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Strongly correlated electron systems constitute a rich reservoir for interesting physical phenomena. The competition and interplay between the localized magnetic moments in partially filled d or f electron systems and the itinerant conduction electrons states lead to novel phenomena such as complex magnetic properties, unconventional superconductivity, non-Fermi-liquid behavior, and the coexistence of superconductivity and magnetism. Such intriguing physical phenomena can be achieved by tuning the system with a control parameter, such as chemical composition, applied pressure, and magnetic field. It is interesting to study the chemical substitution effects on the correlated f electron system along with magnetic field to explore their complex phase diagram. This dissertation work focuses on experimental studies of the Ce and Eu substituted filled skutterudite system PrPt4Ge12 over a wide range of doping, magnetic field, and temperature using heat capacity measurements. The first study will focus on the specific heat and electrical resistivity measurements performed on the Pr[subscript 1-x]Ce[subscript x]Pt4Ge12 crystals. We have found that Ce monotonically suppresses the superconducting transition temperature T[subscript c] and a small Ce concentration of x = 0.14 brings the T[subscript c] to as low as 0.6 K. We further have demonstrate that small Ce substitution does not affect the multiband nature of superconductivity seen previously in the parent compound PrPt4Ge12. On the other hand, our data provide evidence that one of the two gaps is nodal in the parent compound and that Ce substitution gradually suppresses the value of the nodal gap. To understand the possible interplay between superconductivity and magnetism, we study the same parent system PrPt4Ge12, this time substituting Pr with europium. The compound so formed is Pr[subscript 1-x]Eu[subscript x]Pt4Ge12 whose end members are superconductor (x = 0) and antiferromagnetic (x = 1) at lower temperatures, so that there is the possibility of interaction between superconductivity and magnetism in the intermediate doping range. The increase of Eu concentration leads to a suppression of the superconducting transition temperature as in the case of cerium substitution. There is a low temperature heat capacity anomaly present over the whole doping range. Our analysis of the heat capacity data shows that in alloys with x = 0.5 the Schottky peaks in the heat capacity in the superconducting state appear to be due to Zeeman splitting by an internal magnetic field. Our theoretical analysis suggests that this internal magnetic field is a result of short-range antiferromagnetic correlations between the europium ions. We further investigated the effect of Eu substitution on the Pr site through heat capacity measurements on the same system in an applied magnetic field. The low temperature heat capacity peaks seen in the samples with x
The main goal of this program was to explore the possibility of novel states and behaviors in Pr-based system exhibiting quantum critical behavior, PrOs4Sb12. Upon small changes of external parameter, such as magnetic field, physical properties of PrOs4Sb12 are drastically altered from those corresponding to a superconductor, to heavy fermion, to field-induced ordered phase with primary quadrupolar order parameter. All these states are highly unconventional and not understood in terms of current theories thus offer an opportunity to expand our knowledge and understanding of condensed matter. At the same time, these novel states and behaviors are subjects to intense international controversies. In particular, two superconducting phases with different transition temperatures were observed in some samples and not observed in others leading to speculations that sample defects might be partially responsible for these exotic behaviors. This work clearly established that crystal disorder is important consideration, but contrary to current consensus this disorder suppresses exotic behavior. Superconducting properties imply unconventional inhomogeneous state that emerges from unconventional homogeneous normal state. Comprehensive structural investigations demonstrated that upper superconducting transition is intrinsic, bulk, and unconventional. The high quality of in-house synthesized single crystals was indirectly confirmed by de Haas-van Alphen quantum oscillation measurements. These measurements, for the first time ever reported, spanned several different phases, offering unprecedented possibility of studying quantum oscillations across phase boundaries.
Superconductivity and Magnetism in Skutterudites discusses superconducting and magnetic properties of a class of materials called skutterudites. With a brief introduction of the fundamental structural features of skutterudites, the book then provides a detailed assessment of the superconducting and magnetic properties, focusing particularly on the rare earth-filled skutterudites where a plethora of fascinating properties and ground states is realized due to interactions of the filler species with the framework ions. Such interactions underpin the exciting forms of superconductivity and magnetism, most notably realized in the exotic heavy fermion superconductor of composition PrOs4Sb12. The two main topics of superconductivity and magnetism are provided with a concise introduction of superconducting and magnetic properties so that a reader can appreciate and understand the main arguments in the text. This book would appeal to graduate students, postdoctoral students, and anyone interested in superconducting and magnetic properties of a large family of minerals called skutterudites. Key Features: • Gives a thorough account of the superconducting and magnetic properties of skutterudites. • Each topic is accompanied by introductory sections to assist in the understanding of the text. • Supported by numerous figures and all key references.
Superconductivity in Highly Correlated Fermion Systems documents the proceedings of the Yamada Conference XVIII on Superconductivity in Highly Correlated Fermion Systems held in Sendai, Japan, from August 31 to September 3, 1987. This book compiles selected papers on the experimental and theoretical advances in the study of superconductivity. The topics include the superconductivity and magnetism in heavy-electron materials, magneto-resistance of heavy-fermion compounds, and magnetic fluctuations and order in exotic superconductors. The fabrication and properties of thin superconducting oxide films, bipolaron models of superconductors, superconducting properties of superlattices, and flux quantization on quasi-crystalline networks are also covered. This publication is recommended for physicists and students researching on the superconductivity in highly correlated fermion systems.
The heavy fermions (HF) are strongly correlated electron systems consisting of intermetallic compounds of lanthanides and actinides ions with f -electrons unfilled shells. These systems are very rich in physics and the interplay between competing interactions results in various interesting physical phenomena such as heavy fermion behavior, unconventional superconductivity, non-Fermi-liquid behavior, coexistence of superconductivity and magnetism, and quantum criticality. The origin of such phenomena comes from the interaction of itinerant conduction states with the partially filled 4f - or 5f -electron states of rare earth elements. The study of such important physical phenomena can be possible by tuning the system using nonthermal control parameters, such as chemical composition, magnetic field, and applied pressure. So, studying the chemical pressure effect on heavy fermion systems with or without magnetic field is an intriguing idea to construct various phase diagrams and study their phase transitions. We performed heat capacity (HC), magnetoresistance (MR), and resistivity measurements on the Ce-based 115 and U-based 122 heavy fermion materials at low temperatures. We studied the nature of the quantum critical point, second-order phase transition, and the possible interplay between superconductivity and magnetism. First, we were motivated by the possibility of observing the coexistence of magnetism and unconventional superconductivity in the heavy fermion Ce1-xSmxCoIn5 alloys. We performed specific heat, MR, and resistivity measurements in different magnetic fields. We investigated how the samarium substitution on the cerium site affects the magnetic-field-tuned quantum criticality of stoichiometric CeCoIn5. We have observed Fermi-liquid to non-Fermi-liquid crossovers in the temperature dependence of the electronic specific heat and resistivity at higher external magnetic fields. We obtained the magnetic-field-induced quantum critical point (HQCP) by extrapolating the crossover temperature to zero temperature. Furthermore, we performed a scaling analysis of the electronic specific heat and confirmed the existence of the QCP. According to our findings, the magnitude of (HQCP) decreases as the samarium content rises and ultimately becomes zero. The electronic specific heat and resistivity data reveal a zero-field QCP for xcr = 0.15, which falls inside the antiferromagnetic and superconducting coexistence region. Next, we performed measurements of the heat capacity as a function of temperature in a single crystals URu2-xOsxSi2. Our experimental results show that the critical temperature of the second-order phase transition increases while the value of the Sommerfeld coefficient in the ordered state decreases with an increase in osmium concentration. We also observed the increase in the magnitude of the heat capacity at the critical temperature and a broadening of the critical fluctuations region with an increase in Os concentration. We analyze the experimental data using the Haule- Kotliar model, which identifies the 'hidden order' transition in the parent material URu2Si2 as a transition to a state with nonzero hexadecapolar moment. We showed that our experimental results are consistent with this model. In conclusion, we studied the interplay between superconductivity and magnetism in Ce based 115 and U based 122 single crystal alloys using heat capacity, magnetoresistivity, and resistivity measurements in both cryogenic systems including He-4 and He-3. The understating of various phenomena in these heavy fermions could be helpful in developing higher transition temperature superconductors, energy storage devices, quantum computers, and memory devices in the future.
This volume contains invited and contributed papers of eminent scientists who are deeply involved in the field of strongly correlated electron systems and high-Tc superconductivity. The topics of the papers include the Hubbard model, the t-J model, the polaronic model for high-Tc superconductivity, Fermi liquid and non-Fermi liquid theory, and heavy fermion systems.
The discoveries of new superconducting materials, most of them during the last 30 years, have served very much as the context for further developments in theory which continue to the present. In many of these cases, the observations of superconductivity in new materials were completely unexpected and therefore may be regarded as real discoveries. Even the most visible progress, which followed a search using, to some extent, conventional wisdom, was finally rather unexpected – the discovery of high-Tc superconductivity in copper oxides. This book presents superconductivity in this materials context and displays some of the underlying simplicity in the materials record that provided fuel for the theoretical developments. Not only is the phenomenon deeply interesting, the metallic systems where it plays out are as well, and superconductivity gives a very interesting window from which to view the nature of electrically conducting materials. The level is not advanced, yet allows the serious reader to access the current developments in the literature. Addresses in detail the exciting developments after 1980. Demonstrates that progress in superconductivity is to a large extent due to progress in materials synthesis and characterization. Gateway to the current developments in the literature.