Heavy Fermion Compound Research Articles

Metamagnetism in itinerant ferromagnetic CeCrGe3 Kondo system

Magneto-resistance (MR), isothermal magnetization M (H) and specific heat (CP) of heavy fermion, dense Kondo and itinerant ferromagnetic compound CeCrGe3 (TC ~ 70 K) at low temperatures and high magnetic fields is being reported. Phase purity and elemental analysis were done by XRD and EDX. Electrical resistivity, ρ (T) vs. field H curves show hysteresis above 5 T and below 15 K. Hystersis was also observed in the M vs. H curves. This hysteresis in MR and M (H) is attributed to the magnetic field induced metamagnetism. The metamagnetism in CeCrGe3 occurs probably due to the peculiar density of states (DOS) with the Kondo resonance peak and the enhancement of DOS at the Fermi level as a result of the partial Kondo breakdown.

Orbital-Selective Kondo Entanglement and Antiferromagnetic Order in USb_{2}.

In heavy-fermion compounds, the dual character of f electrons underlies their rich and often exotic properties like fragile heavy quasiparticles, a variety of magnetic orders and unconventional superconductivity. 5f-electron actinide materials provide a rich setting to elucidate the larger and outstanding issue of the competition between magnetic order and Kondo entanglement and, more generally, the interplay among different channels of interactions in correlated electron systems. Here, by using angle-resolved photoemission spectroscopy, we present the detailed electronic structure of USb_{2} and observe two different kinds of nearly flat bands in the antiferromagnetic state of USb_{2}. Polarization-dependent measurements show that these electronic states are derived from 5f orbitals with different characters; in addition, further temperature-dependent measurements reveal that one of them is driven by the Kondo correlations between the 5f electrons and conduction electrons, while the other reflects the dominant role of the magnetic order. Our results on the low-energy electronic excitations of USb_{2} implicate orbital selectivity as an important new ingredient for the competition between Kondo correlations and magnetic order and, by extension, in the rich landscape of quantum phases for strongly correlated f electron systems.

Hybridization Effects Revealed by Angle-Resolved Photoemission Spectroscopy in Heavy-Fermion Ce2IrIn8**Supported by the National Key Research and Development Program of China under Grant Nos 2016YFA0401000, 2015CB921300, 2016YFA0300303, 2016YFA0401002 and 2017YFA0303103, the National Natural Science Foundation of China under Grant Nos 11674371, 11774401 and 11874330, the Strategic Priority Research Program (B) of the Chinese Academy of Sciences under Grant No XDB07000000, and

We utilize high-resolution resonant angle-resolved photoemission spectroscopy (ARPES) to study the band structure and hybridization effect of the heavy-fermion compound Ce2IrIn8. We observe a nearly flat band at the binding energy of 7 meV below the coherent temperature , which characterizes the electrical resistance maximum and indicates the onset temperature of hybridization. However, the Fermi vector and the Fermi surface volume have little change around Tcoh, which challenges the widely believed evolution from a high-temperature small Fermi surface to a low-temperature large Fermi surface. Our experimental results of the band structure fit well with the density functional theory plus dynamic mean-field theory calculations.

Crystal electric field splitting and f -electron hybridization in heavy-fermion CePt2In7

We use high-resolution angle-resolved photoemission spectroscopy to investigate the electronic structure of the antiferromagnetic heavy fermion compound CePt2In7, which is a member of the CeIn3-derived heavy fermion material family. Weak hybridization among 4f electron states and conduction bands was identified in CePt2In7 at low temperature much weaker than that in the other heavy fermion compounds like CeIrIn5 and CeRhIn5. The Ce 4f spectrum shows fine structures near the Fermi energy, reflecting the crystal electric field splitting of the 4f^1_5/2 and 4f^1_7/2 states. Also, we find that the Fermi surface has a strongly three-dimensional topology, in agreement with density-functional theory calculations.

Microscopic Nature of Magnetic Ground State in CeAuSb2

The synergistic investigation of ground‐state magnetic correlation in the single‐crystal heavy fermion compound CeAuSb2 using detailed neutron scattering measurements and density functional calculations is reported. Unlike the previous reports of single antiferromagnetic transition at TN ≈ 6 K, three successive transitions with distinct critical exponents at TN = 5.3, 4.46, and 3.76 K, respectively, are detected in CeAuSb. The low‐temperature ground‐state magnetic correlation is described by the spin density wave order in the basal plane with propagation wave vector τ = (0.135, 0.135, 0.5) rlu. The spin density wave order arises due to the nesting of hole pockets in the Fermi surface, with parallel surfaces being separated by the experimentally found propagation vector τ. The comprehensive investigation of magnetic ground‐state properties is expected to provide new insights in understanding the emerging quantum magnetism in this system, including the debated quantum critical state and magnetic field‐induced metamagnetic transitions at low temperature.

Exactly solvable Kondo lattice model in the anisotropic limit

In this paper we introduce an exactly solvable Kondo lattice model without any fine-tuning local gauge symmetry. This model describes itinerant electrons interplaying with a localized magnetic moment via only longitudinal Kondo exchange. Its solvability results from conservation of the localized moment at each site, and is valid for arbitrary lattice geometry and electron filling. A case study on square lattice shows that the ground state is a N\'{e}el antiferromagnetic insulator at half-filling. At finite temperature, paramagnetic phases including a Mott insulator and correlated metal are found. The former is a melting antiferromagnetic insulator with a strong short-range magnetic fluctuation, while the latter corresponds to a Fermi liquid-like metal. Monte Carlo simulation and theoretical analysis demonstrate that the transition from paramagnetic phases into the antiferromagnetic insulator is a continuous $2D$ Ising transition. Away from half-filling, patterns of spin stripes (inhomogeneous magnetic order) at weak coupling, and phase separation at strong coupling are predicted. With established Ising antiferromagnetism and spin stripe orders, our model may be relevant to a heavy fermion compound CeCo(In$_{1-x}$Hg$_{x}$)$_{5}$ and novel quantum liquid-crystal order in a hidden order compound URu$_{2}$Si$_{2}$.

CeAu2Bi : A new nonsymmorphic antiferromagnetic compound

Here we report the structural and electronic properties of CeAu$_{2}$Bi, a new heavy-fermion compound crystallizing in a nonsymmorphic hexagonal structure ($P63/mmc$). The Ce$^{3+}$ ions form a triangular lattice which orders antiferromagnetically below $T_{N} = 3.1$~K with a magnetic hard axis along the c-axis. Under applied pressure, $T_{N}$ increases linearly at a rate of $0.07$~K/kbar, indicating that the Ce $f$-electrons are fairly localized. In fact, heat capacity measurements provide an estimate of 150(10) mJ/mol.K$^{2}$ for the Sommerfeld coefficient. The crystal-field scheme obtained from our thermodynamic data points to a ground state with dominantly $|j_{z}=\pm1/2\rangle$ character, which commonly occurs in systems with a hard c-axis. Finally, electronic band structure calculations and symmetry analysis in $k$-space reveal that CeAu$_{2}$Bi hosts symmetry-protected crossings at $k_{z} = \pi$ in the paramagnetic state

Electronic structure study of LaCoIn5 and its comparison with CeCoIn5

A central key to understanding heavy-fermion systems involves revealing how itinerant low-energy excitations emerge from local $f$ moments. An effective way to understand the $f$-electron behavior is to compare the electronic structure of $f$-electron compounds with a reference isostructural compound with no $f$ electrons. Here we present a systematic electronic structure study of ${\mathrm{LaCoIn}}_{5}$, which is a non-$f$ reference compound of the heavy-fermion superconductor ${\mathrm{CeCoIn}}_{5}$. Our angle-resolved photoemission spectroscopy study of the three-dimensional Fermi surface and band structure of ${\mathrm{LaCoIn}}_{5}$ highlights the rather three-dimensional electronic character of this compound. The conduction bands of ${\mathrm{LaCoIn}}_{5}$ are almost identical to that of ${\mathrm{CeCoIn}}_{5}$, though obvious differences can be found in the low-energy electronic structure. Finally, we give a quantitative analysis of the Fermi pocket change in ${\mathrm{LaCoIn}}_{5}$ and ${\mathrm{CeCoIn}}_{5}$, which may be helpful for understanding the important ``large'' and ``small'' Fermi surface issue in heavy-fermion compounds.

Magnetic transport properties of heavy fermionic EuNi2P2

Electrical resistivity, magnetoresistance (MR) and Hall resistivity have been measured for Eu-based heavy fermion compound EuNi2P2 (exact composition is Eu1.2Ni2P2.1). It is found that the 4f electron contribution to the resistivity shows −log T above 200 K. The MR is positive at the lowest temperature and shows a negative minimum at around 60 K. The Hall resistivity shows a broad positive peak at around 120 K. These results are very similar to those of typical heavy fermions, CeCu6 and CeAl3. We discuss the results in terms of the Kondo model for heavy fermions. Measurements under high pressures suggest that the scaling of the Hall effect is valid below 1.5 GPa in EuNi2P2.

Evolution of the propagation vector of antiferroquadrupolar phases in Ce3Pd20Si6 under magnetic field

Hidden-order phases that occur in a number of correlated f-electron systems are among the most elusive states of electronic matter. Their investigations are hindered by the insensitivity of standard physical probes, such as neutron diffraction, to the order parameter that is usually associated with higher-order multipoles of the f-orbitals. The heavy-fermion compound Ce3Pd20Si6 exhibits magnetically hidden order at subkelvin temperatures, known as phase II. Additionally, for magnetic field applied along the [001] cubic axis, another phase II' was detected, but the nature of the transition from phase II to phase II' remained unclear. Here we use inelastic neutron scattering to demonstrate that this transition is associated with a change in the propagation vector of the antiferroquadrupolar order from (111) to (100). Despite the absence of magnetic Bragg scattering in phase II', its ordering vector is revealed by the location of an intense magnetic soft mode at the (100) wave vector, orthogonal to the applied field. At the II-II' transition, this mode softens and transforms into quasielastic and nearly Q-independent incoherent scattering, which is likely related to the non-Fermi-liquid behavior recently observed at this transition. Our experiment also reveals sharp collective excitations in the field-polarized paramagnetic phase, after phase II' is suppressed in fields above 4 T.

Orientation of the ground-state orbital inCeCoIn5andCeRhIn5

We present core level non-resonant inelastic x-ray scattering (NIXS) data of the heavy fermion compounds CeCoIn$_5$ and CeRhIn$_5$ measured at the Ce $N_{4,5}$-edges. The higher than dipole transitions in NIXS allow determining the orientation of the $\Gamma_7$ crystal-field ground-state orbital within the unit cell. The crystal-field parameters of the Ce$M$In$_5$ compounds and related substitution phase diagrams have been investigated in great detail in the past; however, whether the ground-state wavefunction is the $\Gamma_7^+$ ($x^2\,-\,y^2$) or $\Gamma_7^-$ ($xy$ orientation) remained undetermined. We show that the $\Gamma_7^-$ doublet with lobes along the (110) direction forms the ground state in CeCoIn$_5$ and CeRhIn$_5$. For CeCoIn$_5$, however, we find also some contribution of the first excited state crystal-field state in the ground state due to the stronger hybridization of 4$f$ and conduction electrons, suggesting a smaller $\alpha^2$ value than originally anticipated from x-ray absorption. A comparison is made to the results of existing density functional theory plus dynamical mean-field theory calculations.

Quantum Griffiths phase near an antiferromagnetic quantum critical point: Muon spin relaxation study of Ce(Cu1−xCox)2Ge2

The antiferromagnetic order in the heavy-fermion compound ${\mathrm{CeCu}}_{2}{\mathrm{Ge}}_{2}$ can be suppressed by Co-doping, and at critical composition ${x}_{c}=0.6$ (${T}_{N}\phantom{\rule{0.16em}{0ex}}\ensuremath{\rightarrow}0$ K) a quantum critical point has been observed. We have performed zero-field (ZF) and longitudinal-field muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) measurements on polycrystalline samples of $\mathrm{Ce}{({\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{Ge}}_{2}$ ($x=0,0.2,0.6,1$) over a temperature range of 100 mK to 10 K and in applied fields from 0 up to 3000 G. Above any ordering temperature, the muon relaxation spectra can be described by a Gaussian-Kubo-Toyabe times exponential line shape. Below the magnetic ordering temperature (i.e., for $x<0.6$), an additional Gaussian relaxation is observed. The zero-field muon relaxation rate suggests the presence of antiferromagnetic ordering below 4 and 0.8 K for $x=0$ and 0.2 samples, respectively. For $x=0.6$, the magnetic order is completely suppressed, and the quantum critical point is accompanied by non-Fermi-liquid behavior, manifested in the power-law divergence of exponential depolarization, i.e., $\ensuremath{\lambda}\phantom{\rule{0.16em}{0ex}}\ensuremath{\propto}\phantom{\rule{0.16em}{0ex}}{T}^{0.55}$. The relaxation rate of $x=0.6$ obeys the time-field scaling relation ${G}_{z}(t,H)={G}_{z}(t/{H}^{\ensuremath{\gamma}})$, which is considered to be a characteristic feature of quantum critical magnetic fluctuations. Furthermore, for $x=0.6$, the exponent of isotherm magnetization, $M\ensuremath{\sim}{H}^{\ensuremath{\eta}}$, and magnetization-field-temperature scaling is consistent with the ZF-$\ensuremath{\mu}\mathrm{SR}$ data. These results provide strong evidence for the formation of a quantum Griffiths phase near the antiferromagnetic quantum phase transition.

Enhanced thermoelectric performance of heavy-fermion compounds YbTM 2Zn20 (TM = Co, Rh, Ir) at low temperatures.

Thermoelectricity allows direct conversion between heat and electricity, providing alternatives for green energy technologies. Despite these advantages, for most materials the energy conversion efficiency is limited by the tendency for the electrical and thermal conductivity to be proportional to each other and the Seebeck coefficient to be small. Here we report counter examples, where the heavy fermion compounds YbTM 2Zn20 (TM = Co, Rh, Ir) exhibit enhanced thermoelectric performance including a large power factor (PF = 74 μW/cm-K2; TM = Ir) and a high figure of merit (ZT = 0.07; TM = Ir) at 35 K. The combination of the strongly hybridized electronic state originating from the Yb f-electrons and the novel structural features (large unit cell and possible soft phonon modes) leads to high power factors and small thermal conductivity values. This demonstrates that with further optimization these systems could provide a platform for the next generation of low temperature thermoelectric materials.

Phenomenological Mean-Field Theory of Metamagnetism in Heavy Fermions

Metamagnetism observed in some heavy fermion compounds is an intriguing phenomena. To understand this phenomena, we explore the phenomenological mean-field theory for heavy fermion systems without microscopic mechanism. We write an extension of the Landau-like free energy density with the Bragg-Williams entropy. By minimising this free energy density with respect to the sub-lattice magnetizations, we can obtain thermal equilibrium quantities. As examples, the magnetic behaviors of CeRu2Si2(the metamagnetism with crossover) and CeTiGe (the first order metamagnetism) are explained qualitatively, where the scaling hypothesis is used to consult with experimental data under the constant pressure condition.

The Sub-mK Transition of CeCu $$_{6}$$ 6 in a New Light

Following our recent discovery of superconductivity in YbRh $$_2$$ Si $$_2$$ at 2 mK, nearly coinciding with a hybrid electronic–nuclear magnetic moment ordering (Schuberth et al. in Science 351:485, 2016), we re-analyzed previous data on the magnetic susceptibility of CeCu $$_{6}$$ at ultra-low temperatures. We found that there is a direct analogy between the magnetic susceptibilities of both compounds which raises the question if there is the same hybrid transition in CeCu $$_{6}$$ , possibly also followed by superconductivity. Data taken some 20 years ago by groups in Garching and at Cornell University revealed an antiferromagnetic transition around 3 mK and further anomalies around 0.5 mK, very similar to the case of YbRh $$_2$$ Si $$_2$$ . If the above speculation is correct, it would suggest that superconductivity is a general phenomenon, occurring even in a heavy-fermion compound like CeCu $$_{6}$$ which for a long time was considered as “non-ordering.”

Electronic Structure in Heavy Fermion Compound UPd2Al3 through Directional Compton Profile Measurement

We have measured directional Compton profiles on the (0001) plane in the UPd2Al3 single crystal with hexagonal symmetry in order to investigate the change in electronic structure related to the cro...

Fermi Volume Evolution and Crystal-Field Excitations in Heavy-Fermion Compounds Probed by Time-Domain Terahertz Spectroscopy.

We measure the quasiparticle weight in the heavy-fermion compound CeCu_{6-x}Au_{x} (x=0, 0.1) by time-resolved terahertz spectroscopy for temperatures from 2 up to 300K. This method distinguishes contributions from the heavy Kondo band and from the crystal-electric-field satellite bands by different terahertz response delay times. We find that the formation of heavy bands is controlled by an exponentially enhanced, high-energy Kondo scale once the crystal-electric-field states become thermally occupied. We corroborate these observations by temperature-dependent dynamical mean-field calculations for the multiorbital Anderson lattice model and discuss consequences for quantum-critical scenarios.

Direct visualization of coexisting channels of interaction in CeSb.

Our understanding of correlated electron systems is vexed by the complexity of their interactions. Heavy fermion compounds are archetypal examples of this physics, leading to exotic properties that weave magnetism, superconductivity and strange metal behavior together. The Kondo semimetal CeSb is an unusual example where different channels of interaction not only coexist, but have coincident physical signatures, leading to decades of debate about the microscopic picture describing the interactions between the f moments and the itinerant electron sea. Using angle-resolved photoemission spectroscopy, we resonantly enhance the response of the Ce f electrons across the magnetic transitions of CeSb and find there are two distinct modes of interaction that are simultaneously active, but on different kinds of carriers. This study reveals how correlated systems can reconcile the coexistence of different modes on interaction-by separating their action in momentum space, they allow their coexistence in real space.

Realization of Kondo chain in CeCo2Ga8

We revisited the anisotropy of the heavy-fermion material CeCo$_2$Ga$_8$ by measuring the electrical resistivity and magnetic susceptibility along all the principal $\mathbf{a}$-, $\mathbf{b}$- and $\mathbf{c}$-axes. Resistivity along $\mathbf{c}$-axis ($\rho_c$) shows clear Kondo coherence below about 17 K, while both $\rho_{a}$ and $\rho_{b}$ remain incoherent down to 2 K. The magnetic anisotropy is well understood within the theoretical frame of crystalline electric field effect in combination with magnetic exchange interactions. We found the anisotropy ratio of these magnetic exchange interactions, $|J_{ex}^c/J_{ex}^{a,b}|$, reaches a large value of 4-5. We, therefore, firmly demonstrate that CeCo$_2$Ga$_8$ is a quasi-one-dimensional heavy-fermion compound both electrically and magnetically, and thus provide a realistic example of \textit{Kondo chain}.

Electronic structure of the quadrupolar ordered heavy-fermion compound YbRu2Ge2 measured by angle-resolved photoemission

We studied the electronic structure of the heavy fermion compound Yb(Ru$_{1-x}$Rh$_{x}$)$_2$Ge$_2$ with $x=0$ and nominally $x=0.125$ using ARPES and LDA calculations. We find a valence band structure of Yb corresponding to a non-integer valence close to $3+$. The three observed crystal electric field levels with a splitting of 32 and 75 meV confirm the suggested configuration with a quasi-quartet ground state. The experimentally determined band structure of the conduction electrons with predominantly Ru $4d$ character is well reproduced by our calculations. YbRu$_2$Ge$_2$ undergoes a non-magnetic phase transition into a ferroquadrupolar ordered state below 10.2\,K and then to an antiferromagnetically ordered state below 6.5\,K. A small hole Fermi surface shows nesting features in our calculated band structure and its size determined by ARPES is close to the magnetic ordering wave vector found in neutron scattering. The transitions are suppressed when YbRu$_2$Ge$_2$ is doped with 12.5% Rh. The electron doping leads to a shift of the band structure and successive Lifshitz transitions.