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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 book offers a new approach to the long-standing problem of high-Tc copper-oxide superconductors. It has been demonstrated that starting from a strongly correlated Hamiltonian, even within the mean-field regime, the “competing orders” revealed by experiments can be achieved using numerical calculations. In the introduction, readers will find a brief review of the high-Tc problem and the unique challenges it poses, as well as a comparatively simple numerical approach, the renormalized mean-field theory (RMFT), which provides rich results detailed in the following chapters. With an additional phase picked up by the original Hamiltonian, some behaviors of interactive fermions under an external magnetic field, which have since been experimentally observed using cold atom techniques, are also highlighted.
This volume comprises 8 articles written by lecturers of 50th Karpacz Winter School of Theoretical Physics and their collaborators. The contributing lecturers are outstanding researchers, and experts in various fields of physics, in particular in condensed matter, quantum phase transitions and quantum critical phenomena. The articles reflect the contents of the lectures given for school participants and cover various aspects of quantum phase transitions and quantum critical phenomena, from experimental to mathematical ones. For those wishing to get acquainted with the subject, extensive lists of references are provided in the articles.
In this book the author presents two important findings revealed by high-precision magnetic penetration depth measurements in iron-based superconductors which exhibit high-transition temperature superconductivity up to 55 K: one is the fact that the superconducting gap structure in iron-based superconductors depends on a detailed electronic structure of individual materials, and the other is the first strong evidence for the presence of a quantum critical point (QCP) beneath the superconducting dome of iron-based superconductors. The magnetic penetration depth is a powerful probe to elucidate the superconducting gap structure which is intimately related to the pairing mechanism of superconductivity. The author discusses the possible gap structure of individual iron-based superconductors by comparing the gap structure obtained from the penetration depth measurements with theoretical predictions, indicating that the non-universal superconducting gap structure in iron-pnictides can be interpreted in the framework of A1g symmetry. This result imposes a strong constraint on the pairing mechanism of iron-based superconductors. The author also shows clear evidence for the quantum criticality inside the superconducting dome from the absolute zero-temperature penetration depth measurements as a function of chemical composition. A sharp peak of the penetration depth at a certain composition demonstrates pronounced quantum fluctuations associated with the QCP, which separates two distinct superconducting phases. This gives the first convincing signature of a second-order quantum phase transition deep inside the superconducting dome, which may address a key question on the general phase diagram of unconventional superconductivity in the vicinity of a QCP.
While the macroscopic phenomenon of superconductivity is well known and in practical use worldwide, the current theoretical paradigm for superconductivity suffers from a number of limitations. For example, there is no currently accepted theoretical explanation for the pattern of superconductor critical temperatures in the periodic table. Historical developments in condensed matter were strongly focused on the similarities of all metals and the electron gas model, with little attention paid to their real differences. Accessible by a wide audience, Superconductivity Revisited explores the work of those who investigated the differences, and laid the foundation for all current and future work. Topics Include Pattern of Elemental Superconductors in the Periodic Table High-Temperature Superconductors Electron Spin in Superconductors Heat Capacity and Magnetic Susceptibility in Superconductors Quantum Foundations of Molecular Electricity and Magnetism Metals and Insulators Electron Transport in Metals Magnetoresistance Quantum Hall Effect Type I and Type II Superconductivity Superconductivity Revisited starts from the foundations and shows that the current theory of the subject cannot explain the pattern of superconductors in the periodic table, as the theory depends on a theory of resistivity not congruent with the Sommerfeld equation. Partial wave scattering is introduced as a route to deal with these issues. The book develops a theory of superconductivity that includes the periodic table. The new, coherent, understandable theory of superconductivity is directly based on thermodynamics, scattering theory, and molecular quantum mechanics.
Atomic thin quantum materials that host exotic quantum phases such as unconventional superconductivity, correlated magnetic insulating, and quantum anomalous Hall insulating states have become a new research frontier due to their intriguing physical phenomena and potential applications that may revolutionize human life. Atomic-thin quantum materials reduce the dimensionality into two, which leads to many unique properties. For example, monolayer or bilayer thin films reduce interlayer coupling compared with their 3D bulk counterparts, thus disentangling the interlayer interaction from interlayer interaction; weakened Coulomb screening at the 2D limit enhances the electron correlations; increased phase fluctuations of the order parameter make it possible for studying quasi-long range order, and quantum confinement from the third dimension may introduce quantum size effect. Thanks to the advancement of material engineering techniques, synthesis or separation of atomic thin quantum materials have become possible and popular. Among these techniques, such as molecular beam epitaxy, physical/chemical vapor deposition, and mechanical exfoliationmolecular beam epitaxy is very powerful due to its precise control of layer thickness and cleanness across a macroscopic wafer scale. Combining molecular beam epitaxy with other in-situ characterization techniques such as scanning tunneling microscopy and double-coil mutual inductance system allows us to design, control, and characterize the atomic thin quantum materials in both microscopic and macroscopic length scales. Here in this dissertation, I first briefly introduce the background motivations of the atomic thin quantum materials in the first chapter. In the second chapter, I cover the research techniques that have been employed during my graduate studies such as molecular beam epitaxy, scanning tunneling microscopy, and double-coil mutual inductance system. The third chapter is devoted to superconductivity in the 2D limit, I use monolayer indium thin film as a platform to discuss how the geometric arrangement of a monolayer indium thin-film affects the superconductivity transition temperature and superfluid density. In the fourth chapter, I use monolayer 1T phase NbSe2 as a material example to discuss the manifestation of strong electron correlation and how it leads to magnetic charge-transfer insulators in its charge density wave phase. I also discuss the interplay of local magnetic moments with metallic/superconducting states, which lead to Kondo resonances and Yu-Shiba-Rusinov-like bound states. The fifth chapter focuses on the concept of band topology and its accompanied surface states. I use intrinsic magnetic topological insulator MnBi2Te4 as a material platform to discuss the interplay of the Dirac mass gap with magnetism and its theoretical understanding. Finally, I make some concluding remarks, including so far confronted obstacles in these topics, and comment on the further steps to make for future advancement
Ruthenate materials have come into focus recently because of their very interesting magnetic and superconducting properties. From the first international conference on this topic, the present volume has emerged as a first coherent account of the considerable body of work, both theoretical and experimental, gathered in this field within a short time span. The book has been written in the form of a set of lectures and tutorial reviews with the aim of providing the research community with both a comprehensive and modern source of reference and a tutorial introduction for postgraduate students and nonspecialists working in related areas.
The Committee to Assess the Current Status and Future Direction of High Magnetic Field Science in the United States was convened by the National Research Council in response to a request by the National Science Foundation. This report answers three questions: (1) What is the current state of high-field magnet science, engineering, and technology in the United States, and are there any conspicuous needs to be addressed? (2) What are the current science drivers and which scientific opportunities and challenges can be anticipated over the next ten years? (3) What are the principal existing and planned high magnetic field facilities outside of the United States, what roles have U.S. high field magnet development efforts played in developing those facilities, and what potentials exist for further international collaboration in this area? A magnetic field is produced by an electrical current in a metal coil. This current exerts an expansive force on the coil, and a magnetic field is "high" if it challenges the strength and current-carrying capacity of the materials that create the field. Although lower magnetic fields can be achieved using commercially available magnets, research in the highest achievable fields has been, and will continue to be, most often performed in large research centers that possess the materials and systems know-how for forefront research. Only a few high field centers exist around the world; in the United States, the principal center is the National High Magnetic Field Laboratory (NHMFL). High Magnetic Field Science and Its Application in the United States considers continued support for a centralized high-field facility such as NHFML to be the highest priority. This report contains a recommendation for the funding and siting of several new high field nuclear magnetic resonance magnets at user facilities in different regions of the United States. Continued advancement in high-magnetic field science requires substantial investments in magnets with enhanced capabilities. High Magnetic Field Science and Its Application in the United States contains recommendations for the further development of all-superconducting, hybrid, and higher field pulsed magnets that meet ambitious but achievable goals.
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.