Heavy-fermion Ground State Research Articles

Magnetism, heat capacity, magnetocaloric effect, and magneto-transport properties of heavy fermion antiferromagnet CeGaSi

We synthesize high-quality single crystal of CeGaSi by a Ga self-flux method and investigate its physical properties through magnetic susceptibility, specific heat and electrical resistivity measurements as well as high pressure effect. Magnetic measurements reveal that an antiferromagnetic order develops below T m ∼ 10.4 K with magnetic moments orientated in the ab plane. The enhanced electronic specific heat coefficient and the negative logarithmic slope in the resistivity of CeGaSi indicate that the title compound belongs to the family of Kondo system with heavy fermion ground states. The max magnetic entropy change −ΔSMmax (μ 0 H⊥c, μ 0 H = 7 T) around T m is found to reach up to 11.85 J⋅kg−1⋅K−1. Remarkably, both the antiferromagnetic transition temperature and −lnT behavior increase monotonically with pressure applied to 20 kbar (1 bar = 105 Pa), indicating that much higher pressure will be needed to reach its quantum critical point.

Two-dimensional heavy fermions in the van der Waals metal CeSiI.

Heavy-fermion metals are prototype systems for observing emergent quantum phases driven by electronic interactions1-6. A long-standing aspiration is the dimensional reduction of these materials to exert control over their quantum phases7-11, which remains a significant challenge because traditional intermetallic heavy-fermion compounds have three-dimensional atomic and electronic structures. Here we report comprehensive thermodynamic and spectroscopic evidence of an antiferromagnetically ordered heavy-fermion ground state in CeSiI, an intermetallic comprising two-dimensional (2D) metallic sheets held together by weak interlayer van der Waals (vdW) interactions. Owing to its vdW nature, CeSiI has a quasi-2D electronic structure, and we can control its physical dimension through exfoliation. The emergence of coherenthybridization of f and conduction electrons at low temperature is supported by the temperature evolution of angle-resolved photoemission and scanning tunnelling spectra near the Fermi level andby heat capacity measurements. Electrical transport measurements on few-layer flakes reveal heavy-fermion behaviour and magnetic order down to the ultra-thin regime. Our work establishes CeSiI and related materials as a unique platform for studying dimensionally confined heavy fermions in bulk crystals and employing 2D device fabrication techniques and vdW heterostructures12 to manipulate the interplay between Kondo screening, magnetic order and proximity effects.

On the Ytterbium Valence and the Physical Properties in Selected Intermetallic Phases.

The present review summarizes important aspects of the crystal chemistry of ytterbium-based intermetallic compounds along with a selection of their outstanding physical properties. These originate in many cases from the ytterbium valence. Different valence states are possible here, divalent (4f14), intermediate-valent, or trivalent (4f13) ytterbium, resulting in simple diamagnetic, Pauli or Curie-Weiss paramagnetic, or valence fluctuating behavior. Especially, some of the Yb3+ intermetallics have gained deep interest due to their Kondo or heavy Fermion ground states. We have summarized their property investigations using magnetic and transport measurements, specific heat data, NMR, ESR, and Mössbauer spectroscopy, elastic and inelastic neutron scattering, and XAS data as well as detailed thermoelectric measurements.

Low-temperature antiferromagnetic order in orthorhombic CePdAl3

We report the magnetization, ac susceptibility, and specific heat of optically float-zoned single crystals of ${\mathrm{CePdAl}}_{3}$. In comparison to the properties of polycrystalline ${\mathrm{CePdAl}}_{3}$ reported in the literature, which displays a tetragonal crystal structure and no long-range magnetic order, our single crystals exhibit an orthorhombic structure ($Cmcm$) and antiferromagnetic order below a N\'eel temperature ${T}_{1}=5.6$ K. The specific heat at zero field shows two anomalies, namely, a broad transition at ${T}_{1}=5.6$ K followed by a $\ensuremath{\lambda}$-anomaly at ${T}_{2}=5.4$ K. A conservative estimate of the Sommerfeld coefficient of the electronic specific heat, $\ensuremath{\gamma}=121\phantom{\rule{0.28em}{0ex}}\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{mol}}^{\ensuremath{-}1}$, indicates a moderately enhanced heavy-fermion ground state. A twin microstructure evolves in the family of planes spanned by the basal plane lattice vectors ${\mathbf{a}}_{\mathrm{o}}$ and ${\mathbf{c}}_{\mathrm{o}}$ , with the magnetic hard axis ${\mathbf{b}}_{\mathrm{o}}$ common to all twins. The antiferromagnetic state is characterized by a strong a${}_{\mathrm{o}}$, c${}_{\mathrm{o}}$ easy-plane magnetic anisotropy where the a${}_{\mathrm{o}}$ direction is the easy axis in the easy plane. A spin-flop transition induced under magnetic field along the easy directions, results in complex magnetic phase diagrams. Taken together, our results reveal a high sensitivity of the magnetic and electronic properties of ${\mathrm{CePdAl}}_{3}$ to its structural modifications.

Anisotropy-driven quantum criticality in an intermediate valence system

Intermetallic compounds containing f-electron elements have been prototypical materials for investigating strong electron correlations and quantum criticality (QC). Their heavy fermion ground state evoked by the magnetic f-electrons is susceptible to the onset of quantum phases, such as magnetism or superconductivity, due to the enhanced effective mass (m*) and a corresponding decrease of the Fermi temperature. However, the presence of f-electron valence fluctuations to a non-magnetic state is regarded an anathema to QC, as it usually generates a paramagnetic Fermi-liquid state with quasiparticles of moderate m*. Such systems are typically isotropic, with a characteristic energy scale T0 of the order of hundreds of kelvins that require large magnetic fields or pressures to promote a valence or magnetic instability. Here we show the discovery of a quantum critical behaviour and a Lifshitz transition under low magnetic field in an intermediate valence compound α-YbAlB4. The QC origin is attributed to the anisotropic hybridization between the conduction and localized f-electrons. These findings suggest a new route to bypass the large valence energy scale in developing the QC.

Antiferromagnetic order in the honeycomb Kondo lattice CePt6Al3 induced by Pd substitution

The cerium-based compound $\mathrm{Ce}{\mathrm{Pt}}_{6}{\mathrm{Al}}_{3}$, in which Ce atoms form a honeycomb lattice hosting magnetic frustration, has a heavy-fermion ground state. We have observed development of magnetic order in partially Pd-substituted $\mathrm{Ce}{({\mathrm{Pt}}_{1\ensuremath{-}x}{\mathrm{Pd}}_{x})}_{6}{\mathrm{Al}}_{3}$ series up to $x$ = 0.3 by the measurements of magnetic susceptibility, electrical resistivity \ensuremath{\rho}, and specific heat $C$. In the whole range of $x$, the unit cell volume remains unchanged within 0.2%, and the effective magnetic moment stays at $2.4\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{B}/\mathrm{Ce}$. For $x$ = 0.05, both $C/T$ and $\ensuremath{\rho}(T)$ jump on cooling at ${T}_{m}=1.8\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. With increasing $x$ to 0.2, ${T}_{m}$ increases to 3.8 K, where $C/T$ shows a pronounced \ensuremath{\lambda}-type anomaly. Application of magnetic fields suppresses ${T}_{m}$, which is indicative of an antiferromagnetic (AFM) ordered state. Thus, a long-range AFM order is induced by the substitution of isovalent Pd for Pt in $\mathrm{Ce}{\mathrm{Pt}}_{6}{\mathrm{Al}}_{3}$ without carrier doping and chemical pressure. We attribute the emergence of AFM order in $\mathrm{Ce}{({\mathrm{Pt}}_{1\ensuremath{-}x}{\mathrm{Pd}}_{x})}_{6}{\mathrm{Al}}_{3}$ to the randomness in the spin-orbit interaction in the Pt-Pd sublattice, which weakens both the coherent Kondo effect and magnetic frustration in the honeycomb Kondo lattice.

Structural study of the pressure-induced metal-insulator transition in LiV2O4

At ambient pressure the spinel $\mathrm{Li}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$ is a metal with a heavy fermion ground state. However, under applied pressure a transition to an insulating, nonmagnetic state occurs. Powder x-ray diffraction has been used to study structural changes associated with this transition, and reveals that the ambient-pressure $Fd\overline{3}m$ spinel structure distorts to a monoclinic cell of $C2/m,C2$, or $Cm$ symmetry above 11 GPa at low temperatures. The changes of structure and properties provide evidence for orbital molecule formation in the cubic phase at high pressures and low temperatures that leads to long-range orbital molecule order in the monoclinic phase.

Real space manifestations of coherent screening in atomic scale Kondo lattices

The interaction among magnetic moments screened by conduction electrons drives quantum phase transitions between magnetically ordered and heavy-fermion ground states. Here, starting from isolated magnetic impurities in the Kondo regime, we investigate the formation of the finite size analogue of a heavy Fermi liquid. We build regularly-spaced chains of Co adatoms on a metallic surface by atomic manipulation. Scanning tunneling spectroscopy is used to obtain maps of the Kondo resonance intensity with sub-atomic resolution. For sufficiently small interatomic separation, the spatial distribution of Kondo screening does not coincide with the position of the adatoms. It also develops enhancements at both edges of the chains. Since we can rule out any other interaction between Kondo impurities, this is explained in terms of the indirect hybridization of the Kondo orbitals mediated by a coherent electron gas, the mechanism that causes the emergence of heavy quasiparticles in the thermodynamic limit.

Two-state Weiss model for the anomalous thermal expansion in EuNi2P2

It has recently been found that the single crystalline EuNi2P2 shows an anormalous thermal expansion and its scaling relation to the average 4f electron number. We point out that a phenomenological 2-state Weiss model can explain these behaviors. We also show that the model leads to the observed effective Bohr magneton number per atom (7.4μB) in the high-temperature limit, and is consistent with the temperature dependence of experimental susceptibility. Moreover the model predicts that the 4f contribution to the thermal expansion coefficient is proportional to that of the specific heat. The results suggest that the strong temperature dependence found in these experimental data originates in the excitations from the heavy-fermion ground state to the independent 4f7 atomic states.

Mean-field study of the heavy-fermion metamagnetic transition

We investigate the evolution of the heavy-fermion ground state under application of a strong external magnetic field. We present a richer version of the usual hybridization mean-field theory that allows for hybridization in both the singlet and triplet channels and incorporates a self-consistent Weiss field. We show that for a magnetic field strength ${B}^{*}$, a filling-dependent fraction of the zero-field hybridization gap, the spin up quasiparticle band becomes fully polarized---an event marked by a sudden jump in the magnetic susceptibility. The system exhibits a kind of quantum rigidity in which the susceptibility (and several other physical observables) is insensitive to further increases in field strength. This behavior ends abruptly with the collapse of the hybridization order parameter in a first-order transition to the normal metallic state. We argue that the feature at ${B}^{*}$ corresponds to the ``metamagnetic transition'' in $\mathrm{Yb}{\mathrm{Rh}}_{2}{\mathrm{Si}}_{2}$. Our results are in good agreement with recent experimental measurements.

Crystal Structure and Physical Properties of Polymorphs of LnAlB4 (Ln = Yb, Lu)

Single crystals of YbAlB4 were grown in excess Al flux. Plate- and needle-shaped crystals were found. The plates are found to be s-YbAlB4, which crystallizes with the ThMoB4 structure type in space group Cmmm (No. 65), Z = 4, with lattice parameters of a = 7.3080(4), b = 9.3150(5), and c = 3.4980(2) A. The needle-shaped crystals were identified as the first form of YbAlB4 which crystallizes with the YCrB4 structure type in space group Pbam (No. 55), Z = 4, with lattice parameters of a = 5.9220(2), b = 11.4730(3), and c = 3.5060(5) A. While both compounds have heavy fermion ground states with Isinglike magnetic anisotropy, the electronic specific heat coefficients (γ) differ. The s-phase has a γ value near 300 mJ mol -1 K-2, more than twice that of the α-phase, γ = 130 mJ mol-1 K-2. A comparison of the structures and physical properties of both polymorphs is presented. © 2007 American Chemical Society.

Reversible Tuning of the Heavy-Fermion Ground State inCeCoIn5

Cadmium doping the heavy-fermion superconductor CeCoIn(5) at the percent level acts as an electronic tuning agent, sensitively shifting the balance between superconductivity and antiferromagnetism and opening new ambient-pressure phase space in the study of heavy-fermion ground states.

Loss of coherence in Ce1−La Pd3 studied by high-frequency transport

Abstract We have studied the destruction of the coherent heavy-fermion ground state in a correlated electron system by doping with Kondo impurities. Substitution of Ce by the non-magnetic La in the intermediate valence system CePd3 leads to scattering of the renormalized electrons on these impurities. The loss of coherence is observed by measuring the temperature dependence of the microwave properties in Ce1−xLaxPd3 for x=0, 0.5%, 1%, 3%, and 5%.

Ce 4f-LIGAND DEHYBRIDIZATION IN CeT2X2-BASED KONDO LATTICE/HEAVY FERMION SYSTEMS

The lattice parameters of two CeT2X2 ( T =3d transition elements and X=Si and Ge) based Kondo lattice systems, the CeNi2-xCux Si2 and Ce(NiSi)2-x (CuGe)x, have been studied. The analysis of the Ce-ligand distances indicates that the variation of the a-parameter dominantly affects Ce 4f-X ligandn orbital hybridization whereas the variation of c-parameter dominantly affects Ce 4f-T ligand orbital hybridization. The average Ce-ligand distance varies linearly with x across the whole series in spite of the abnormal increase of the c-parameter in the region 1.6 ≤ x ≤ 2.0. Thus, our analysis shows that the average Ce 4f-ligand orbital hybridization decreases uniformly with x across the entire range of x for the CeNi2-xCuxSi 2 and Ce(NiSi)2-x(CuGe)x series. The means that the Ce 4f-ligand orbital dehybridization may not be a key factor to the formation of the heavy-fermion ground state in CeCu2Si2.

Magnetic, thermodynamic, and electrical transport properties of ternary equiatomic ytterbium compoundsYbTM (T=transition metal,M=Snand Bi)

Physical behavior of several $\mathrm{Yb}\mathrm{TM}$ intermetallics has been studied by means of x-ray powder diffraction, magnetization, dc magnetic susceptibility, heat capacity, and electrical resistivity measurements. The compounds $\mathrm{Yb}T\mathrm{Bi}\mathrm{}$ with $T=\mathrm{Cu},$ Ag, Au and $\mathrm{Yb}T\mathrm{Sn}$ with $T=\mathrm{Ag},$ Au, Zn were shown to be nonmagnetic due to the presence of divalent ytterbium ions. The bismuthide YbPdBi as well as the stannides YbRhSn and YbPtSn were found to exhibit localized magnetism of almost trivalent Yb ions. The electrical behavior of these three phases is characteristic of dense Kondo systems, and their low-temperature specific heat data indicate a possible heavy fermion ground state.

Magnetic-field dependence of the low-temperature specific heat inPrInAg2: Support for a nonmagnetic heavy-fermion ground state

In order to elucidate the nature of the ground state in the heavy-electron system PrInAg_2, we measured its specific heat for temperatures between 60 mK and 2 K in magnetic fields up to 9 T. The peak maximum of the low temperature anomaly in the specific heat (interpreted as Kondo anomaly) shifts from 0.42 K at zero field to 0.45 K at 6 T, and to 0.5 K at 9 T. The data at 3 T (after subtracting the low temperature tail due to a Schottky anomaly) is practically indistinguishable from the zero field data. A low temperature nuclear Schottky anomaly from a hyperfine enhancement of the Pr nuclei was observed in field. Both enhanced hyperfine interaction and the insensitivity of the specific heat anomaly to fields support the hypothesis that the Kondo effect in this system has a non-magnetic origin.

Strongly correlated electron phenomena in filled skutterudite compounds

We report on a search for intermediate valence and heavy fermion behavior in filled skutterudite compounds. The compound YbFe 4Sb 12 was synthesized and, to our knowledge, is the first example of a filled skutterudite of a heavy lanthanide. Transport, magnetic, and thermal measurements on CeFe 4Sb 12 and YbFe 4Sb 12 indicate that these materials have a moderately heavy fermion ground state.

Magnetic behaviour of Yb 2Co 3T 9 (T=Ga and Al)

Orthorhombic Yb 2Co 3Ga 9 has a Pauli paramagnetic, moderate heavy-fermion ground state. In the isotypic new compound Yb 2Co 3Al 9, Yb ions order magnetically near 1.2 K with a possible modest Kondo coupling with the conduction band electrons.

Heavy fermion behaviour of the cerium-filled skutterudites and

The low temperature properties of polycrystalline samples of the filled skutterudites and , as well as the unfilled skutterudites and , have been investigated by means of electrical resistivity, specific heat and magnetic susceptibility measurements. The resistivity of exhibits a rather abrupt drop-off with decreasing temperature near 100 K; this drop-off temperature increases with increasing applied hydrostatic pressure, which is reminiscent of the onset of coherence in so-called Kondo lattice materials. The compounds and exhibit values of the electronic specific heat coefficient and Pauli susceptibility at low temperature which are enhanced over those of the lanthanum-filled and unfilled skutterudites. These quantities yield a Wilson ratio of order unity, which indicates that they both correspond to the properties of itinerant electrons. These transport, magnetic and thermodynamic properties suggest a moderately heavy fermion ground state in and .

μSR magnetic response of CeNiSn on impurity content

Recent results of thermodynamic and transport measurements on CeNiSn at low temperatures show a strong dependence on sample purity. A lower impurity content leads to a more metallic behavior of CeNiSn. We present muon spin relaxation (μSR) spectra at T < 1 K for two samples of CeNiSn which show quite different behavior in low temperature specific heat. No significant difference could be detected in decisive μSR parameters for the two specimen. This strengthens the view that μSR probes the intrinsic magnetic properties of the heavy fermion ground state of CeNiSn.