Features of the electronic structure of cerium-based systems

Ce-based systems were studied by means of x-ray photoelectron spectroscopy. The density of states was calculated by the TB-LMTO-ASA and APW+LO band methods. A correlation was found between 4f level localization and x-ray photoelectron spectra with characteristics of the heavy fermion state. It was shown that a high value of interatomic interaction is characteristic of Ce-based systems with heavy fermions under normal conditions.     

Unique spin dynamics and unconventional superconductivity in the layered heavy fermion compound CeIrIn5: NQR evidence

We report measurements of the 115In nuclear spin-lattice relaxation rate ( 1/T1) between T = 0.09 and 100 K in the new heavy fermion (HF) compound CeIrIn5. At 0.4 < or = T< or = 100 K, 1/T1 is strongly T-dependent, which indicates that CeIrIn5 is much more itinerant than known Ce-based HFs. We find that 1/T1T, subtracting that for LaIrIn5, follows a (1 / T+straight theta)3/4 variation with straight theta = 8 K. We argue that this novel feature points to anisotropic, due to a layered crystal structure, spin fluctuations near a magnetic ordering. The bulk superconductivity sets in at 0.40 K below which the coherence peak is absent and 1/T1 follows a T3 variation, which suggests unconventional superconductivity with line-node gap.     

Electronic structure and magnetic properties of YbCuAl

The electronic band structure of YbCuAl has been calculated using the self-consistent full potential nonorthogonal local orbital minimum basis scheme based on the density functional theory. We investigated the electronic structure with the spin–orbit interaction and on-site Coulomb potential for the Yb-derived 4f orbitals to obtain the correct ground state of YbCuAl. The exchange interaction between local f electrons and conduction electrons play an important role in the heavy fermion characters of them. The fully relativistic band structure scheme shows that spin–orbit coupling splits the 4f states into two manifolds, the 4f_7/2 and the 4f_5/2 multiplet.     

Electronic structure and magnetism of YbRhSn

The electronic band structure of YbRhSn has been calculated using the self-consistent full potential nonorthogonal local orbital minimum basis scheme based on the density functional theory. We investigated the electronic structure with the spin-orbit interaction and on-site Coulomb potential for the Yb-derived 4f orbitals to obtain the correct ground state of YbRhSn. The exchange interaction between local f electrons and conduction electrons play an important role in the heavy fermion characters of them. The fully relativistic band structure scheme shows that spin-orbit coupling splits the 4f states into two manifolds, the 4f_7/2 and the 4f_5/2 multiplet.     

Model band structure calculations for heavy fermion systems

We present calculations of the quasiparticle band structure for simple heavy fermion systems, based on a mean-field approximation of the Anderson Hamiltonian. The aim of this investigation is to study the influence of the parameters of the Anderson Hamiltonian, position of thef-level and hybridization, on the quasiparticle bands and the form of the Fermi surface. We also calculate the static susceptibility.     

含铀化合物UAl3和USn3电子结构的密度泛函研究

用第一性原理的全势能LMTO密度泛函能带计算方法研究了具有简单立方Cu₃Au 结构的含U化 合物UX₃(X=Al, Sn)的电子结构.对于重原子U的相对论效应，除了用标量相对论 加以修正 外，还加入了自旋-轨道耦合的修正.研究结果定性地说明了由于不同的交换关联电子势场的 作用，在这两种结构相同的含U合金中，U的5f电子态具有完全不同的性质，即在UAl_{3< /sub>和US n3中U的5f态分别表现为巡游扩展态和局域态行为，通过St 关键词： 铀化合物 电子态 密度泛函理论 自旋-轨道耦合}     

Energy scales and the non-Fermi liquid behavior in YbRh<Subscript>2</Subscript>Si<Subscript>2</Subscript>

Multiple energy scales are detected in measurements of the thermodynamic and transport properties in heavy fermion metals. We demonstrate that the experimental data on the energy scales can be well described by the scaling behavior of the effective mass at the fermion condensation quantum phase transition, and show that the dependence of the effective mass on temperature and applied magnetic fields gives rise to the non-Fermi liquid behavior. Our analysis is placed in the context of recent salient experimental results. Our calculations of the non-Fermi liquid behavior, of the scales and thermodynamic and transport properties are in good agreement with the heat capacity, magnetization, longitudinal magnetoresistance and magnetic entropy obtained in remarkable measurements on the heavy fermion metal YbRh₂Si₂.     

Electronic structure of La (0001) thin films on W (110) studied by photoemission spectroscopy and first principle calculations

Surface states that have a dz2 symmetry around the center of the surface Brillouin zone(BZ)have been regarded common in closely-packed surfaces of rare-earth metals.In this work,we report the electronic structure of dhcp La(0001)thin films by ultrahigh energy resolution angle-resolved photoemission spectroscopy(ARPES)and first principle calculations.Our first principle analysis is based on the many-body approach,therefore,density function theory(DFT)combined with dynamic mean-field theory(DMFT).The experimentally observed Fermi surface topology and band structure close to the Fermi energy qualitatively agree with first principle calculations when using a renormalization factor of between 2 and 3 for the DFT bands.Photon energy dependent ARPES measurements revealed clear kZ dependence for the hole-like band around the BZ center,previously regarded as a surface state.The obtained ARPES results and theoretical calculations suggest that the major bands of dhcp La(0001)near the Fermi level originate from the bulk La 5d orbits as opposed to originating from the surface states.     

Electronic structure and magnetism of CeCoGe3

The electronic band structure of CeCoGe₃ has been calculated using the self-consistent full potential nonorthogonal local orbital minimum basis scheme based on density functional theory. We investigated the electronic structure with the spin-orbit interaction and on-site Coulomb potential for the Ce-derived 4f orbitals to obtain the correct ground state of CeCoGe₃. The exchange interaction between local f electrons and conduction electrons play an important role in their heavy fermion characters. The fully relativistic band structure scheme shows that spin-orbit coupling splits the 4f states into two manifolds, the 4f_7/2 and the 4f_5/2 multiplets.     

Phases of renormalized lattice gauge theories with fermions

Starting from the formulation of gauge theories on a lattice we derive renormalization group transformation of the Migdal-Kadanoff type in the presence of fermions. We consider the effect of the fermion vacuum polarization on the gauge Lagrangian but we neglect fermion mass renormalization. We work out the weak coupling and strong coupling expansion in the same framework. Asymptotic freedom is recovered for the non-Abelian case provided the number of fermion multiplets is lower than a critical number. Fixed points are determined both for the U(1) and SU(2) case. We determine the renormalized trajectories and the phases of the theory.     

Heavy fermion quantum criticality

During the last few years, investigations of rare-earth materials have made clear that heavy fermion quantum criticality exhibits novel physics not fully understood. In this work, we write for the first time the effective action describing the low energy physics of the system. The f fermions are replaced by a dynamical scalar field whose nonzero expected value corresponds to the heavy fermion phase. The effective theory is amenable to numerical studies as it is bosonic, circumventing the fermion sign problem. Via effective action techniques, renormalization group studies, and Callan-Symanzik resummations, we describe the heavy fermion criticality and predict the heavy fermion critical dynamical susceptibility and critical specific heat. The specific heat coefficient exponent we obtain (0.39) is in excellent agreement with the experimental result at low temperatures (0.4).     

Fermi surface and antiferromagnetism in the Kondo lattice: an asymptotically exact solution in d>1 dimensions

Interest in the heavy fermion metals has motivated us to examine the quantum phases and their Fermi surfaces within the Kondo lattice model. We demonstrate that the model is soluble asymptotically exactly in any dimension d>1, when the Kondo coupling is small compared with the RKKY interaction and in the presence of antiferromagnetic ordering. We show that the Kondo coupling is exactly marginal in the renormalization group sense, establishing the stability of an ordered phase with a small Fermi surface AFS. Our results have implications for the global phase diagram of the heavy fermion metals, suggesting a Lifshitz transition inside the antiferromagnetic region and providing a new perspective for a Kondo-destroying antiferromagnetic quantum critical point.     

Anomalies in the Fermi surface and band dispersion of quasi-one-dimensional CuO chains in the high-temperature superconductor YBa2Cu4O8

We have investigated the electronic states in quasi-one-dimensional CuO chains by microprobe angle resolved photoemission spectroscopy. We find that the quasiparticle Fermi surface consists of six disconnected segments, consistent with recent theoretical calculations that predict the formation of narrow, elongated Fermi surface pockets for coupled CuO chains. In addition, we find a strong renormalization effect with a significant kink structure in the band dispersion. The properties of this latter effect [energy scale (～40 meV), temperature dependence, and behavior with Zn-doping] are identical to those of the bosonic mode observed in CuO2 planes of high-temperature superconductors, indicating they have a common origin.     

Superconducting gap structure of the 115s revisited

Density functional theory calculations of the electronic structure of Ce- and Pu-based heavy fermion superconductors in the so-called 115 family are performed. The gap equation is used to consider which superconducting order parameters are most favorable assuming a pairing interaction that is peaked at (π, π, qz)—the wavevector for the antiferromagnetic ordering found in close proximity. In addition to the commonly accepted dx2?y2 order parameter, there is evidence that an extended s-wave order parameter with nodes is also plausible. We discuss whether these results are consistent with current observations and possible measurements that could help distinguish between these scenarios.     

Hybridization Induced Competitive Scanning Tunneling Interference Process into a Heavy Fermion System

We theoretically present the results for a scanning interference tunneling process between a metallic tip and a heavy fermion system. The density of states(DOS) and the differential conductance at zero temperature under different c-f band hybridizations, as well as the interference Fano ratio strength in the heavy fermion system,are calculated. It is found that the hybridization strength gives rise to the splitting effect in the DOS around the Fermi energy. Also the interference Fano ratio strength makes the differential conductance characteristics strongly asymmetric.     

Interaction of phonons and dirac fermions on the surface of Bi2Se3: a strong Kohn anomaly

We report the first measurements of phonon dispersion curves on the (001) surface of the strong three-dimensional topological insulator Bi2Se3. The surface phonon measurements were carried out with the aid of coherent helium beam surface scattering techniques. The results reveal a prominent signature of the exotic metallic Dirac fermion quasiparticles, including a strong Kohn anomaly. The signature is manifest in a low energy isotropic convex dispersive surface phonon branch with a frequency maximum of 1.8 THz and having a V-shaped minimum at approximately 2kF that defines the Kohn anomaly. Theoretical analysis attributes this dispersive profile to the renormalization of the surface phonon excitations by the surface Dirac fermions. The contribution of the Dirac fermions to this renormalization is derived in terms of a Coulomb-type perturbation model.     

Ce 基重费米子材料的电子态研究与维度调控

重费米子材料作为一类典型的强关联电子体系,蕴含着非常规超导、奇异金属、量子临界、 磁有序、重电子态、关联拓扑态等新奇的量子态,而4f 电子在其中扮演着重要的作用。随着高分 辨角分辨光电子能谱和薄膜生长技术的发展,精确探测重费米子材料中4f 电子在能量/动量空间 的色散和谱权重成为了可能,这为从微观上理解这类材料中的电子关联效应和新奇量子现象提供 了重要的基础。本论文总结了几个典型的重费米子单晶和薄膜体系的电子态研究,包括Ce-115 体 系、CeCu2Si2、CeRh6Ge4 以及单晶 Ce 膜等。这些结果为理解重费米子体系中重电子态的形成 和温度演化、近藤杂化的能带/动量依赖、重电子能带与超导的关系、近藤效应与磁性和其它量子 态的竞争、4f 电子的维度调控等重要物理问题提供了谱学证据。     

A POSSIBLE MODEL OF HEAVY FFRMION SUPERCONDUCTIVITY

We used the periodic Anderson Hamiltonian to study the behaviour of heavy fermion systems. It has been argued that=the properly large mixing between f and the conduction electrons, and the related renormalization effect are the main causes of the large effective mass of the quasi-particle. Further, we introduced phenomenoloigcally the BCS attractive interaction between the heavy quasi-particles and explained that the valueof ΔC/γT_c and T_c may be quite different from that of the BCS theory as a result of the interaction between two branches of the quasi-particles.