Session V16: Heavy Fermions- 1-1-5 Systems and Others

Fermi surface of Ce$_{2}$RhIn$_{8}$ studied by angle-resolved photoemission spectroscopy

The superconductivity in heavy fermion compounds is one of most interesting, outstanding problems in condensed matter physics. In this family of materials, Ce$_{2}$RhIn$_{8}$ and related compounds is unique because its electronic properties are believed to be quasi 2D. Here, we use angle-resolved photoemission spectroscopy to measure its electronic properties. The lack of significant $k_{z}$ dispersion confirms the quasi two dimensionality of the electronic structure. Fermi surface is quite complicated and consists of several hole and electron sheets. There are several extended segments of the Fermi surface which are likely nested. This provides clues to understanding of their unusual transport and thermodynamical properties.   

Anomalous superfluid response in the layered heavy fermion compound Ce$_2$PdIn$_8$ near a magnetic quantum critical point

The recently discovered layered heavy fermion superconductor Ce$_2$PdIn$_8$ ($T_c$ = 0.7 K) has attracted a great interest because of the appearance of unconventional superconductivity near a quantum critical point. Here we report high-precision measurements of magnetic penetration depth $\lambda$ down to 60 mK ($\sim0.1T/T_c$) by using a tunnel diode oscillator operating at $\sim$13 MHz. A strong power-law temperature dependence of $\lambda$ demonstrates low-energy excitations of quasiparticles, which is consistent with recent thermal conductivity measurements indicative of nodal superconductivity. The observed $T^{1.5}$ behavior at low temperatures, which is commonly seen in superconductors with line nodes near a magnetic quantum critical point such as CeCoIn$_5$ or organics, can be attributed to the enhancement of effective mass towards zero temperature due to quantum magnetic fluctuations even in the superconducting state.   

Magnetism in 2D Heavy Fermion Superconductor CePt$_{2}$In$_{7}$

CePt$_{2}$In$_{7}$ was first synthesized in 2008, and it was discovered to be a superconductor under pressure shortly thereafter in 2009. It is closely related to the well-studied crystal class CeMIn$_{5}$ (M = Ir, Co, Rh), but CePt$_{2}$In$_{7}$ is considerably more anisotropic with a c/a ratio more than twice as large as the 115's. We present here nuclear quadrupole resonance results for the $^{115}$In nucleus, and follow it from the paramagnetic heavy fermion state into the anti-ferromagnetic state below T$_{N}$ = 5.2 K. The NQR data will show a rich magnetic phase diagram, with a commensurate magnetic order parameter directly below T$_{N}$, and more complex magnetic states emerging as the material is further cooled. Additionally, we will present some additional results on how hydrostatic pressure affects the magnetic order in CePt$_{2}$In$_{7}$.   

Electric transport measurements on micro-structured CePt$_{2}$In$_{7}$ single crystals in a diamond anvil cell

We report Shubnikov--de Haas and resistivity measurements of CePt$_{2}$In$_{7}$ samples under hydrostatic pressures using a diamond anvil cell. CePt$_{2}$In$_{7}$ belongs to the Ce$_{m}M_{n}$In$_{3m+2n}$ heavy fermion family. Compared to the Ce$M$In$_{5}$ members of this group, the structure of CePt$_{2}$In$_{7}$ has a more two dimensional character, but also exhibits an antiferromagnetically ordered and a superconducting phase. Upon increasing pressure the AFM order is suppressed with the N\'{e}el temperature extrapolating to a quantum critical point. The fluctuations associated with the QCP are thought to stabilize the unconventional superconducting phase. To investigate the weight of the different scattering channels the anisotropy of the resistivity above the N\'{e}el temperature was measured for various applied pressures. Shubnikov--de Haas measurements were conducted to deduce the changes in the effective electron masses in the AFM and superconducting phases under applied hydrostatic pressure. To this end we developed a method to conduct four terminal resistance measurements on micro-structured samples inside a diamond anvil cell.   

SmPt$_4$Ge$_{12}$ -- a filled skutterudite with exotic heavy-fermion properties

Ternary samarium-filled platinum-germanium skutterudite SmPt$_4$Ge$_{12}$ was prepared at a pressure of 5.0(0.5)\,GPa and a temperature of 1070(70)\,K. The compound crystallizes in the cubic space group $Im\bar{3}$ ($a$ = 8.6069(4)\,\AA) and is isotypic with LaFe$_4$P$_{12}$. X-ray absorption spectroscopy measurements show that samarium in SmPt$_4$Ge$_{12}$ has a temperature-independent intermediate valence ($\nu = 2.90 \pm 0.03$). Magnetization data reveal van-Vleck paramagnetism above $\sim$50\,K. The low-temperature specific heat displays a broad anomaly centred at 2.9\,K and a large linear coefficient $\gamma'$ = 450 mJ\,mol$^{-1}$K$^{-2}$ suggesting heavy-fermion behaviour. This state is remarkabkly field-independent and resembles the physics of SmOs$_4$Sb$_{12}$. [1] R Gumeniuk et al. New J. Physics 12 (2010) 103035.   

Physical properties and Ce-valence of the filled skutterudite CePt$_4$Ge$_{12}$

Filled skutterudite compounds $M$Pt$_4$Ge$_{12}$ with a Pt-Ge framework structure show intriguing physical ground states (conventional ($M$ = Sr, Ba) and unconventional superconductivity ($M$ = Pr), heavy-fermion behavior ($M$ = Sm), magnetic ordering), similar to the well-known transition-metal pnicogen skutterudites. Here, we report on the electronic, magnetic, and transport properties of CePt$_4$Ge$_{12}$ [1]. High-resolution X-ray absorption spectroscopy (XANES) measurements at the cerium L$_{\mathrm{III}}$ edge demonstrate that Ce in this compound has a temperature-independent valence close to three. However, magnetic susceptibility, thermopower, Hall effect, and electronic specific heat reveal broad maxima at $T_{\mathrm{max}}$ = 65-80\,K, suggesting the presence of valence fluctuations. The Sommerfeld coefficient $\gamma$ = 105 mJ\,mol$^{-1}$\,K$^{-2}$ indicates moderately enhanced band masses for CePt$_4$Ge$_{12}$. We discuss these findings and conclude that CePt$_4$Ge$_{12}$ represents a system at the border between intermediate valence (IV) and Kondo lattice behavior.\\[4pt] [1] R.Gumeniuk et al. J.\ Phys.: Condensed Matter 23 (2011) 456601.   

Cu-NMR and magnetization in disordered nFL system UCu$_4$Ni

We present a study of the NMR spectra in a random powder of UCu$_4$Ni as a function of frequency (40-70 MHz) and temperature (5-300 K). Two types of spectral lines for each of the two isotopes of naturally abundant Cu in the material are clearly evident in the spectra. Their behavior is followed for temperature and field variations and compared/contrasted with the more studied case of UCu$_4$Pd, where only one type of Cu-NMR line has been observed clearly. Unlike in UCu$_4$Pd, the appearance of two types of signal from Cu nuclei in the Ni compound is unambiguous evidence of site disorder in UCu$_4$Ni. This alone is indication that the amount of site disorder in the Ni sample is larger than in the Pd system; however, the NMR line intensities reveal that the Ni ions do not seem to go completely randomly in the two available crystallographic sites of the underlying crystal structure as would be expected from ionic-size considerations alone. The NMR parameters for both types of spectral lines, together with complementary measurements of magnetic susceptibility performed on the same powder samples, will be discussed from the point of view of magnetic disorder and non-Fermi liquid behavior.   

Effect of pressure on CeGe$_{2-x}$ single crystals

At ambient pressure CeGe$_{2-x}$ has an antiferromagnetic (AFM) transition at $T_N \sim 7$ K and, on further cooling, a second transition, to a weak ferromagnetic (WFM) state, at $T_C \sim 4$ K. Under pressure the $T_N$ goes through a broad maximum, and then is completely suppressed with a critical point at $\sim 85$ kbar, grossly following the Kondo-necklace picture. The WFM phase evolves into apparent AFM phase under moderate pressure of $\sim 10$ kbar, with $T_N1$ joining $T_N$ at $\sim 60$ kbar. Resulting $P - T$ phase diagram and comparison with CeRu$_2$Ge$_2$ under pressure will be presented.   

A THz spectroscopy investigation of the heavy fermion state in CeCu$_2$Ge$_2$

We present time-domain THz spectroscopy data of a thin film of the heavy fermion compound CeCu$_2$Ge$_2$. Measurements to obtain the frequency dependent complex conductivity were taken as a function of temperature down to temperatures well below the onset of magnetic order. At low temperatures a narrow Drude-like peak forms, which is likely associated with the heavy fermion or spin density wave state. Using this data in conjunction with DC resistivity measurements, we obtain the frequency dependence of the scattering rate and the mass renormalization through an extended Drude model analysis.   

Influence of charge carrier doping on the $T^\ast$-scale in YbRh$_2$Si$_2$

YbRh$_2$Si$_2$ is a prototype heavy-fermion metal which displays a magnetic field-induced antiferromagnetic (AF) quantum critical point (QCP). It has attracted much attention due to an additional low-energy scale $T^\star(B)$ merging at the QCP, whose origin is controversially discussed. Here, we report measurements of the electrical resistivity $\rho(T,B)$ on different single crystalline samples of charge-carrier doped Yb(Rh$_{1-x}$T$_x$)$_2$Si$_2$ (T=Fe, Ni) at temperatures down to 15~mK and in magnetic fields up to 7~T. The partial substitution of Rh by either Fe or Ni introduces holes or electrons, respectively. The evolution of the single-ion Kondo scale is similar as for isoelectronic Co substitution and in accordance with the chemical pressure effect. However, while chemical pressure has little influence on $T^\star(B)$, we observe a drastic reduction or increase of $B^\star(T=0)$ by Fe- or Ni-doping, respectively. Most interestingly, $B^\star(T=0)$ is always pinned at the field-induced AF QCP, in contrast to chemical pressure results. As AF order is completely suppressed by Fe-doping, a heavy Fermi liquid ground (without $T^\star(B)$ anomaly) is observed.   

Quantum Criticality in the strongly correlated 3d electron system YFe$_{2}$Al$_{10}$

YFe$_{2}$Al$_{10}$ has recently been identified as a strongly correlated 3d -electron system that is located very close to a quantum critical point (QCP). At low temperatures, divergences were found in the ac magnetic susceptibility ($\chi '\propto T^{-1.3})$ and magnetic specific heat ($C_M /T\propto T^{-0.47})$ based on the measurements carried out on single crystals. The magnetic Gruneisen ratio determined from these measurements ($\Gamma /H=-\frac{1}{T}\frac{\partial S/\partial B}{\partial S/\partial T}=-\frac{\partial M/\partial T}{C})$ also shows a strong divergence as $T\to 0$, which is suppressed in fields indicating a quantum critical point at B=0. When applying magnetic field, Fermi liquid like behavior with an enhanced magnetic susceptibility $\chi _{0}$ and Sommerfeld coefficient C/T emerges at T$<$T$_{FL}$, and this enabled us to establish a field temperature (B-T) phase diagram with a crossover temperature T$_{FL}$(B) coming out from the QCP at B=0. This is very similar to the quantum critical behavior observed in many f-electron based heavy fermion (HF) systems, and it makes YFe$_{2}$Al$_{10}$ an interesting 3d- electron candidate for studying quantum criticality.   

Small-angle neutron scattering study of the vortex lattice in PrOs$_4$Sb$_{12}$

We carried out a small angle neutron scattering study of the vortex lattice in the Pr based heavy Fermion superconductor PrSb$_4$Os$_{12}$ with H applied along the c-axis. With a critical temperature T$_c$ of 1.85 K and a H$_{c2}$ of 2.5T, PrOs$_4$Sb$_{12}$ is the first Pr based heavy fermion superconductor (SC). Although PrOs$_4$Sb$_{12}$ crystallizes in a filled Skutterudite structure with a cubic lattice, this structure has a tetrahedral point group symmetry. In consequence, with the application of a magnetic field along the $c$-direction, the $a$ and $b$-axis are no longer equivalent. Measurements of the angular dependence of the thermal conductivity in PrOs$_4$Sb$_{12}$ suggest a change of the superconducting order parameter deep inside the SC phase diagram from a two fold symmetry to a fourfold symmetry. Three ways of applying field were tested and wiggle field cooling was found to produce the highest intensity. We observed a twinned distorted hexagonal VL up to 1T of applied field which is consistent with $s$-wave superconductivity. We did not observe a field dependent phase transition associated with a change in symmetry of the VL. We found a rapidly decreasing form factor |F|$^2$ of the vortex lattice (VL) in PrOs$_4$Sb$_{12}$ with increasing field.   

In plane vortex lattice of the heavy Fermion superconductor CeCoIn5

We present a small angle neutron scattering study of the vortex lattice (VL) in the unconventional superconductor CeCoIn$_5$ for magnetic fields $H$ applied in the crystallographic $ab$-plane. CeCoIn$_5$ is a $d_{x^2-y^2}$ superconductor with the lines nodes of the superconducting gap oriented along the $[110]$ direction. At low fields $H$ applied parallel to $[100]$, and $[110]$, we observed a distorted hexagonal VL with an opening angle of about 80$^{\circ}$. However, for $[110]$ we find a reorientation transition for fields above 9~T where the vortex lattice is rotated by 90$^{\circ}$. The $H$-dependence of the form factor of the vortex lattice is similar to what we observed previously along the $c$-axis: The scattering intensity increases with increasing $H$ due to Pauli paramagnetism. For fields where the transition into the SC state becomes first order, we observe a broadening of the VL.   

Low-energy magnetic excitations in single-crystalline CeCu$_{2}$Ge$_{2}$ in fields up to 10 T

CeCu$_{2}$Ge$_{2}$ is a magnetically ordered (T$_{N}$ = 4.1 K) Kondo lattice with a moderate enhanced Sommerfeld coefficient of 140 mJ/molK$^{2}$. Calculations of the Fermi surface showed that the observed incommensurate antiferromagnetic order can only be explained by involving an itinerant component of the Ce 4f-moments in addition to their local character. We performed low-energy inelastic neutron scattering experiments on a 2 g single crystal of CeCu$_{2}$Ge$_{2}$ using the cold triple-axis spectrometer PANDA at FRM II. Data were taken at 1.5 K and in magnetic fields up to 10 T applied perpendicular to the (110/001) scattering plane. At zero field the magnetic excitations show an energy gap of 0.5 meV at all investigated $\Gamma$ points in the (110/001) plane with similar intensities. Away from the $\Gamma$ points the magnetic excitations become dispersive merging into a band of excitations around 1 meV. For increasing magnetic fields the gap energy decreases indicating opposite action of external and internal fields. The results will be discussed in the framework of local, crystal field related, and non-local, spin-wave-like, magnetic excitations.   

Universal Kondo effect in Ti$_{0.94}$M$_{0.06}$O$_{2}$ (M = Nb, Ta) Thin Films

We show that Ti$_{0.94}$M$_{0.06}$O$_{2}$ (M = Nb, Ta) thin films are Kondo systems by angle dependent magneto-resistance measurement. Surprisingly, the data fitted by both Goldhaber-Gordon and Hamann formula yield comparable carrier dependent Kondo temperatures (10 - 60 K). The Kondo temperature dependence on carrier density was different for Ta and Nb incorporated in TiO$_{2}$ but this result could be understood on the basis of the formation of compensating defects (Ti vacancies) which also act as localized magnetic scattering centers. Using the unitarity-limit resistivity and Kondo temperature estimated from these fits the normalized resistivity versus temperature for all these films collapse into a universal curve consistent with data observed in metallic systems. However, the size of the Kondo scattering is at least an order of magnitude larger than in metallic systems. Further, from the extrapolation of the Kondo temperature we are able to predict that Ta is a better candidate for observation of ferromagnetism versus Nb.