Thermopower of Yb Heavy Fermion Compounds at High Pressure

The absolute thermopower S(T) of magnetic and non-magnetic Yb-based heavy fermions compounds was investigated at ambient pressure. High pressure experiments were performed for YbSiT _N =1.6 K) and YbCu ₂ Si ₂ . For the latter, which is not magnetically ordered at P=0, the chemical pressure effect was studied within the solid-solution Yb(Ni _x Cu _1–x ) ₂ Si ₂ (0x1). All S(T)-curves obtained at ambient pressure are characterised by a giant negative peak at a temperature T _{min, S} , and by an additional positive maximum at low temperature for the magnetic compounds. With increasing pressure, S(T _min,S ) and T _min,S were reduced, reflecting a reduction of the Kondo effect. At low temperature a magnetic transition is observed at P>8 GPa in the case of YbCu ₂ Si ₂ without a sign change in contrast to the alloys Yb(Ni _x Cu _1–x ) ₂ Si ₂.     

Theory of Unusual Superconducting Phase Transitions in Heavy Fermion Metals at High Magnetic Fields

We present a non-perturbative theory of paramagnetically-driven superconducting (SC) phase transitions in heavy-fermion metals, which reliably determines their stable SC phases, treats properly the corresponding finite jumps of the order parameter, and can account for unusual features reported recently for this type of materials. It is found that for quasi-2D heavy-fermion metals, such as CeCoIn₅, at high magnetic fields oriented perpendicular to the highly conducting planes, the effect of the Fulde-Ferrel (FF) modulation is too weak to prevent a direct first-order phase transition from the normal to the uniform SC state. For 3D heavy-fermion metals, such as URu₂Si₂, the FF modulation stabilizes, under a decreasing magnetic field, a non-uniform SC state via a second-order phase transition from the normal state. However, at a slightly lower field the modulated phase becomes unstable, transforming to a uniform SC state via a first-order transition.     

Vortex Dynamics in Heavy Fermion CeCoIn5

Local magnetic measurements utilizing a micro-Hall probe were performed on a superconducting heavy fermion CeCoIn₅ single crystal. We show that the critical current follows a power law as predicted by Ginzburg–Landau theory. This behavior is found to be universal in different heavy fermion and high-T _c superconducting materials. Furthermore, we report on remanent magnetization relaxation showing a high relaxation rate with approximately linear temperature dependence. Although qualitatively similar to another undoped heavy fermion, UBe₁₃, this relaxation rate is significantly higher, providing evidence that CeCoIn₅ can be grown in the clean limit.     

Magnetic Susceptibility of Heavy Fermion 3He-Bilayers

A ³He bilayer film adsorbed on graphite preplated with a bilayer of solid ⁴He has recently been shown to be a new heavy fermion system that can be tuned towards a quantum critical point (QCP) by varying the ³He coverage. We report new measurements of the temperature dependence of the magnetic susceptibility of this system over the temperature range 0.3 to 300 mK. The magnetisation of the ³He film is measured using pulsed NMR at a frequency of 60 kHz, on a SQUID detector setup with an untuned input circuit. The signal sensitivity attainable with this setup enables us to collect high-precision data with reasonable acquisition times, despite the low surface area of the sample (2 m²) and the small liquid susceptibility. The detailed NMR response provides an essential diagnostic tool to precisely adjust the ⁴He preplating. On cooling, the magnetisation evolves from Curie-law, through a distinct Kondo-like maximum at a temperature T ^*, to a weakly rising magnetisation far below the Curie law. This behaviour signifies the formation of the heavy fermion state of the coupled ³He layers. As the ³He coverage is increased towards the putative QCP, T ^* is tuned to lower temperature. These preliminary observations demonstrate the power of the technique, confirm previous results obtained in a different sample cell by a different method, and pave the way for a detailed study close to the QCP.     

Phenomenological Theory of Pyroelectricity

In pyroelectric materials there is a spontaneous dielectric polarization.When raising the temperature,then a voltage appears in the system.In the present manusc...     

Frustration and the Kondo Effect in Heavy Fermion Materials

The observation of a separation between the antiferromagnetic phase boundary and the small-large Fermi surface transition in recent experiments has led to the proposal that frustration is an important additional tuning parameter in the Kondo lattice model of heavy fermion materials. The introduction of a Kondo (K) and a frustration (Q) axis into the phase diagram permits us to discuss the physics of heavy fermion materials in a broader perspective. The current experimental situation is analyzed in the context of this combined “QK” phase diagram. We discuss various theoretical models for the frustrated Kondo lattice, using general arguments to characterize the nature of the f-electron localization transition that occurs between the spin liquid and heavy Fermi liquid ground-states. We concentrate in particular on the Shastry–Sutherland Kondo lattice model, for which we establish the qualitative phase diagram using strong coupling arguments and the large-N expansion. The paper closes with some brief remarks on promising future theoretical directions.     

晶体滑移变形的表象理论及其应用

本文运用塑性理论和晶体学知识,结合前人的工作,提出了一种晶体滑移变形的表象理论。实例分析表明,若己知晶体的滑移系,用该理论可以判断滑移系的独立性;预计独立滑移系的应变分量。     

High Pressure Resistivity of the Heavy Fermion Compound CeCu6

High pressure resistivity measurements up to 8.5 GPa performed on the archetypal heavy fermion compound CeCu₆ are reported down to temperatures of 30 mK. The pressure variation of the different characteristic temperatures and resistivity exponents are analyzed in the framework of recent theories aiming to understand quantum phase transitions.     

Unconventional Superconductivity and Heavy Fermion Behavior in PrOs4Sb12

Research carried out during the last several years on the heavy fermion superconductor PrOs₄Sb₁₂ is reviewed. The compound PrOs₄Sb₁₂ exhibits unconventional strong coupling superconductivity below a superconducting critical temperature T _c = 1.85 K. The superconducting state breaks time reversal symmetry, apparently consists of several distinct superconducting phases, some of which may have point nodes in the energy gap, and may involve triplet spin pairing of electrons. The determination of the Pr³⁺ energy level scheme in the crystalline electric field, as well as the discovery and characterization of the high field-ordered phase (HFOP), which has been identified with antiferroquadrupolar order, are described. Studies of the substitutional systems Pr(Os_1−x Ru _x )₄Sb₁₂, Pr_1−x La _x Os₄Sb₁₂, and Pr_1−x Nd _x Os₄Sb₁₂ are summarized and compared. PACS 71.27.+a; 74.62.Dh; 74.70.Tx.     

First-Principles Study of the Electronic Structure of Heavy Fermion YbNi2

We investigate the electronic properties of YbNi₂ by means of band structure calculations based on the density functional theory within LDA (local density approximation), fully relativistic, and LDA+U schemes. The 4f derived bands are studied within a relativistic framework which yields flat and spin-orbit split bands, and a correlated band method (LDA+U) that includes correlation corrections. In both cases, the 4f bands, which is located roughly 200 meV below the Fermi level (E _F ), hybridize weakly with the dispersive Ni-3d bands. When the fully relativistic scheme is applied, the 4f derived bands split into lower and higher bands due to spin-orbit coupling effects. The 4f electrons are delocalized through the hybridization with conduction electrons, and the hybridization between f and conduction d electrons also plays a important role in YbNi₂. The on-site Coulomb potential is added to the Yb-derived 4f orbitals, the degeneracy between the 4f orbitals would be lifted partially and they are split into three manifolds bands. The Fermi surface splits into three different sheets which are from main the Yb-4f derived bands and Ni-3d bands. Band structure calculations reveal a saddle points existence at the L point in the energy dispersion curve closed to E _F , whereby, we think YbNi₂ might have a superconducting properties. In addition, the quasiparticle mass enhancement inferred by comparing γ to the density of states (DOS) at the Fermi level indicates the effective mass of YbNi₂ enhanced with the fully relativistic results.     

A Phenomenological Theory and Numerical Procedure for Chemo-Mechanical Coupling Behavior of Hydrogel

Coupling and interaction of multi-physical fields exist in hydrogel consisting of a fluid and a solid under external stimulus. In this paper, a phenomenological theory for chemo-mechanical coupling behavior and finite element formulation are developed, based on the thermodynamic laws. The free energy function is constructed and used to derive the constitutive equations and governing equations for a linear coupling system including a chemical effect. Equivalent integral forms of the governing equations and coupled finite element equations are obtained by a variational principle. Numerical examples demonstrate the interaction of chemical and mechanical effects of hydrogel under external force loadings and chemical stimuli. It is shown that the chemo-mechanical coupling behavior of hydrogel can be described by the theory and numerical method presented in this paper.     

Transport Measurements of the Heavy Fermion Superconductor CeCu2Si2 Under Pressure

Superconducting and normal state properties of a single crystal of CeCu₂Si₂ have been investigated by resistivity measurements under hydrostatic pressure and magnetic field. The low temperature resistivity behaviour ( = ₀ + AT ², which is observed up to a temperature T _A) is considered within the self-consistent renormalized (SCR) spin fluctuation model. Contrary to the model predictions, the product AT ² _A increases on approaching the quantum critical point (QCP). The temperature dependence of the upper critical field is analyzed assuming strong coupling and an intermediate regime between the clean and dirty limits. Both the low temperature resistivity behaviour at high pressure and the parameters deduced from the fits of H _c2 point to a pressure induced decrease of the quasi-particle effective mass m*. The anisotropy of the initial slope of H _c2(T) and therefore that of the effective mass was found to change under pressure.     

A Phenomenological Model for Desorption in Polymers

A phenomenological formulation is adopted to investigate desorption in polymers. The speed of the front is studied and the well-posedness of the general model is analyzed. Numerical simulations illustrating the dynamics of the desorption process described by the proposed model are included.     

Controlled Creation of Structural Defects in the Heavy Fermion Superconductor UPt3 and Its Influence on the Superconducting Properties

The superconducting properties of the heavy fermion UPt ₃ have been changed by irradiation with high energy electrons which creates point defects in a reproducible and controled way. Measurements of the residual resistivity, critical temperature, upper critical field and thermal conductivity have been realized on these irradiated samples. The strong suppresion of superconductivity with increasing defect concentration is in agreement with the theory of unconventional superconductivity. However, our thermal conductivity data contradicts the simple predictions derived from the most popular modelsE _1g and E _2u) of the superconducting order parameter in UPt ₃ .     

Effect of Pressure on Transport Properties of Heavy Fermion CePtSi<Subscript>2</Subscript>

We measured the resistivity of heavy fermion CePtSi₂ under pressure. At ambient pressure, CePtSi₂ shows an antiferromagnetic (AF) transition at 2 K and a Fermi liquid like T ² dependence in resistivity below 1.5 K. With increasing pressure, the AF phase and T ² dependence are suppressed. Above 1.4 GPa, a T linear dependence and pressure-induced superconductivity were found with the maximum T _c=0.14 K at 1.7 GPa. Above 2 GPa, the T ² dependence recovers just above T _c.     

Field Tuning and Quantum Criticality in Ytterbium Based Heavy Electron Compounds

We review here the results of magnetization, specific heat, and inelastic neutron scattering measurements conducted on Yb₃Pt₄, Yb₂Pt₂Pb, Yb₅Pt₉, and YbRh₂Pb, which indicate that the Yb moments in these heavy electron compounds are appreciably localized, at least in their paramagnetic states. The magnetic ground states in each are isolated magnetic doublets, and we show that magnetic fields suppress long ranged magnetic order and lead to a characteristic magnetic field-temperature phase diagram where order vanishes suddenly above a critical value for the field. We argue that the stability of magnetic order in these compounds arises from the competition between the Zeeman splitting gμ _B H of the ground state doublet, which favors a spin polarized state with minimal entropy and without long range order, and the exchange splitting Δ of the doublet, which enables long ranged magnetic order.     

New State of Matter: Heavy Fermion Systems,Quantum Spin Liquids,Quasicrystals, Cold Gases,and High-Temperature Superconductors

We report on a new state of matter manifested by strongly correlated Fermi systems including various heavy fermion (HF) metals, two-dimensional quantum liquids such as \(^3\)He films, certain quasicrystals, and systems behaving as quantum spin liquids. Generically, these systems can be viewed as HF systems or HF compounds, in that they exhibit typical behavior of HF metals. At zero temperature, such systems can experience a so-called fermion condensation quantum phase transition (FCQPT). Combining analytical considerations with arguments based entirely on experimental grounds, we argue and demonstrate that the class of HF systems is characterized by universal scaling behavior of their thermodynamic, transport, and relaxation properties. That is, the quantum physics of different HF compounds is found to be universal, emerging irrespective of the individual details of their symmetries, interactions, and microscopic structure. This observed universal behavior reveals the existence of a new state of matter manifest in HF compounds. We propose a simple, realistic model to study the appearance of flat bands in two-dimensional ensembles of ultracold fermionic atoms, interacting with coherent resonant light. It is shown that signatures of these flat bands may be found in peculiarities in their thermodynamic and spectroscopic properties. We also show that the FCQPT, in generating flat bands and altering Fermi surface topology, is an essential progenitor of the exotic behavior of the overdoped high-temperature superconductors represented by \(\hbox {La}_{2-x}\hbox {Sr}_{x}x\hbox {CuO}_4\), whose superconductivity differs from that predicted by the classical Bardeen–Cooper–Schrieffer theory. The theoretical results presented are in good agreement with recent experimental observations, closing the colossal gap between these empirical findings and Bardeen–Cooper–Schrieffer-like theories.     

(p,T,H) Phase Diagram of Heavy Fermion Systems: Some Systematics and Some Surprises from Ytterbium

Pressure is the cleanest way to tune heavy fermion systems to a quantum phase transition in order to study the rich physics and competing phases, and the comparison between ytterbium and cerium systems is particularly fruitful. We briefly review the mechanisms in play and show some examples of expected and unexpected behaviour. We emphasise the importance of the valence changes under pressure and show how modern synchrotron techniques can accurately determine this, including at low temperature.