Yb-based Heavy Fermion Compounds Research Articles

Pressure Evolution of the Magnetism and Fermi Surface of YbPtBi Probed by a Tunnel Diode Oscillator Based Method

A central research topic in condensed matter physics is the understanding of the evolution of various phases and phase transitions under different tuning parameters such as temperature, magnetic field and pressure. To explore the pressure-induced evolution of the magnetism and Fermi surface of the heavy fermion antiferromagnet YbPtBi, we performed tunnel diode oscillator based measurements under pressure at low temperatures in high magnetic fields. Our results reveal that the magnetic order strengthens and the Fermi surface shrinks as the pressure increases, which are consistent with typical observations for Yb-based heavy fermion compounds. In addition, an anomalous change in the quantum oscillation amplitudes is observed above 1.5 GPa, and determining the origin requires further study.

Pressure-induced magnetic transition exceeding 30 K in the Yb-based heavy-fermion β−YbAlB4

Measurements of the electric resistivity $\rho(T)$ under pressure up to 8 GPa were performed on high-quality single-crystals of the Yb-based heavy fermion system $\beta$-YbAlB$_4$ in the temperature range $2<T<$ 300 K. In the resistivity data, we observed pressure-induced magnetic ordering above the critical pressure $P_{\rm c} \sim$ 2 GPa. Clear difference in the phase diagram under pressure using two types of pressure mediums indicates that the transition temperature may be further enhanced under application of uniaxial pressure. With pressure, this phase transition temperature $T_{\rm M}$ is enhanced reaching 32 K at 8 GPa, which is the highest transition temperature so far recorded for the Yb-based heavy fermion compounds. The power-law exponent $\alpha$ in $\rho=\rho_0+ AT^{\alpha}$ below $T_{\rm M}$ gradually changes from 3/2 to 5/2 with increasing pressure from 2 to 8 GPa. In contrast, the resistivity exhibits a $T$-linear behavior in the temperature range 2 $\le T \le$ 20 K and is insensitive to pressure below $P_{\rm c}$. In this pressure regime, the magnetization is also nearly independent of pressure and shows no anomaly above 2 K. Our results indicate that a quantum critical point for $\beta$-YbAlB$_4$ is also located near $P_{\rm c}$ in addition to the strange metal region near the ambient pressure.

High Magnetic Transition Temperature and Semiconductor like Transport Properties of Mn-doped α-YbAlB4

We succeeded in growing single crystals of α-YbAl1–xMnxB4 by the substitution of Mn for the Al site in the intermediate-valence rare-earth heavy fermion system α-YbAlB4. We performed specific heat and resistivity measurement on single crystals of α-YbAl1–xMnxB4 (x = 40). In the specific heat measurement on α-YbAl0.60Mn0.40B4, a sharp peak was observed at magnetic transition temperature TM = 9.7(6) K. The resistivity measurements on x = 0.40 also show a semiconductor-like behavior in the entire temperature range from 2 - 300 K. An inclination change was also observed at 10(1) K in resistivity measurement. These results indicate a magnetic order. The magnetic transition temperature is quite high compared to its temperatures observed in the other Yb-based heavy fermion compounds.

High Pressure Measurements of the Resistivity of β-YbAlB4

The electric resistivity ρ(T) under hydrostatic pressure up to 8 GPa was measured above 2 K using a high-quality single crystal of the Yb-based heavy fermion system β-YbAlB4. We found pressure-induced magnetic ordering above the critical pressure Pc ≈ 2.4 GPa. This phase transition temperature TM is enhanced with pressure and reaches 30 K at a pressure of 8 GPa, which is the highest transition temperature for the Yb-based heavy fermion compounds. In contrast, the resistivity is insensitive to pressure below Pc and exhibits the T-linear behavior in the temperature range between 2 and 20 K. Our results indicate that quantum criticality for β-YbAlB4 is also located near Pc in addition to the ambient pressure.

Elastic anomalies of YbIrGe in magnetic fields

The Yb-based heavy-fermion compound YbIrGe, which has the orthorhombic structure, shows the crystal electric field effect at high temperatures and antiferromagnetic orderings at TN = 2.4 K and Tm = 1.4 K. We previously found anomalous elastic softening in zero magnetic field originating from an indirect quadrupole interaction between the ground doublet and the excited doublets, and determined the crystal electric field level scheme: the ground doublet and an excited doublet at 138 K. To investigate the antiferromagnetic orderings at TN and Tm, we performed ultrasonic measurements under magnetic fields along the a-axis on YbIrGe single crystals. Temperature dependence of elastic modulus C11 exhibits elastic hardening below both TN and Tm at 0 T. As increasing the magnetic field along the a-axis, both TN and Tm decrease monotonically, suggesting that both transitions are antiferromagnetic ordering. TN closes around 2 T, and Tm disappears above 1 T. We clarified the magnetic field- temperature phase diagram along the a-axis.

Intermediate valence behavior of Yb2Ni12P7 studied by using 31P NMR

The Yb-based heavy-fermion compound Yb2Ni12P7 with a hexagonal Zr2Fe12P7-type crystal structure was investigated by using the 31P nuclear magnetic resonance (NMR) technique. The complicated NMR line changes its shape gradually with decreasing temperature, implying the presence of some Knight shift components. The temperature dependences of the Knight shift and the nuclear spin-lattice relaxation rate 1/T 1 suggest the delocalization of 4f electrons.

Thermoelectric Response Near a Quantum Critical Point ofβ−YbAlB4andYbRh2Si2: A Comparative Study

The thermoelectric coefficients have been measured down to a very low temperature for the Yb-based heavy-fermion compounds β-YbAlB4 and YbRh2Si2, often considered as model systems for the local quantum criticality case. We observe a striking difference in the behavior of the Seebeck coefficient S in the vicinity of their respective quantum critical point (QCP). Approaching the critical field, S/T is enhanced in β-YbAlB4, but drastically reduced in YbRh2Si2. The ratio of thermopower to specific heat remains constant for β-YbAlB4, but it is significantly reduced near the QCP in YbRh2Si2. In both systems, on the other hand, the Nernst coefficient shows a diverging behavior near the QCP. The interplay between valence and magnetic quantum criticality and the additional possibility of a Lifshitz transition crossing the critical field under magnetic field are discussed as the origin of the different behaviors of these compounds.

Mechanism of Field Induced Fermi Liquid State in Yb-Based Heavy-Fermion Compound: X-ray Absorption Spectroscopy and Nuclear Magnetic Resonance Studies of YbCo2Zn20

We have investigated the field dependences of Yb valence as well as of the magnetic properties of the heavy-fermion (HF) compound YbCo 2 Zn 20 by X-ray absorption spectroscopy, 59 Co nuclear magnet...

Evidence for a Non-Fermi-Liquid Phase in Ge-SubstitutedYbRh2Si2

The canonical view of heavy fermion quantum criticality assumes a single quantum critical point separating the paramagnet from the antiferromagnet. However, recent experiments on Yb-based heavy fermion compounds suggest the presence of non-Fermi liquid behavior over a finite zero-temperature region. Using detailed susceptibility and transport measurements we show that the classic quantum critical system, Ge-substituted YbRh(2)Si(2), also displays such behavior. We advance arguments that this is not due to a disorder-smeared quantum critical point, but represents a new class of metallic phase.

Ultrasonic investigation close to quantum critical point in YbTr2Zn20 (Tr: Co, Rh, and Ir)

We performed ultrasonic measurements on high quality single crystals of the Yb-based heavy fermion compounds YbTr2Zn20 (Tr: Co, Rh and Ir) over a temperature range from 200 K to 0.5 K, which seem to be close to a quantum critical point (QCP). A sharp contrast of the temperature dependence of elastic constants was found at low temperature among the three compounds, reflecting the 4f electronic state stemmed from Yb ion. The results indicate that a crystalline electric field (CEF) effect seems to be dominant in the systems YbRh2Zn20 and YbIr2Zn20 at low temperatures. On the other hand, the CEF effect is much less, but an additional effect would be dominant which is most probably ascribable to non Fermi liquid characteristics formed close to the QCP. We discuss briefly each 4f electronic state developed at the low temperatures and physical parameters relating to a renormalized band model in YbTr2Zn20 in the framework of a deformation potential approximation.

Ultrasonic study of the Yb-based heavy fermion compound YbRh2Zn20

We report ultrasonic measurements on the high quality single crystal of the Yb-based heavy fermion compound YbRh2Zn20 over a temperature range from 200 K to 0.5 K. A shallow, but clear minimum was observed in the temperature dependent elastic constants C11, (C11 – C12)/2 and C44 around 15 K, probably attributed to the ground state and low-lying excited states of Yb3 in the cubic CEF. We discuss the low-temperature elastic properties and possible energy level scheme of localized 4f state of Yb3 ions in YbRh2Zn20.CEF ground state developed at the low temperatures and physical parameters relating to a quadrupolar moment in YbRh2Zn20

Electron spin resonance of dense Yb-based heavy-fermion compounds: New experimental data

We report the more recent advances in electron spin resonance (ESR) of few undoped Yb-based intermetallic compounds with heavy fermions (HF). The X-band ESR spectra of the Kondo lattices YbBiPt, YbRh 2Pb, and YbT 2Zn 20 (T = Fe, Co) are presented. A comparison with earlier ESR studies in YbRh 2Si 2 and YbIr 2Si 2 shows that the exchange interactions between the Yb 4f electrons and relevant conduction electron (d-, s-, or p-like) bands as well as the hybridization crystalline electric field (CEF) effects should be taken into account in order to develop a reliable model of spin dynamics in the Yb-based HF systems.

Non-Fermi liquid phenomena and intermediate valence in Yb-based compounds located close to the quantum critical point

In this paper, we focus on the mechanism of non-Fermi liquid phenomena in the vicinity of the magnetic field-tuned quantum critical point in Yb-based heavy fermion compounds YbAuCu 4, YbRh 2Si 2 and YbFe 4P 12. A comparative study of YbAuCu 4 and YbPdCu 4 with differently magnetically ordered ground states of antiferromagnetic (AFM) and ferromagnetic (FM) indicates that the competition between the field-destabilized AFM and field-stabilized FM correlations plays an important role on the field-tuned quantum critical behavior. The non-integer valence of these compounds leads to a consideration that the experimental susceptibility is dominated by the weakly broadened 4f hole level above the Fermi sea that extends to the cf hybridized band located at the Fermi level. Whilst, the monotonic increase of T 0 (an estimate of Kondo temperature) with field, derived from the transport and spin-fluctuation measurements, is ascribed to the variation of conduction bands from a weakly cf hybridized band toward the originally underlying conduction bands. The important role of the crossover transition upon cooling from the mixed valence localized f hole state to an itinerant electron state with the intermediate valence is also suggested.

Myriad of correlated electron effects found in the [formula omitted] family

The dilute, rare earth bearing family of RT 2 Zn 20 (R = rare earth, T = transition metal) intermetallic compounds can be tuned to a nearly ferromagnetic Fermi liquid that is closer to the Stoner limit than Pd (for R = Y, Lu and T = Fe). The submersion of moment bearing rare earths (Gd–Tm) into this highly polarizable matrix gives rise to exceptionally high temperature ferromagnetism (e.g. T C = 86 K for GdFe 2 Zn 20 ) for a compound with only one moment bearing ion out of 23 in the formula unit. In addition to this d-shell correlated electron behavior, 4f hybridization gives rise to six, new, Yb-based heavy fermion compounds: YbT 2 Zn 20 (T = Fe, Ru, Os, Co, Rh, Ir). These half dozen compounds manifest a range of Yb ion degeneracy values ( N ) for T ∼ T K , ranging from N = 4 – 8 . Not only do these compounds offer a clear testing ground for the effects of low temperature degeneracy on the correlated electron state, they also present an exceptionally clear experimental confirmation of the generalized Kadowaki–Woods formalism.

Six closely related YbT 2 Zn 20 (T = Fe, Co, Ru, Rh, Os, Ir) heavy fermion compounds with large local moment degeneracy

Heavy fermion compounds represent one of the most strongly correlated forms of electronic matter and give rise to low temperature states that range from small moment ordering to exotic superconductivity, both of which are often in close proximity to quantum critical points. These strong electronic correlations are associated with the transfer of entropy from the local moment degrees of freedom to the conduction electrons, and, as such, are intimately related to the low temperature degeneracy of the (originally) moment bearing ion. Here we report the discovery of six closely related Yb-based heavy fermion compounds, YbT(2)Zn(20), that are members of the larger family of dilute rare earth bearing compounds: RT(2)Zn(20) (T = Fe, Co, Ru, Rh, Os, Ir). This discovery doubles the total number of Yb-based heavy fermion materials. Given these compounds' dilute nature, systematic changes in T only weakly perturb the Yb site and allow for insight into the effects of degeneracy on the thermodynamic and transport properties of these model correlated electron systems.

Pressure-Induced Magnetic Phase Transitions in Yb-Based Heavy-Fermion Compounds

We report a detailed study of the pressure–temperature ( P – T ) phase diagrams of two Yb-based heavy-fermion compounds, namely, the antiferromagnet YbAgGe and weak ferromagnet YbRhSb. YbAgGe with a quasikagome lattice of Yb ions undergoes two magnetic phase transitions at T M1 = 0.8 K (second order) and T M2 =0.65 K (first order). With increasing pressure ( P ) up to 0.5 GPa, the transitions at T M1 and T M2 merge into one transition. For 0.5 < P < 1.6 GPa, T M2 remains constant and abruptly rises above 1.6 GPa. This rise occurs with a significant change in the specific heat anomaly from a small cusp at T M2 into a large peak at T M3 . This finding suggests that the suppressed magnetic entropy is released due to geometrical frustration at ambient pressure. On the other hand, YbRhSb undergoes a transition into an unusual magnetic state at T M1 = 2.7 K, which is associated with a small spontaneous moment of 3 ×10 -3 µ B /Yb. Measurements of the resistivity, specific heat, and ac magnetic susceptibility χ ac reveal that the application of pressure above 0.9 GPa induces another magnetic transition at T M2 below T M1 . T M2 ( P ) exhibits a V-shaped minimum of 1.8 K at P C = 1.6 GPa. With increasing pressure up to 2 GPa, the significant enhancement of the peak height of χ ac suggests that the canted antiferromagnetic state below P C changes to a ferromagnetic state with a large magnetic moment above P C . These complex P – T phase diagrams for YbAgGe and YbRhSb are explained in terms of the competition among distinct types of magnetic interactions, magnetic frustration, and crystal field anisotropy.

Crystal Electric Field Excitations in the Non-Fermy Liquid Compound YbRh2Si2

We have calculated the crystalline electric field (CEF) splitting of the energy levels of Yb3+ (4f13) in the clean Yb-based heavy fermion compound YbRh2Si2. The data of inelastic neutron scattering and electron spin resonance measurements in YbRh2Si2, together with relevant structural, thermodynamic, and magnetic properties, were used as input in the calculations of the possible CEF level scheme in this non-Fermi-liquid compound. Two possible sets of the CEF parameters with the Γ6 or Γ7 ground-state symmetry are discussed.

Unconventional metallic state in YbRh 2Si 2 — a high-pressure study

YbRh 2Si 2 is identified to be one of the few stoichiometric and the first Yb-based heavy fermion compound showing non-Fermi-liquid (NFL) effects at ambient pressure. We present results of a detailed study of the electrical resistivity ρ( T) and the specific heat C( T) under hydrostatic pressure and map out the magnetic phase diagram up to about 2.5 GPa. While at p<1.5 GPa only one anomaly is observed in ρ( T), two subsequent anomalies are found at p>1.5 GPa. These anomalies are interpreted as signature of (presumably antiferro) magnetic phase transitions. The results of our specific heat measurements prove that those phase transitions are a true bulk effect. In addition, NFL-behavior is found to exist over a broad range of temperature and pressure for temperatures above the magnetic instability.

Nuclear magnetic relaxation of dense Kondo system YbAgCu 4

The Yb-based heavy-fermion compound YbAgCu 4 with the C15b crystal structure and the isostructural compound LuAgCu 4 were investigated with the Cu nuclear quadropole resonance (NQR) technique. The nuclear spin-lattice relaxation rate, 1 T 1 , of YbAgCu 4 decreases markedly with decreasing temperature, which is consistent with the delocalization of 4f electrons. The 4f spin relaxation rate deduced from T 1 shows the typical temperature dependence for the dense Kondo system, i.e., it has a minimum around 110±20 K, which is considered to be the characteristic temperature of the system.