Magnetic and Superconducting Quantum Critical Behavior of Itinerant Electronic Systems PDF Download

Author: Rastko Sknepnek
Publisher:
ISBN:
Category : Phase transformations (Statistical physics)
Languages : en
Pages : 246

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"Quantum phase transitions occur at zero temperature as a function of some non-thermal parameter, e.g., pressure or chemical composition. In addition to being of fundamental interest, quantum phase transitions are important because they are believed to underlie a number of interesting low temperature phenomena. Quantum phase transitions differ from the classical phase transitions in many important aspects, two of them being (i) the mode-coupling effects and (ii) the behavior in the presence of disorder. We devote two projects of this dissertation to each of the two."--Abstract, p. iv.

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Category :
Languages : en
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Fermi-liquid theory is one of the standard models of condensed matter physics, supplying a valid explanation of the low temperature properties of many metals. However, non-Fermi-liquid behaviours arise in many itinerant systems that exhibit a zero temperature magnetic phase transition. This thesis is mainly concerned with such quantum critical points and is an investigation into the various phenomena seen in the phase diagram of itinerant ferromagnetic systems. We apply a standard theory of itinerant quantum criticality to a quantum-critical end-point in a three-dimensional ferromagnet, before speculating on ZrZn\(_2\) being a test-bed of our results. Then we consider two explanations for the appearance of a first-order phase transition at low temperatures and attempt to reconcile them with ZrZn\(_2\). Finally we concentrate on the wide range of novel states that appear instead of a pure quantum critical point. Such exotic phases are superconducting or magnetic in nature and we investigate whether the onset of ferromagnetic quantum critical fluctuations can give rise to a certain class of such states.

Author: Dom Lal Kunwar
Publisher:
ISBN:
Category :
Languages : en
Pages : 0

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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.

Author: Andreas W Rost
Publisher: Springer Science & Business Media
ISBN: 3642145248
Category : Science
Languages : en
Pages : 154

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Our department nominated this thesis for a Springer award because we regard it as an outstanding piece of work, carried out with a remarkable level of independence. Andreas Rost joined us in 2005, as one of the inaugural Prize Students of the Scottish Universities Physics Alliance. Our research group has been working on Sr Ru O , in collaboration with our colleagues in the group of Professor Y. Maeno 3 2 7 at Kyoto, since 1998. By early 2005 we had tantalising evidence that a novel phase was forming at very low temperatures, in an overall phase diagram dominated by quantum ?uctuations. We knew that comprehensive thermodynamic information would be needed in order to understand how this was happening, and that the demanding constraints of low temperature and high magnetic ?eld meant that bespoke apparatus would need to be constructed. Andreas had studied the speci?c heat of glasses below 50 mK during his diploma thesis work at Heidelberg, and was brimming with ideas about how to proceed. We gave him advice, and constantly discussed the physics with him, but quickly realised that the best way to proceed practically was to give him a budget, and let him take the main design decisions, double-checking with us from time to time.

Author: K.H.J. Buschow
Publisher: Elsevier
ISBN: 0080457657
Category : Technology & Engineering
Languages : en
Pages : 1361

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Magnetic and superconducting materials pervade every avenue of the technological world – from microelectronics and mass-data storage to medicine and heavy engineering. Both areas have experienced a recent revitalisation of interest due to the discovery of new materials, and the re-evaluation of a wide range of basic mechanisms and phenomena. This Concise Encyclopedia draws its material from the award-winning Encyclopedia of Materials and Engineering, and includes updates and revisions not available in the original set -- making it the ideal reference companion for materials scientists and engineers with an interest in magnetic and superconducting materials. Contains in excess of 130 articles, taken from the award-winning Encyclopedia of Materials: Science and Technology, including ScienceDirect updates not available in the original set Each article discusses one aspect of magnetic and superconducting materials and includes photographs, line drawings and tables to aid the understanding of the topic at hand Cross-referencing guides readers to articles covering subjects of related interest

Author: Yogesh Pratap Singh
Publisher:
ISBN:
Category : Fermions
Languages : en
Pages : 137

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The current research interest in heavy fermion (HF) materials is for their unconventional superconductivity, their quantum critical behavior and the breakdown of Fermi liquid (FL) theory. Presently, we lack a universal understanding of the breakdown of the FL behavior in these materials. However, there are evidences which suggest that the breakdown of the FL behavior and the unconventional superconducting (SC) pairing could be the result of a zero temperature phase transition, taking place at a quantum critical point (QCP). Heavy fermions are f-electron materials in which local moments at each lattice site interact with the spin of the conduction electrons sitting at that site via an exchange coupling. There are two energy scales that result from this interaction, the Kondo temperature TK (temperature below which the local moments are screened by the spins of the conduction electrons), and the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, which characterize the induced coupling between two local moments. These two energy scales can be tuned by external parameters such as magnetic field, pressure, and chemical substitution. Such a tuning provides an opportunity to study the rich physics of these materials. This dissertation work presents experimental and theoretical studies on ab unique member of Ce-115 family of heavy fermions, i.e., Ce1-xYbxCoIn5. In the Ce-115 family of HFs, Cerium (Ce) contributes the f-electrons to form a Kondo lattice. The substitution of Ce ions by other rare earths is a widely used approach to study this system. Our selection of ytterbium for substituting Ce-site is unique in the sense that Yb appears in the intermediate valence state in this system (unlike any other substitution), thus giving rise to many of the unusual properties, which helped in the understanding the underlying physics of SC pairing, quantum criticality and non-Fermi liquid behavior in this HF. For the purpose of the studies presented in this dissertation, we utilized electronic, magneto- and thermal transport measurements. These measurements were done under high magnetic fields and pressures wherever needed. We developed a new method to identify the field-induced QCP in this material by studying its normal state. We utilized this method to locate QCP in the parent compound CeCoIn5 and determined its evolution with Yb doping in Ce1-xYbxCoIn5. Our findings show that quantum criticality in this system is suppressed by doping with Yb and a zero field QCP is obtained for the x = 0.20 Yb-doping level. Our studies also show the evolution of the many-body electronic state as the Kondo lattice of Ce moments is transformed into an array of Ce impurities with Yb-doping. Specifically, we observe a crossover from the predominantly localized Ce moment regime to the predominantly itinerant Yb f-electronic state regime. In the crossover regime, the magneto-transport behavior of the system indicates single impurity behavior of Ce ions. This result is surprising because the resistivity and specific heat measurements suggest significant amount of coherent scattering in the system. We attribute this unusual behavior to the hybridization of conduction electrons with mixed valence Yb ions, giving rise to an intermediate energy scale (TK ~ 14 K) between the single impurity regime of Ce and Ce Kondo lattice regime. Even more intriguing are the results at even higher Yb-doping levels. Large enough Yb concentrations show an increased coherence, unlike any other member of the Ce-115 family. We also identified another QCP at a higher Yb concentration of x = 0.75. An equally interesting feature in the doping dependence of this compound is the survival of NFL behavior throughout the phase diagram. The sub-linear temperature dependence of resistivity across the whole range of Yb concentrations suggests the presence of an unconventional scattering mechanism for the conduction electrons. Thus although the quantum spin fluctuations are suppressed at around 20 % of Yb doping, the NFL behavior is observed for the whole family. Our finding of an additional high doping QCP very well explains the large value of the Sommerfeld's coefficient and the persistent NFL behavior over the whole Yb-doping range. Given the complete suppression of the antiferromagnetic fluctuations for x > 0.20 and the very robust coherence and superconductivity, the possible electron pairing mechanism may involve an exchange of virtual magnetic fluctuations or a more unconventional mechanism involving virtual fluctuations into higher lying Ce crystalline field multiplets. We analyze theoretically the dependence of the superconducting critical temperature and Kondo lattice coherence temperature on pressure for both cases of clean and disordered systems. We use the approach of the large-N mean field theory, which works very well for Kondo lattice systems.

Author: V.P. Mineev
Publisher: CRC Press
ISBN: 9789056992095
Category : Science
Languages : en
Pages : 204

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Unconventional superconductivity (or superconductivity with a nontrivial Cooper pairing) is believed to exist in many heavy-fermion materials as well as in high temperature superconductors, and is a subject of great theoretical and experimental interest. The remarkable progress achieved in this field has not been reflected in published monographs and textbooks, and there is a gap between current research and the standard education of solid state physicists in the theory of superconductivity. This book is intended to meet this information need and includes the authors' original results.

Author: Dieter Wagner
Publisher: Springer Science & Business Media
ISBN: 940115080X
Category : Science
Languages : en
Pages : 455

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A summary of recent developments in theoretical and experimental studies of fluctuation effects in itinerant electron magnets, focusing on novel physical phenomena: soft-mode spin fluctuations and zero-point effects, strong spin anharmonicity, magnetic frustrations in metals, fluctuation effects in Invar alloys and low-dimensional systems. All of these may be important for novel high-technology applications.

Author:
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ISBN:
Category : Physics
Languages : en
Pages : 920

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Languages : en
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This proposal is for theoretical work on strongly correlated electron systems, which are at the center of experimental and theoretical activities in condensed-matter physics. The interest to this field is driven fascinating variety of observed effects, universality of underlying theoretical ideas, and practical applications. I propose to do research on Iron-based superconductors (FeSCs), which currently attract high attention in the physics community. My goal is to understand superconductivity and magnetism in these materials at various dopings, the interplay between the two, and the physics in the phase in which magnetism and superconductivity co-exist. A related goal is to understand the origin of the observed pseudogap-like behavior in the normal state. My research explores the idea that superconductivity is of electronic origin and is caused by the exchange of spin-fluctuations, enhanced due to close proximity to antiferromagnetism. The multi-orbital/multi-band nature of FeSCs opens routes for qualitatively new superconducting states, particularly the ones which break time-reversal symmetry. By all accounts, the coupling in pnictdes is below the threshold for Mott physics and I intend to analyze these systems within the itinerant approach. My plan is to do research in two stages. I first plan to address several problems within weak-coupling approach. Among them: (i) what sets stripe magnetic order at small doping, (ii) is there a preemptive instability into a spin-nematic state, and how stripe order affects fermions; (iii) is there a co-existence between magnetism and superconductivity and what are the system properties in the co-existence state; (iv) how superconductivity emerges despite strong Coulomb repulsion and can the gap be s-wave but with nodes along electron FSs, (v) are there complex superconducting states, like s+id, which break time reversal symmetry. My second goal is to go beyond weak coupling and derive spin-mediated, dynamic interaction between fermions, understand what sets the upper scale for attractive interaction, compute T_c, and then obtain and solve matrix non-linear gap equation for spin-mediated pairing and study various feedbacks from the pairing on fermions on ARPES spectra, optical and thermal conductivity, and other observables, The problems I have chosen are quite generic, and the understanding of magnetically-mediated superconductivity in the strong-coupling regime will not only advance the theory of superconductivity in FeSCs, but will contribute to a generic understanding of the pairing of fermions near quantum-critical points -- the problems ranging from s-wave pairing by soft optical phonons to to color superconductivity of quarks mediated by a gluon exchange.