Crystal Growth and Investigation of CeCu2Si2 and YbRu2Ge2: Competition/Co-existence of Superconducting, Dipolar and Quadrupolar order

Strongly correlated systems represent one of the major topics in modern solid-state physics. The rare-earth intermetallic compounds belonging to this class provide rich grounds for investigation of various phenomena. They show one of the most fascinating types of ground states in condensed-matter physics. Among them are: Kondolattice effects, heavy fermion behavior, superconductivity, magnetic order, non-Fermi liquid behavior, and quantum phase transition. Those properties occur mainly due to two competing interactions, the Kondo effect and the Ruderman-Kittel-Kasuya-Yosida interaction. The study of unconventional superconductivity in heavy fermion systems attracted great interest over the last two decades. The exotic pairing mechanism (e.g. mediated by spin fluctuations) and the symmetry of the order parameter have been intensively discussed especially for superconducting Ce- and U-based compounds. The discovery of superconductivity below 0.65 K in the heavy-electron system CeCu2Si2 appeared unexpected as magnetic moments were known to destroy superconductivity. The pronounced anomaly of the electronic specific heat at Tc, however, strongly suggests that the unusual low temperature properties of heavy-electron systems indicate an unconventional origin of the superconducting phase. Since the discovery of superconductivity in CeCu2Si2, the question of the exact nature and origin of this phenomenon has been the subject of great interest in research. It has been postulated, that the superconductivity in these materials is not caused primarily by the usual electronphonon mechanism but rather by some magnetic interaction. CeCu2Si2 shows a rich phase diagram with different phases competing, depending on slight changes of the interactions. These properties are also strongly sample dependent. Small changes in composition eventually lead to changes in the electron interactions. These unique properties make this compound a fascinating subject of study. On the other hand it is difficult to synthesis the single crystals with defined physical properties. During the last three decades CeCu2Si2 has been an active research topic, from single crystal growth to sophisticated experiments like high-pressure measurements, neutron experiments etc. This thesis involved systematic investigations of the phase diagram, starting with the single crystal growth of different ground state and catheterized their physical properties including neutron experiments. The second part of the thesis contains, for the first time (to our knowledge), detailed investigations of the very interesting physical properties on YbRu2Ge2, which shows a quasiquartet crystalelectric-field ground state with quadrupolar ordering at 10 K. The first chapter is an overview of the underlying physics of heavy- fermion systems, including a description of the Doniach phase diagram. The second part of this chapter gives a brief introduction of crystalline-electric-field effect in rare-earth intermetallic compounds. Chapter 2. describes the experimental methods and crystal growth details. This chapter provides the main focus of this dissertation, presenting detailed experimental results for the different types of CeCu2Si2 crystals. Magnetic, thermodynamic and transport measurements on the new generation of large highquality single crystals were conducted by our research group. Furthermore, complimentary neutron investigations have been performed, which allowed to conclude that both magnetic and superconducting phases compete with each other. The effect of Ge doping on the Si site and possible coexistence of magnetic and superconducting phase is discussed in chapter 4. Chapter 5 provides a detailed investigation of the physical properties of YbRu2Ge2 single crystals. In addition, neutron experiments as well as the determination the magnetic structure and crystalline-electric-field scheme of YbRu2Ge2 are presented. The μSR experiments were also performed as a complimentary method to the neutron experiments. Chapter 6 ends the dissertation with a conclusion and summary

Interplay of magnetism and superconductivity in EuFe2_{2}(As1−x_{1-x}Px_{x})2_{2} single crystals probed by muon spin rotation and 57{}^{57}Fe M\"ossbauer spectroscopy

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

Super-heavy electron material as metallic refrigerant for adiabatic demagnetization cooling

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

Fully gapped superconductivity with no sign change in the prototypical heavy-fermion CeCu2Si2

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

Super-heavy electron material as metallic refrigerant for adiabatic demagnetization cooling

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.This article is published as Tokiwa, Yoshifumi, Boy Piening, Hirale S. Jeevan, Sergey L. Bud’ko, Paul C. Canfield, and Philipp Gegenwart. "Super-heavy electron material as metallic refrigerant for adiabatic demagnetization cooling." Science Advances 2, no. 9 (2016): e1600835. DOI: 10.1126/sciadv.1600835. Posted with permission.</p

Local and collective magnetism of EuFe2As2

We present an experimental study of the local and collective magnetism of EuFe2As2 that is isostructural with the high-temperature superconductor parent compound BaFe2As2. In contrast to BaFe2As2, where only Fe spins order, EuFe2As2 has an additional magnetic transition below 20 K due to the ordering of the Eu2+ 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. FeKbeta x-ray emission experiments on EuFe2As2 single crystals reveal a local magnetic moment of 1.3 ± 0.15 muB at 15 K that increases slightly to 1.45 +- 0.15 muB at 300 K. Resonant inelastic x-ray scattering experiments performed on the same crystals show dispersive broad (in energy) magnetic excitations along (0,0) -> (1,0) and (0,0) -> (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 AFe2As2 series (A = Ba, Ca, and Sr), especially the well-characterized BaFe2As2. 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

Experimental evidence for importance of Hund's exchange interaction for incoherence of charge carriers in iron-based superconductors

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.Comment: 5 pages; 3 figure