Magnetism and Superconductivity inCeCu2(Si1−xGex)2 Probed by Cu NQR

We report Cu-NQR results on Ge-doped heavy-fermion superconductor CeCu₂(Si_1–x Ge _x )₂ (0<x0.2) and undoped Ce_0.99Cu_2.02Si₂. The main effect of the Ge doping is considered to be a negative pressure, since the strength of hybridization decreases by the Ge doping. With increasing x, the dynamical characteristics of the magnetic order at x=0 change to more static ones which suggests a localized regime above x0.25. From the derived T–x phase diagram, it is suggested that the magnetic and the superconducting phases are almost degenerate in undoped CeCu₂Si₂. An exotic interplay between the magnetism characterized by the slow fluctuations and the superconductivity is implied in the region of small x.     

Unconventional magnetic and transport properties in Gd1−xLaxMn2Ge2 with two-dimensional arrangement of Mn atoms

In this paper, we present our recent experimental results of magnetic and transport properties of Gd_1−xLa_xMn₂Ge₂ intermetallic compounds with the ThCr₂Si₂-type layered structure. The results obtained indicate that, in GdMn₂Ge₂, a first-order transition from a collinear antiferromagnetic to a collinear ferrimagnetic state appears with decreasing temperature at T_t3, below the Néel temperature T_N. In Gd_1−xLa_xMn₂Ge₂ compounds with x=0.05 and 0.075, after ordering ferrimagnetically at T_t1, two kinds of first-order transitions from a canted ferrimagnetic to a non-collinear antiferromagnetic state and from a non-collinear antiferromagnetic to a reentrant canted ferrimagnetic state occur at T_t2 and T_t3. In Gd_0.925La_0.075Mn₂Ge₂, a field-induced metamagnetic transition from non-collinear antiferromagnetism to canted ferrimagnetism occurs at relatively low fields, accompanied by fractal like multi-step transitions, the so called “devil's stair-case”. Furthermore, a negative giant magnetoresistance (GMR) effect (Δρ/ρ15%) was observed at the field-induced metamagnetic transition. The mechanism of this negative GMR was clarified by comprehensive measurements of the resistivity on single crystals Gd_0.925La_0.075Mn₂Ge₂ and TbMn₂Ge₂. With further increasing x, only canted ferrimagnetism appears with a compensation temperature for 0.10<x<0.40, whereas no compensation behavior appears for x>0.50. The phase diagram obtained indicates that the overall magnetism is controlled by the Mn–Mn intralayer distance in the tetragonal c-plane, reflecting the two-dimensional arrangement of Mn atoms.     

Universal behavior of CePd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Rh<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> ferromagnet at the quantum critical point

The heavy-fermion metal CePd_1−x Rh _x can be tuned from ferromagnetism at x = 0 to the nonmagnetic state at some critical concentration x _c . The non-Fermi liquid behavior (NFL) at x ≃ x _c is recognized by the power-law dependence of the specific heat C(T) given by the electronic contribution susceptibility X(T) and volume expansion coefficient α(T) at low temperatures: C/T ∝ X(T) ∝ α(T)/T∝ 1/ √T. We also demonstrate that the behavior of the normalized effective mass M _N ^* observed in CePd_1−x Rh _x at x ≃ 0.8 agrees with that of M _N ^* observed in paramagnetic CeRu₂Si₂ and conclude that these alloys exhibit the universal NFL thermodynamic behavior at their quantum critical points. We show that the NFL behavior of CePd_1−x Rh _x can be accounted for within the frameworks of the quasiparticle picture and fermion condensation quantum phase transition, while this alloy exhibits a universal thermodynamic NFL behavior that is independent of the characteristic features of the given alloy such as its lattice structure, magnetic ground state, dimension, etc. The text was submitted by the authors in English.     

Electron spin resonance investigation of the Heavy-Fermion compound CeCu2(Si1−x Ge x )2

The Kondo-lattice system CeCu₂(Si_1?x Ge _x )₂ exhibits an alloying induced transition from a coherent Fermi-liquid (x=0) with strongly enhanced effective masses to an antiferromagnetically ordered heavy-fermion system (x=1). This transition is studied by Gd³⁺ ESR in oriented powder samples. The temperature dependence of the ESR line width follows a characteristic pattern which allows one to distinguish between the different ground states. The results obtained in polycrystalline CeCu₂Si₂ are in good agreement with the measurements performed in single crystals. Finally we compare our results with⁶³Cu-NMR data obtained from CeCu₂Si₂ and CeCu₂Ge₂.     

Synchrotron X-ray diffraction and absorption studies of CeM2X2 (M=Cu, Ni and X=Si, Ge) at high pressure

The equations of state of CeCu₂Si₂ and CeCu₂Ge₂ to about 60 GPa, as well as that of CeNi₂Ge₂ to 22 GPa and the valence state of Ce in CeCu₂Ge₂ to 20 GPa have been studied at room temperature in a diamond-anvil cell using synchrotron radiation sources. In each compound, the ambient-pressure phase (tetragonal ThCr₂Si₂-type structure) persisted to the highest pressure studied. The unit cell volumes of CeNi₂Ge₂ at ∼5 GPa and CeCu₂Ge₂ at ∼7 GPa, respectively, approached that of CeCu₂Si₂ taken at ambient pressure. From the equation-of-state data, the bulk modulus was derived to be 112.0±5.1 GPa for CeCu₂Si₂, 125.6±4.3 GPa for CeCu₂Ge₂, and 178.4±14.3 GPa for CeNi₂Ge₂. The valence state of Ce in CeCu₂Ge₂ remained trivalent throughout the pressure range investigated.     

Reduction of hysteresis loss and large magnetocaloric effects in substituted compounds of itinerant-electron metamagnets La(FexSi1−x)13

The maximum value of hysteresis loss E_h^MAX due to the itinerant-electron metamagnetic (IEM) transition of La(Fe_xSi_1−x)₁₃ and the partially substituted compounds La_1−zCe_z(Fe_0.86Si_0.14)₁₃ and La_1−z′Pr_z′(Fe_0.86Si_0.14)₁₃ increases when the magnetocaloric effects (MCEs) become large. It should be noted that the reduction of E_h^MAX without the decrease of large MCEs is achieved in La_1−zCe_z(Fe_0.86Si_0.14)₁₃ and La_1−z′Pr_z′(Fe_0.86Si_0.14)₁₃. For both the compound systems mentioned above, the critical temperature T₀ for the IEM transition decreases and the difference between T₀ and the Curie temperature T_C becomes larger with decreasing T_C. These results are consistent with the magnetic phase diagram of La(Fe_0.86Si_0.14)₁₃ under hydrostatic pressure. Consequently, the reduction of E_h^MAX in La_1−zCe_z(Fe_0.86Si_0.14)₁₃ and La_1−z′Pr_z′(Fe_0.86Si_0.14)₁₃ is closely related with the magnetovolume effects.     

Interplay between unconventional superconductivity and heavy-fermion quantum criticality: CeCu2Si2 versus YbRh2Si2

In this paper the low-temperature properties of two isostructural canonical heavy-fermion compounds are contrasted with regards to the interplay between antiferromagnetic (AF) quantum criticality and superconductivity. For CeCu₂Si₂, fully-gapped d-wave superconductivity forms in the vicinity of an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum critical point (QCP). Inelastic neutron scattering results highlight that both quantum critical SDW fluctuations as well as Mott-type fluctuations of local magnetic moments contribute to the formation of Cooper pairs in CeCu₂Si₂. In YbRh₂Si₂, superconductivity appears to be suppressed at T???10?mK by AF order (T_N?=?70?mK). Ultra-low temperature measurements reveal a hybrid order between nuclear and 4f-electronic spins, which is dominated by the Yb-derived nuclear spins, to develop at T_A slightly above 2?mK. The hybrid order turns out to strongly compete with the primary 4f-electronic order and to push the material towards its QCP. Apparently, this paves the way for heavy-fermion superconductivity to form at T_c?=?2?mK. Like the pressure – induced QCP in CeRhIn₅, the magnetic field – induced one in YbRh₂Si₂ is of the local Kondo-destroying variety which corresponds to a Mott-type transition at zero temperature. Therefore, these materials form the link between the large family of about fifty low-T unconventional heavy – fermion superconductors and other families of unconventional superconductors with higher T_cs, notably the doped Mott insulators of the cuprates, organic charge-transfer salts and some of the Fe-based superconductors. Our study suggests that heavy-fermion superconductivity near an AF QCP is a robust phenomenon.     

Quantum critical behaviour in the CePd2Si2/CeNi2Ge2 system

We have explored the vicinity of the antiferromagnetic quantum critical point in the related heavy fermion metals CePd₂Si₂ and CeNi₂Ge₂ as a function of hydrostatic pressure. The normal state resistivity of the antiferromagnet CePd2Si2 near the critical pressure, at which magnetic order disappears, varies as ρ ～ T^χ(1:1 < χ < 1:4) over nearly two orders of magnitude in temperature up to about 30 K. This anomalous form for the resistivity appears to defy not only Fermi-liquid theory, but also simple phenomenological models for the effect of spin fluctuations close to a quantum critical point. An analogous unconventional behaviour is observed in the ambient pressure resistivity of the electronically and structurally equivalent, non-magnetic metal CeNi₂Ge₂. At pressures above 15 kbar, a new and unexpected superconducting transition appears in CeNi₂Ge₂ below 220 mK, which rises to higher temperatures with increasing pressure, reaching 400 mK at 26 kbar.     

A non-magnetic Fe probe of multiple magnetic phase transitions in RMn2Si2−x Ge x,R=rare earth

Summary Dilute⁵⁷Fe M?ssbauer-spectroscopy studies of RMn₂X₂ (X=Si and/or Ge, R=La, Ce, Pr, Nd, Sm, Eu and Gd) at 4.2 to 650 K yield the following results: Fe in RMn₂X₂ does not carry a magnetic moment. It reveals the magnetic order in the Mn and R sublattices through transferred hyperfine fields (∼100 kOe). The compounds LaMn₂Si₂, LaMn₂Ge₂, CeMn₂Ge₂, PrMn₂Ge₂, NdMn₂Ge₂ and SmMn₂Ge₂, known to be ferromagnets withT _c=300–350 K, are antiferromagnetically ordered above their correspondingT _c. TherT _N values extend from 385 K (SmMn₂Ge₂) to 470 K (LaMn₂Si₂). At the ferromagnetic-antiferromagnetic phase transition, a sharp reorientation of the Mn magnetic moments relative to the crystalline axes occurs. In RMn₂Si_2−x Ge _x with intermediatex values, the Ge is much more dominant in determining the magnetic properties of the Mn sublattice. While PrMn₂Si₂ is an antiferromagnet (T _N=365 K) and PrMn₂Ge₂ is a ferromagnetantiferromagnet (T _c=328 K,T _N=415 K), we find that in PrMn₂SiGe, the magnetic behaviour is similar to that in pure PrMn₂Ge₂,T _c=305 K andT _N=395 K. Paper presented at ICAME-95, Rimini, 10–16 September 1995.     

Studies of the quantum Hall to quantum Hall insulator transition in InSb-based 2DESs

The temperature dependence of ρ_xx is studied in the vicinity of the quantum Hall to quantum Hall insulator transition (ν=1→0) in InSb/InAlSb based 2DESs. ρ_xx displays a symmetric temperature dependence about the transition with on the QH side and on the insulating side. A plot of 1/T₀ for successive ν displays power-law divergence with 1/T₀∝|ν−ν_c|^−γ,² with γ=2.2±0.3. This critical behavior in addition to the behavior expected of the quantum transport regime confirms that the QH/QHI transition is indeed a good quantum phase transition.     

Optical properties of Stranski–Krastanov grown three-dimensional Si/Si1−xGex nanostructures

Detailed Raman and photoluminescence (PL) measurements are reported for Si/Si_1−xGe_x nanostructures grown by molecular beam epitaxy under near Stranski–Krastanov (S–K) growth mode conditions. In samples with x ranging from 0.096 to 0.53, we observe that an increase in the Raman signal related to Ge–Ge vibrations correlates with (i) a red shift in the PL peak position, (ii) an increase in the activation energy of PL thermal quenching, and (iii) an increase in the PL quantum efficiency. The results indicate that for x>0.5 Ge atoms form nanometer size clusters with a nearly pure Ge core surrounded by a SiGe shell. Time-resolved PL measurements reveal a stretched-exponential long-lived PL component that is associated with compositional and dimensional fluctuations in the SiGe dots.     

Metals near a magnetic instability

Zero-temperature magnetic phase transitions exhibit an abundance of nearly critical magnetic fluctuations that allow to probe the traditional concepts of the metallic state. For the prototypical heavy-fermion compound, CeCu_6−x Au _x , a breakdown of the Fermi-liquid properties may be tuned by Au concentration, hydrostatic pressure, or magnetic field. The d-electron weak itinerant ferromagnet ZrZn₂, on the other hand, was recently found to display superconductivity in coexistence with ferromagnetism.     

Spin fluctuation effects in substituted CeRu2Si2 and YbPd2Si2 alloys

Effects of chemical substitution in CeRu₂Si₂, a well-studied heavy fermion system and YbPd₂Si₂ have been investigated through magnetic susceptibility and x-ray diffraction in the systems CeRu _x Si₂, CeRu_2−x Os _x Si₂, CeRu₂Si_2−x Ge _x and YbPd₂Si_2−x Ge _x . Replacing silicon by germanium generates normal chemical pressure effect, namely, Ce and Yb atoms in CeRu₂Si₂ and YbPd₂Si₂ became more and less magnetic respectively. With increasing Ge concentration, CeRu₂Si_2−x Ge _x exhibits larger susceptibility at low temperature, goes to an antiferromagnetic state and finally becomes ferromagnetic. In YbPd₂Si_2−x Ge _x , increasing Ge concentration drives Yb atoms to more divalent state. Electronic effects are more pronounced in CeRu_2−x Os _x Si₂ though CeRu₂Si₂ and CeOs₂Si₂ have very nearly the same lattice parameters. It is conjectured that CeRu₂Si_2−x Ge _x may be the first Ce-based heavy fermion having a magnetic ground state. The authors felicitate Prof. D S Kothari on his eightieth birthday and dedicate this paper to him on this occasion.     

Kondo behaviour in magnetic (Ce−La)Pd2Si2

Susceptibility, specific heat and resistivity studies on CePd₂Si₂ and its pseudoternary system Ce_xLa_1–xPd₂Si₂ are reported. Replacement of Ce by La leads to a near dilution like decrease of the Néel temperaturesT _N (x): long range magnetic order in the ground state doublet exists in the wholex range explored (0.2x1). The present data indicate a concomitant Kondo effect, with Kondo temperaturesT _K decreasing linearly from 9 to 3 K, competing with the magnetic interaction. A close relationship between the value of the specific heat jump atT _N and the ratio between the two characteristic temperaturesT _N andT _K is demonstrated.     

Thermodynamic properties of CeCu6-xAux: Fermi-liquid vs. non-Fermi-liquid behaviour

We report on a detailed comparison of the thermodynamic properties of the heavy-fermion system CeCu₆ which can be described as a Fermi liquid at low temperatures T < 0.1 K, and CeCu_5.9Au_0.1 where strong deviations from the Fermi-liquid behaviour were found previously in the T dependence of the specific heat C, magnetization M and electrical resistivity p. The specific heat, magnetization and elastic constants are investigated in a large range of magnetic fields, corroborating the idea that the non-Fermi-liquid behaviour arises from low-lying spin excitations. For the elastic constants, a striking linear T dependence is found for CeCu_5.9Au_0.1 in contrast to the T² Fermi-liquid behaviour of CeCu₆.     

Potential of Si1−xGex alloys for Auger generation in highly efficient solar cells

Recent investigation on Si solar cells demonstrated the utility of Auger generation for the creation of more than merely one electron/hole pair per absorbed photon. The semiconductor Si requires a minimum photon energy of about 3.4 eV for this internal carrier multiplication. The current of a Si cell is therefore not significantly increased by Auger generation when the cell is illuminated by an air mass 1.5 spectrum, which contains only few photons with energies above 3.4 eV. Use of Si_1–x Ge _x alloys promises a lower onset energy. Unfortunately, incomplete data on band structures ofrandom Si_1–x Ge _x alloys preclude a detailed quantitative discussion of the full potential for these materials. Nevertheless, (i) analogies to our own quantum efficiency data from pure Si, (ii) the calculated band structure of the hypothetical,ordered zincblende type Si_0.5Ge_0.5 crystal, and (iii) optical data for Si_1–x Ge _x alloys indicate an optimum Ge content ofx=0.6 tox=0.7.     

Chemical pressure effects on the Kondo behaviour of CePt2Ge2

The compound, CePt₂Ge₂, recently reported to crystallize in a monoclinic modification of the CaBe₂Ge₂ structure, has been identified to be an antiferromagnetic Kondo lattice (T _N =2.2 K). The influence of positive and negative chemical pressure on the Kondo behavior is investigated by measurements of magnetic susceptibility and electrical resistivity () on the solid solutions, Ce_1–x La _x Pt₂Ge₂ and Ce_1–z Y _z Pt₂Ge₂. Y and La substituents cause nearly similar effects on the magnetic and electrical resistivity behaviour of CePt₂Ge₂, though the unit-cell volumes vary in opposite directions. We propose that the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction may contribute to decide the temperature at which attains a maximum at low temperatures (5 K) in CePt₂Ge₂.     

Generic phase diagram of “electron-doped” T′ cuprates

We investigated the generic phase diagram of the electron doped superconductor, Nd_2−xCe_xCuO₄, using films prepared by metal organic decomposition. After careful oxygen reduction treatment to remove interstitial O_ap atoms, we found that the T_c increases monotonically from 24 K to 29 K with decreasing x from 0.15 to 0.00, demonstrating a quite different phase diagram from the previous bulk one. The implication of our results is discussed on the basis of tremendous influence of O_ap “impurities” on superconductivity and also magnetism in T′ cuprates. Then we conclude that our result represents the generic phase diagram for oxygen-stoichiometric Nd_2−xCe_xCuO₄.     

Strain compensated Si/SiGe quantum well and quantum cascade on Si0.5Ge0.5 pseudosubstrate

This study follows up our previous investigation of the valence band (VB) intersubband emission from quantum cascade structures grown lattice matched on Si substrates. Here, Si/Si_1−xGe_x (x=80%) heterostructures are investigated which are deposited by MBE on a virtual substrate of relaxed SiGe containing 50% of Ge. TEM analysis reveal flat and abrupt interfaces for structures grown at temperatures T_growth≈300°C. Intersubband absorption and photoluminescence emission manifest well-defined interfaces and good material quality. The observed intersubband line positions are found to be in good agreement with k·p model calculations for the VB. This is in contrast to the observed type II no phonons recombination which is found at consistently lower energy than expected. Finally, electrically excited intersubband emission from a strain compensated cascade structure containing three periods is presented.