two heat baths of lattice vibrations, which are more weakly and strongly coupled to transiently excited electron population. Still the energy transfer from electrons to the strongly coupled phonons results in a rather weak, momentum-averaged electron-phonon coupling quantified by values for \lambda between 30 and 70 meV^2. The chemical potential is found to present a transient modulation induced by the coherent phonons. This change in the chemical potential is particularly strong in a two band system like in the 122 Fe-pnictide compounds investigated here due to the pronounced variation of the electrons density of states close to the equilibrium chemical potential., Comment: 10 pages, 6 figures

We study the in-plane anisotropy of the thermoelectric power and electrical resistivity on detwinned single crystals of isovalent substituted EuFe$_{2}$(As$_{1-x}$P$_{x}$)$_2$. Compared to the resistivity anisotropy the thermopower anisotropy is more pronounced and clearly visible already at temperatures much above the structural and magnetic phase transitions. Most remarkably, the thermopower anisotropy changes sign below the structural transition. This is associated with the interplay of two contributions due to anisotropic scattering and orbital polarization, which dominate at high- and low-temperatures, respectively., Comment: 8 pages, including supplemental Material

We report bulk superconductivity (SC) in Eu$_{0.2}$Sr$_{0.8}$(Fe$_{0.86}$Co$_{0.14}$)$_{2}$As$_{2}$ single crystals by means of electrical resistivity, magnetic susceptibility, and specific heat measurements with $T$$_{\mathrm{c}}$ $\simeq$ 20 K with an antiferromagnetic (AFM) ordering of Eu$^{2+}$ moments at $T$$_{\mathrm{N}}$ $\simeq$ 2.0 K in zero field. $^{75}$As NMR experiments have been performed in the two external field directions (H$\|ab$) and (H$\|c$). $^{75}$As-NMR spectra are analyzed in terms of first order quadrupolar interaction. Spin-lattice relaxation rates (1/$T_{1}$) follow a $T^{3}$ law in the temperature range 4.2-15 K. There is no signature of Hebel-Slichter coherence peak just below the SC transition indicating a non s-wave or s$_{\pm}$ type of superconductivity. The increase of 1/$T_{1}T$ with lowering the temperature in the range 160-18 K following $\frac{C}{T+\theta}$ law reflecting 2D AFM spin fluctuations.

The phase diagrams of EuFe$_{2-x}$Co$_x$As$_2$ $(0 \leq x \leq 0.4)$ and EuFe$_2$As$_{2-y}$P$_y$ $(0 \leq y \leq 0.43)$ are investigated by Eu$^{2+}$ electron spin resonance (ESR) in single crystals. From the temperature dependence of the linewidth $��H(T)$ of the exchange narrowed ESR line the spin-density wave (SDW) $(T < T_{\rm SDW})$ and the normal metallic regime $(T > T_{\rm SDW})$ are clearly distinguished. At $T > T_{\rm SDW}$ the isotropic linear increase of the linewidth is driven by the Korringa relaxation which measures the conduction-electron density of states at the Fermi level. For $T < T_{\rm SDW}$ the anisotropy probes the local ligand field, while the coupling to the conduction electrons disappears. With increasing substitution $x$ or $y$ the transition temperature $T_{\rm SDW}$ decreases linearly accompanied by a linear decrease of the Korringa-relaxation rate from 8 Oe/K at $x=y=0$ down to 3 Oe/K at the onset of superconductivity at $x \approx 0.2$ or at $y \approx 0.3$, above which it remains nearly constant. Comparative ESR measurements on single crystals of the Eu diluted SDW compound Eu$_{0.2}$Sr$_{0.8}$Fe$_2$As$_2$ and superconducting (SC) Eu$_{0.22}$Sr$_{0.78}$Fe$_{1.72}$Co$_{0.28}$As$_2$ corroborate the leading influence of the ligand field on the Eu$^{2+}$ spin relaxation in the SDW regime as well as the Korringa relaxation in the normal metallic regime. Like in Eu$_{0.5}$K$_{0.5}$Fe$_2$As$_2$ a coherence peak is not detected in the latter compound at $T_{\rm c}=21$ K, which is in agreement with the expected complex anisotropic SC gap structure.

The magnetic- and quadrupolar-ordered states of polycrystalline YbRu${}_{2}$Ge${}_{2}$ have been investigated using zero-field (ZF) muon spin relaxation (\ensuremath{\mu}SR) and neutron diffraction measurements. Specific heat measurements show three successive phase transitions with decreasing temperature from a paramagnetic to a quadrupolar state at T${}_{0}$ \ensuremath{\sim} 10 K, from the quadrupolar to a magnetic state at T${}_{1}$ \ensuremath{\sim} 6.5 K, and a possible change in the magnetic ground state at T${}_{2}$ \ensuremath{\sim} 5.5 K. Clear evidence for the magnetic transition below 7 K (spectrum at 8 K reveals paramagnetic state) and a likely change in the magnetic structure near 5.8 K is observed in the ZF \ensuremath{\mu}SR measurements. The \ensuremath{\mu}SR data, however, do not reveal any signature of magnetic order in the temperature range 8--45 K. This result is further supported by neutron-diffraction measurements, where clear magnetic Bragg peaks have been observed below 8 K, but not above it. Below 8 K, the magnetic Bragg peaks can be characterized by an incommensurate antiferromagnetic (AF) ordering with the propagation vector q = [0.352, 0, 0] and the magnetic moment 2.9(3) \ensuremath{\mu}${}_{\mathrm{B}}$ of Yb along the $b$-axis. These results are discussed in terms of quadrupolar-ordered and magnetically ordered states.

Based on low-field magnetization measurements on a series of single crystals, we present a scheme of the Eu${}^{2+}$ spin alignment in EuFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$. We explain observations of the Eu${}^{2+}$ ordering previously reported, reconciling different existing phase diagrams. The magnetic moments of the Eu${}^{2+}$ ions are slightly canted, yielding a ferromagnetic contribution along the $c$ direction that becomes stronger with pressure, until superconductivity sets in. The spin-density wave as well as the superconducting phase coexist with an antiferromagnetic interlayer coupling of the canted spins. Reducing the interlayer distance finally leads to a ferromagnetic Eu${}^{2+}$ interlayer coupling and to the suppression of superconductivity.

We have studied the electronic structure of EuFe2As2-xPx using high resolution angle-resolved photoemission spectroscopy. Upon substituting As with the isovalent P, which leads to a chemical pressure and to superconductivity, we observe a non-rigid-band like change of the electronic structure along the center of the Brillouin zone (BZ): an orbital and kz dependent increase or decrease in the size of the hole pockets near the Gamma - Z line. On the other hand, the diameter of the Fermi surface cylinders at the BZ corner forming electron pockets, hardly changes. This is in stark contrast to p and n-type doped iron pnictides where, on the basis of ARPES experiments, a more rigid-band like behavior has been proposed. These findings indicate that there are different ways in which the nesting conditions can be reduced causing the destabilization of the antiferromagnetic order and the appearance of the superconducting dome., Comment: 5 pages, 3 figures

We carried out specific-heat and ac-susceptibility experiments under hydrostatic pressure to investigate the interplay of spin-density-wave antiferromagnetism (A) and superconductivity (S) in single-crystalline AS-type CeCu2Si2. We find evidence for a line of magnetic-field- and pressure-tuned quantum critical points in the normal state in the zero-temperature magnetic field-pressure plane. Our analysis suggests an extension of this line into the superconducting state and corroborates the close connection of the underlying mechanisms leading to the formation of the antiferromagnetic and the superconducting states in AS-type CeCu2Si2., 4 pages, 4 figures

We report on the magnetic excitation spectrum in the normal state of the heavy-fermion superconductor CeCu92)Si(2) on approaching the quantum critical point (QCP). The magnetic response in the superconducting state is characterized by a transfer of spectral weight to energies above a spin excitation gap. In the normal state, a slowing-down of the quasielastic magnetic response is observed, which conforms to the scaling expected for a QCP of spin-density-wave type. This interpretation is substantiated by an analysis of specific heat data and the momentum dependence of the magnetic excitation spectrum. Our study represents the first direct observation of an almost critical slowing-down of the normal state magnetic response at a QCP when suppressing superconductivity. The results strongly imply that the coupling of Cooper pairs in CeCu(2)Si(2) is mediated by overdamped spin fluctuations.

The tetragonal heavy-fermion compound ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$ exhibits a superconducting ground state ($S$ type, ${T}_{c}=0.67\text{ }\text{ }\mathrm{K}$) close to a magnetic instability. Here, we present angle-resolved resistivity measurements of the upper critical field ${H}_{c2}$. In-plane rotation of $S$-type ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$ single crystals reveals a fourfold oscillation of ${H}_{c2}$. An extended weak-coupling BCS model for a $d$-wave symmetry including strong Pauli-limiting effects confirms the aforementioned angular dependence and points towards ${d}_{xy}$ symmetry of the order parameter.