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Elemental tellurium is a small band-gap semiconductor, which is always p-doped due to the natural occurrence of vacancies. Its chiral non-centrosymmetric structure, characterized by helical chains arranged in a triangular lattice, and the presence of a spin-polarized Fermi surface, render tellurium a promising candidate for future applications. Here, we use a theoretical framework, appropriate for describing the corrections to conductivity from quantum interference effects, to show that a high-quality tellurium single crystal undergoes a quantum phase transition at low temperatures from an Anderson insulator to a correlated disordered metal at around 17 kbar. Such insulator-to-metal transition manifests itself in all measured physical quantities and their critical exponents are consistent with a scenario in which a pressure-induced Lifshitz transition shifts the Fermi level below the mobility edge, paving the way for a genuine Anderson-Mott transition. We conclude that previously puzzling quantum oscillation and transport measurements might be explained by a possible Anderson-Mott ground state and the observed phase transition. Elemental tellurium is a natural p-type semiconductor with a chiral structure and spin-polarized Fermi surface. Here, the authors show that the pressure-induced topological change of the Fermi surface at 17 kbar triggers an Anderson-Mott insulator-to-metal transition.

The delicate balance between spin-orbit coupling, Coulomb repulsion, and crystalline electric field interactions observed in Ir-based oxides is usually manifested as exotic magnetic behavior. Here we investigate the evolution of the exchange coupling between Co and Ir for partial La substitution by Ca in ${\mathrm{La}}_{2}{\mathrm{CoIrO}}_{6}$. A great advantage of the use of ${\mathrm{Ca}}^{2+}$ as a replacement for ${\mathrm{La}}^{3+}$ is the similarity of their ionic radii. Thus, the observed magnetic changes can more easily be associated with electronic variations. A thorough investigation of the structural, electronic, and magnetic properties of the ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{CoIrO}}_{6}$ system was carried out by means of synchrotron x-ray powder diffraction, muon spin rotation and relaxation $(\ensuremath{\mu}\mathrm{SR})$, AC and DC magnetization, x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism, Raman spectroscopy, electrical resistivity, and dielectric permittivity. Our XAS results show that up to 25% Ca substitution at the La site results in the emergence of ${\mathrm{Co}}^{3+}$, possibly in a high-spin state, while the introduction of a larger amount of Ca leads to an increase in the Ir valence. The competing magnetic interactions resulting from the mixed valences lead to the coexistence of a magnetically ordered and an emerging spin-glass (SG) state for the doped samples. Our $\ensuremath{\mu}\mathrm{SR}$ results indicate that for ${\mathrm{La}}_{2}{\mathrm{CoIrO}}_{6}$ a nearly constant fraction of a paramagnetic (PM) phase persists down to low temperatures, possibly related to the presence of a small amount of ${\mathrm{Ir}}^{3+}$ and to the antisite disorder at Co/Ir sites. For doped compounds the PM phase freezes below 30 K, but there is still some dynamics associated with the SG. The dielectric data obtained for the parent compound and the one with 25% Ca doping indicate a possible magnetodielectric effect, which is discussed in terms of the electron hopping between the transition-metal ions, the antisite disorder at Co/Ir sites, and the distorted crystalline structure.

We explore the evolution of the structural phase transition of ${(\mathrm{Sr},\mathrm{Ca})}_{3}{\mathrm{Ir}}_{4}{\mathrm{Sn}}_{13}$, a model system to study the interplay between structural quantum criticality and superconductivity, by means of high-energy x-ray diffraction measurements at high pressures and low temperatures. Our results confirm a rapid suppression of the superlattice transition temperature ${T}^{*}$ against pressure, which extrapolates to zero at a critical pressure of $\ensuremath{\approx}1.79(4)\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. The temperature evolution of the superlattice Bragg peak in ${\mathrm{Ca}}_{3}{\mathrm{Ir}}_{4}{\mathrm{Sn}}_{13}$ reveals a drastic decrease of the intensity and an increase of the linewidth when $T\ensuremath{\rightarrow}0\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and $p\ensuremath{\rightarrow}{p}_{c}$. Such anomaly is likely associated with the emergence of quantum fluctuations that disrupt the formation of long-range superlattice modulation. The revisited temperature-pressure phase diagram of ${(\mathrm{Sr},\mathrm{Ca})}_{3}{\mathrm{Ir}}_{4}{\mathrm{Sn}}_{13}$ thus highlights the intertwined nature of the distinct order parameters present in this system and demonstrates some similarities between this family and the unconventional superconductors.

Bulk ceramic samples with chemical composition (Y 1-x Nd x )Ba 2 Cu 3 O 7-δ , where x = 0, 0.1, 0.2, 0.5, and 1 were sintered by a proposed acetate method. The samples were then submitted to structural characterization by X-ray diffraction with crystal structure refining using the Rietveld method. AC electrical resistivity measurements from ambient temperatures down to 4.2 K were performed, as well as magnetic measurements of magnetic moment as function of temperature and magnetic hysteresis loop. Results have shown superconductivity with critical temperatures of about 91 and 60 K for pure YBa 2 Cu 3 O 7-δ (x = 0) and pure NdBa 2 Cu 3 O 7-δ (x = 1) samples, respectively. Scanning electron microscopy images were also analyzed, revealing a regular parallel-piped grain shape. The proposed method showed as an advantage to reduce the grinding time without any considerable degradation on structural or superconducting properties of pure YBa 2 Cu 3 O 7-δ (x = 0) or pure NdBa 2 Cu 3 O 7-δ ( x = 1), as related to samples prepared by a conventional solid state route.

In the zero-field-cooled exchange bias (ZEB) effect, the unidirectional magnetic anisotropy is set at low temperatures even when the system is cooled in the absence of an external magnetic field. ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoMnO}}_{6}$ stands out as presenting the largest ZEB reported so far, while for ${\mathrm{La}}_{1.5}{\mathrm{Ca}}_{0.5}{\mathrm{CoMnO}}_{6}$ the exchange bias field (${H}_{\text{EB}}$) is one order of magnitude smaller. Here we show that ${\mathrm{La}}_{1.5}{\mathrm{Ba}}_{0.5}{\mathrm{CoMnO}}_{6}$ also exhibits a pronounced shift of its magnetic hysteresis loop, with an intermediate ${H}_{\text{EB}}$ value with respect to Ca- and Sr-doped samples. To figure out the microscopic mechanisms responsible for this phenomenon, these compounds were investigated by means of synchrotron x-ray powder diffraction, Raman spectroscopy, muon spin rotation and relaxation, ac and dc magnetization, x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD). The parent compound ${\mathrm{La}}_{2}{\mathrm{CoMnO}}_{6}$ was also studied for comparison as a reference of a non-ZEB material. Our results show that the Ba-, Ca-, and Sr-doped samples present a small amount of phase segregation, and that the ZEB effect is strongly correlated to the system's structure. We also observed that mixed valence states ${\mathrm{Co}}^{2+}/{\mathrm{Co}}^{3+}$ and ${\mathrm{Mn}}^{4+}/{\mathrm{Mn}}^{3+}$ are already present at the ${\mathrm{La}}_{2}{\mathrm{CoMnO}}_{6}$ parent compound, and that ${\mathrm{Ba}}^{2+}/{\mathrm{Ca}}^{2+}/{\mathrm{Sr}}^{2+}$ partial substitution at the ${\mathrm{La}}^{3+}$ site leads to a large increase of Co average valence, with a subtle augmentation of Mn formal valence. Estimates of the Co and Mn valences from the $L$-edge XAS indicate the presence of oxygen vacancies in all samples ($0.05\ensuremath{\le}\ensuremath{\delta}\ensuremath{\le}0.1$). Our XMCD results show a great decrease of Co moment for the doped compounds, and they indicate that the shift of the hysteresis curves for these samples is related to uncompensated antiferromagnetic coupling between Co and Mn.

Elemental tellurium is a semiconductor with a small band gap of around 330 meV. Under the application of hydrostatic pressure, the band gap narrows, and it has been suggested that below its crystal phase transition at 40 kbar, the gap might close, leading to a transition into a topologically non-trivial state. Here, we present a pressure dependent study of the gap size of tellurium up to 22kbar, studied by temperature dependent electrical transport. We identify the gap size to be still well above 100 meV at our maximum pressure. Extrapolation of our data, assuming a non-linear gap narrowing (in agreement with previous studies), leaves us to assume that the gap does not close within the ambient pressure crystal phase. However, when assuming a linear narrowing of the gap, we leave a small possibility that there exists a minor pressure window at which a topological phase transition could occur. Furthermore, as the second systematic study of the gap size via transport, we see that historical inconsistencies between the measured gap size via transport and the measured optical gap size seem to be systematic and probably reflect some deeper physics of the material.

The semimetal-to-semiconductor transition in fcc-Yb under modest pressure can be considered a picture book example of a metal–insulator transition of the Lifshitz type. We have performed transport measurements at low temperatures in the closest vicinity of the transition and related DFT calculations of the Fermi surface. Our resistivity measurements show a linear temperature dependence with an unusually low d ρ / dT at low temperatures approaching the MIT. The calculations suggest fcc-ytterbium being an ultra-multi valley system with 24 electron and 6 hole pockets in the Brillouin zone. Such Fermi surface topology naturally supports the appearance of strongly correlated phases. An estimation of the quasiparticle-enhanced effective mass shows that the scattering rate is by at least two orders of magnitude lower than in other materials which exhibit linear-in-T behavior at a quantum critical point. However, we cannot exclude an excessive effective mass enhancement, when the van Hove singularity touches the Fermi level.

La3Co4Sn13 is a superconducting material with transition temperature at Tc = 2.70 K, which presents a superlattice structural transition at T* ~ 150 K, a common feature for this class of compounds. However, for this material, it is not clear that at T* the lattice distortions arise from a charge density wave (CDW) or from a distinct microscopic origin. Interestingly, it has been suggested in isostructural non-magnetic intermetallic compounds that T* can be suppressed to zero temperature, by combining chemical and external pressure, and a quantum critical point is argued to be observed near these critical doping/pressure. Our study shows that application of pressure on single-crystalline La3Co4Sn13 enhances Tc and decreases T*. We observe thermal hysteresis loops for cooling/heating cycles around T* for P > 0.6 GPa, in electrical resistivity measurements, which are not seen in x-ray diffraction data. The hysteresis in electrical measurements may be due to the pinning of the CDW phase to impurities/defects, while the superlattice structural transition maintains its ambient pressure second-order transition nature under pressure. From our experiments we estimate that T* vanishes at around 5.5 GPa, though no quantum critical behavior is observed up to 2.53 GPa., 6 pages, 5 figures

Polycrystalline samples of the series La2−xCaxCoIrO6 ( 0 ≤ x ≤ 1.2 ) have been synthesized and its structural and magnetic properties were investigated using experiments of X-ray powder diffraction, temperature dependent magnetic susceptibility and magnetization as a function of applied magnetic field. All grown polycrystalline compounds form in a monoclinic structure (space group P 2 1 / n ) with a partially ordered distribution of transition metal cations Co/Ir over the six-coordinate sites. Our results indicate variation of the magnetic interactions between Co and Ir magnetic sublattices possibly due to valence changes of the transition metals, induced by doping. The undoped La2CoIrO6 presents magnetic ordering at ~ 97 K , while for Ca-doped compounds a decrease of net magnetization and ordering temperatures was observed. For x=0.2 and 0.5 the emergence of a second magnetic interaction, due to the presence of high spin Co3+, becomes evident. Interestingly, for x=0.8 the large effective magnetic moment indicates strong orbital contribution and spin-orbit coupling and/or the presence of Co4+ in high spin state.

Here, we present a study on $\mathrm{Si}(111)/\mathrm{Ta}(150\phantom{\rule{0.28em}{0ex}}\AA{})/\mathrm{IrMn}(150\phantom{\rule{0.28em}{0ex}}\AA{})/\mathrm{NiFe}(50\phantom{\rule{0.28em}{0ex}}\AA{})/\mathrm{Nb}(x)/\mathrm{NiFe}(50\phantom{\rule{0.28em}{0ex}}\AA{})/\mathrm{Ta}(50\phantom{\rule{0.28em}{0ex}}\AA{})$ spin valves with $x=100\ensuremath{-}500\phantom{\rule{0.28em}{0ex}}\AA{}$. Samples with a Nb layer thickness of $300\phantom{\rule{0.28em}{0ex}}\AA{}$ or higher exhibit an unusual magnetization behavior below ${T}_{c}$. The superconducting Nb layer contributes strongly to the magnetization, and manifests itself in an anomalous hysteresis loop. While the hysteresis loop is seemingly similar to what is generally expected from hard superconductors and many superconductor/ferromagnet hybrid systems, its direction is inverted when compared to what is generally observed. Thus, the samples show paramagnetic behavior for upsweeping fields and diamagnetic behavior for downsweeping fields. This means that the respective samples exhibit a magnetization, which is contrary to what Lenz's rule dictates.

Highlights: • Effect of Mn-site doping by Ru has been studied in La{sub 1.24}Sr{sub 1.76}Mn{sub 2-y}Ru{sub y}O{sub 7}. • Electrical resistance, magnetoresistance and magnetic properties were measured. • Ru substitution enhances the ferromagnetism and metallicity. • Results were interpreted by the ferromagnetically coupled Ru with Mn ions in Mn–O–Ru network. - Abstract: The effect of Mn-site doping on magnetic and transport properties in the bilayer manganites La{sub 1.24}Sr{sub 1.76}Mn{sub 2-y}Ru{sub y}O{sub 7} (y = 0.0, 0.04, 0.08 and 0.15) has been studied. The undoped compound La{sub 1.24}Sr{sub 1.76}Mn{sub 2}O{sub 7} exhibits a ferromagnetic metal to paramagnetic insulator transition at T{sub C} = 130 K and the substitution of Ru shifts the transition temperatures to higher temperature values. The increased metal–insulator transition by Ru substitution, obtained from temperature dependence of resistivity measurements, indicates that the Ru substitution enhances the metallic state at low temperature regime and favours the Mn–Ru pairs in the Ru doped samples. Moreover, the activation energy values calculated from the temperature dependence of resistivity curves suggest that the Ru substitution weakens the formation of polarons. The increased magnetoresistance ratio from 108% to 136% by Ru substitution, measured at 5 K, points out that the Ru substitution also enhancesmore » the inter-grain tunneling magnetoresistance. Thus, the ferromagnetic order and metallic state in La{sub 1.24}Sr{sub 1.76}Mn{sub 2}O{sub 7} system have been enhanced by the presence of Ru in the Mn-site. These reinforcements of ferromagnetic metallic state and magnetoresistance have been interpreted by the ferromagnetically coupled high spin states of Ru with Mn ions in the Mn–O–Ru network.« less

We have performed measurements of temperature dependent electrical resistivity ρ ( T ) under pressures up to 27 kbar and down to 0.1 K on single crystals of the Ce3Ir4Sn13 heavy fermion compound. At ambient pressure (P=0) we have identified in the ρ ( T ) data interesting features associated with the presence of crystalline field effects, magnetic correlations, Kondo single impurity scattering and, possibly, a low temperature structural phase transition. All these features were mapped as a function of pressure which allowed us to construct a pressure–temperature phase diagram with these temperature scales. We have also carried out measurements of ρ ( T ) as a function of magnetic fields up to H=8 T and the important temperature scales in ρ ( T ) were followed with field. Enlightened also by temperature dependent specific heat experiments we discuss the possible microscopic origins of the features found in our ρ ( T ) data.

We report the low temperature magnetization, electrical resistance, and magnetoresistance of La1.24Sr1.76Mn2O7 bilayer manganite system under hydrostatic pressure. At ambient pressure, the compound shows a sharp ferromagnetic transition (TC) accompanied by a metal–insulator transition (TMI) at 130 K. We observe that the TC and TMI increase with hydrostatic pressure at a rate of dTC/dP = 2.08 K/kbar and dTMI/dP = 2 K/kbar, respectively. Also, we observe an appreciable increase of magnetic moments at low temperatures with increasing pressure. The high temperature regime of temperature dependence of resistivity curves was fitted with the Emin–Holstein’s polaron hopping model and the calculated activation energy values suggest that the applied pressure weakens the formation of Jahn–Teller polarons. The magnetoresistance ratio (MRR) was measured at TC and at 4.2 K upon an external magnetic field of 5 T. The observed MRR at TC is about 210 % and the applied pressure increases the MRR significantly. These results can be interpreted by the pressure-enhanced overlap between the orbitals of Mn–O–Mn, which facilitates the charge transfer and hence enhances the ferromagnetism and metallicity.

The La 1.32 Sr 1.68 Mn 2 O 7 layered manganite system has been studied by the low temperature electrical resistance and magnetoresistance under hydrostatic pressure up to 25 kbar. We have observe both, a Curie temperature ( T C ) and a metal–insulator transition ( T MI ) at 118 K in the ambient pressure. The applied pressure shifts the T MI to higher temperature values and induces a second metal–insulator transition ( T 2 MI ) at 90 K, in the temperature dependence of resistivity measurements. Also, the pressure suppresses the peak resistance abruptly at T C . When an external field of 5 T is applied, we have observed a large negative magnetoresistance of 300% at the transition temperature and a 128% at 4.5 K. However, the increased pressure decreases the magnetoresistance ratio gradually. When the pressure reaches its maximum available value of 25 kbar, the magnetoresistance ratio decreases at a rate of 1.3%/kbar. From our experimental results, the decrease of magnetoresistance ratio with pressure is explained by the pressure induced canted spin state which is not favor for the spin polarized intergrain tunneling in layered manganites.

Reproducibility, sensibility and selectivity are standard parameters on the development of sensors. In this article we described a planar electrochemical sensor based silk-screened to detect nitrite and its interferers - paracetamol, ascorbic acid and uric acid. We modified the working electrode gold surface by using 1-2 diaminobenzeno (DAB) membrane in order to reduce the interferers. Preliminary results showed which using DAB there is decrease of interference signal about three times. We obtained a sensibility about 310 μA/(mol.L-1.mm2) for nitrite, using plain electrode and 90 μA/(mol.L-1.mm2) after DAB membrane. Although the actual results are promising, we are evaluating several combinations of membranes to reduce more the influence of the interferers in the nitrite detection.

Phase separation and suppression of critical dynamics at quantum phase transitions of MnSi and (Sr 1− x Ca x )RuO 3

The preparation and characterization of new ternary composites formed by the redox/intercalative reaction of pyrrole with the lamellar bentonite/V 2 O 5 composite are reported. The generation of a conducting polymer combined with a mixed semiconductor oxide-clay material provided an interesting material where the conductivity depends upon the polypyrrole content, and on the particular way it influences the Li + ions migration in the films.

57Fe Mossbauer spectroscopy has been used to study rare earth intermetallic compounds having strongly correlated electronic properties, where the transition metal carries no magnetic moment. In this case 57Fe is a local probe to detect very small transferred hyperfine fields (Bthf) from the rare earth sites.

In this work we present a magnetoresistance study on the CeRu2Ge2 compound. We analyze the ρ(T) curves for several applied magnetic fields using the electron–magnon scattering model for a ferromagnetic spin arrangement. From this analysis, the field dependence of the energy gap of the magnon spectrum is obtained. The magnetoresistance ρ(H) at various temperatures arises from a normal metal contribution with an additional scattering mechanism due to electron–magnon interaction.

We present a study of the transport and magnetic properties of a series of $R\mathrm{NiBC}$ compounds $(R=\mathrm{Er},$ Ho, Dy, Tb, and Gd). All the materials investigated have long range magnetic order at sufficiently low temperatures. Magnetoresistance measurements are presented for a large range of temperatures (T) and magnetic fields (H). We show that below the critical temperature, the temperature dependence of the resistivity is determined by electron scattering due to the elementary excitations (spin waves) of the ordered magnetic phase and the values of the gap in the magnon spectra were derived. Finally we discuss the $H\ifmmode\times\else\texttimes\fi{}T$ phase diagram of these materials.

We report pressure and temperature dependent electrical resistivity measurements in the Ce 2 Rh 1 - x Ir x In 8 alloys. Our results revealed two phases with zero resistance. One of these phases is induced by pressure, occurring for a range of compositions situated near the Rh-rich extreme, consistent with the behavior observed for the pure Ce 2 RhIn 8 compound. The second transition occurs for intermediate concentrations ( x ∼ 0.5 – 0.7 ) , and in sharp contrast with the first transition, is progressively eliminated by the application of pressure. We compare the findings in the Ce 2 Rh 1 - x Ir x In 8 alloys to its related CeRh 1 - x Ir x In 5 , discussing how the occurrence of the superconducting phases may become unfavorable for the bi-layer alloys due to higher dimensionality and stronger disorder.

We report on the structures of the magnetic phases of CeRu2Ge2 and their evolution in the series Ce(Ru1-xFex)2Ge2. The crossover behavior between the ferromagnetic ground state of CeRu2Ge2 to the paramagnetic ground state of CeFe2Ge2 is compared to the behavior in the series CeRu2(Ge1-xSix)2. A profound difference is observed between these two series, which is in contradiction to the uniform behavior predicted by the Doniach-necklace model.

Evolution of the structural, magnetic, transport and magneto-transport properties of the series is studied by powder x-ray diffraction, Mossbauer spectroscopy, magnetization and resistivity in zero and applied magnetic field. Changes from concentrated Kondo system-like to single Kondo ion-like behaviour are observed. Magneto-resistivity behaviour in both regimes is discussed within the existing theoretical views which include non-negligible `normal' metal contributions.

The magnetic order of ${\mathrm{TbNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ below ${T}_{N}=15\mathrm{K}$ has been studied via ${}^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy and muon spin relaxation (\ensuremath{\mu}SR). It is shown that only below 8 K all components of the Tb moment are locked. This is traced from an unusual increase and a turning of the transferred magnetic hyperfine field at the ${}^{57}\mathrm{Fe}$ dopants, and also from a decrease of the fluctuation rate of electronic moments detected from \ensuremath{\mu}SR. We relate this locking of the moment to the ferromagnetically ordered component which was concluded from neutron diffraction experiments.

In this work a preliminary pressure–temperature–composition phase diagram for the Ce 2 Rh ( 1 - x ) Ir x In 8 heavy-fermions (HF) materials is reported, built from pressure-dependent electrical resistivity ( ρ ( T ) ) measurements on single-crystalline samples. Although neither of the two end members is an ambient-pressure SC, a transition to a zero-resistance (ZR) state was found at ambient pressure ( T C ≈ 600 mK ) in a narrow Ir concentration range (around x = 0.5 ). Pressures not higher than 10 kbar are sufficient to suppress this ZR state for all measured concentrations, in contrast to the 115 materials.

Mossbauer spectroscopy has been used to investigate the magnetic structure of doped with 1 at.% . Our experiments on polycrystalline reveal the known spin-rearrangement magnetic phase transition at , which until now has not been observed in polycrystalline samples. A spiral-like spin structure with a variable angle between the moment direction and the b- or c-axis seems to be the correct model for the spin structure of .

Experiments of electrical resistivity as a function of hydrostatic pressure for single crystals of Sn-doped CeCoIn 5 are reported. Due to the subtle Sn-doping, T c is strongly suppressed from T c = 2.3 K of undoped CeCoIn 5 to T c ∼ 0.7 K for the reported concentration ( x ∼ 0.12 ). As for pure CeCoIn 5 , superconductivity (SC) seems to evolve out of a non-Fermi liquid (NFL) normal state just above T c . A temperature–pressure phase diagram is constructed from our results and compared with the properties of pure CeCoIn 5 under pressure. Effects of chemical pressure and/or hybridization tuning associated with Sn-doping are discussed.

The system CeCoGe3−xSix is of special interest since there is evidence that a quantum critical point of the antiferromagnetic phase can be approached by alloying as well as by application of high pressure. With new results of AC-susceptibility and specific heat measurements on CeCoGe2.1Si0.9 under high pressure presented here, we show by comparison with former data that the tuning parameters composition and pressure, respectively, are equivalent. Alloy and pressure data fit in a common phase diagram. The quantitative behavior of c/T under pressure, too, corresponds to that under changes of composition.

We deposited amorphous Bi films with a thickness between 3 and 6.5 nm at 4.2 K on top of previously deposited Co clusters having a mean size of ∼4.5 nm. The Co cluster layers thickness was between 2.3 and 5 nm. In-situ electrical transport measurements were performed between 2 and 100 K. Measurements on as-prepared samples having a Bi layer thickness of 3.0 nm show hopping (tunneling) conductivity as σ(T) = σ 0 exp[−(T 0/T)1/2] above the superconducting transition temperature T C and re-entrance behavior again with hopping (tunneling) conductivity below T C . Annealing of films having a Bi layer thickness of 5.5 nm results in a decrease of resistivity, with variable-range hopping conduction behavior as σ(T) = σ 0 exp[−(T 0/T)1/3 ]. Quite different are the findings for films having a Bi layer thickness of 6.5 nm: annealing of these films results in a power-law behavior as σ(T) = σ 0 T α with α = 2/3, indicating that these films are close to a quantum critical point separating superconducting and insulating phases. A phase diagram including all experimental observations is proposed.

The effect of hydrostatic pressure and alloying on the magnetic ordering temperature in the series was studied. The observed behaviour was related to changes in the density of states at the Fermi level caused by pressure or Ni substitution. In addition, the elastic constants of , which is a representative member of the family, were measured.

The effect of hydrostatic pressure on the resistivity of Ce(Ru 1− x Fe x ) 2 Ge 2 was studied. Chemical pressure (alloying) and hydrostatic pressure produce similar changes in the physical properties of the series driving it from the ferromagnetic (CeRu 2 Ge 2 ) to the heavy fermion (CeFe 2 Ge 2 ) ground state with (decreasing) unit cell volume as a unique parameter; f-p(CeGe) hybridization is, probably, also giving some contribution to the changes.

Alloying influences on the superconducting and magnetic properties of the (Gd x Y 1-x Ni 2 B 2 C series are studied. For x 0.25 and the coexistence of superconductivity with long-range antiferromagnetic order for 0.1 < x < 0.25 are discussed.

Magnetic properties of Y 1− x Gd x Co 2 B 2 C series of compounds were studied. Two magnetically ordered phases were observed for 0.7 ≤ x ≤ 1. Change of the magnetic ordering temperatures T 1 and T 2 with the Gd concentration is argued to be related to the dilution of magnetic Gd by nonmagnetic Y and to the reduction of the distance between ( Y 1− x Gd x ) C planes. However, significant contribution to the changes in T 1 between pure GdNi 2 B 2 C and GdCo 2 B 2 C samples is suggested to be due to the different 3 d band filling in these compounds.

We have performed measurements of electrical resistivity, AC susceptibility, magnetization, Mossbauer effect and NMR on the series of intermetallic compounds Ce(Ru 1− x M x ) 2 Ge 2 , M  Fe, Au for 0 ⩽ x ⩽ 0.1. The parent compound CeRu 2 Ge 2 orders ferromagnetically below 7.5 K, with f-electrons showing only a small mass-enhancement at low temperatures. However, its crystal structure and Fermi surface are closely related to the heavy-electron superconductor CeRu 2 Si 2 . Analysis of X-ray diffraction patterns confirmed the structure to be of thCr 2 Si 2 -type, with no foreign phases detected. The results of our Mossbauer studies indicate that Fe goes into the Ru sites with no magnetic moment. NMR signals were observed in the Fe-doped samples with quadrupolar interaction that can be related to 73 Ge nucleus ( I = 9/2).

Structural and superconducting properties of Y(Ni1−xMx)2B2C compounds with MCo, Fe and Ru have been studied. A fast decrease of Tc with the concentration of the dopants was found. This effect can be attributed to the shift of the Fermi level induced by the dopant concentration with the additional contributions from the chemical-pressure effects and from the decrease of the electronic mean free path.

The influence of external pressure on the electrical transport and magnetic properties of EuCu(2)As(2), crystallizing in a ThCr(2)Si(2)-type structure, is reported. The system is known to be an antiferromagnet below T(N) ≈ 15 K in the absence of external magnetic fields. We find that there is a gradual reduction of T(N) with the application of a magnetic field with an extrapolated value of the critical field of around 18 kOe which can drive T(N) to zero. Electrical resistivity under pressure (11 GPa) reveals that the magnetic ordering temperature is pushed up dramatically to higher temperatures which is quite interesting if compared with the behavior in isostructural FeAs-based systems containing Eu. Above 7 GPa, the pressure-induced state appears to be ferromagnetic. The results thus reveal interesting changes in the magnetic ordering behavior of this compound with increasing pressure and magnetic fields.

We have investigated the magnetic and magnetoresistance (MR) properties of [Py/Cu 9 A]20, by addition of thin Co and Fe interface layers, and by varying the thicknesses of Py from 10 to 80 A. The temperature dependence of MR and resistivity from 4.2 to 300 K has been analyzed using a semiclassical theoretical model. The increase of giant magnetoresistance amplitudes for [Co 3 A/Py/Co 3 A/Cu 9 A]20 has been interpreted in terms of a reinforcement of the magnetic ordering of the Py/Co/Cu interfaces. It also has been attributed to a more selective transmission of the spin-down through the interfaces of Py/Co and Co/Cu, compared to the one through the Py/Cu interface. Nevertheless, with the addition of 3 A of Fe at the interfaces Py/Cu, one observes a strong reduction of the magnetoresistance amplitudes.

W. T. Herrera,1,* Y. T. Xing,1 S. M. Ramos,1 P. Munayco,1 M. B. Fontes,1 E. M. Baggio-Saitovitch,1 and F. J. Litterst2 1Centro Brasileiro de Pesquisas Fisicas, Rua Dr. Xavier Sigaud 150, 22290-180, Rio de Janeiro, Brazil 2Institute for Physics of Condensed Matter, Technische Universitat Braunschweig, Mendelssohnstrasse 3, D-38106 Braunschweig, Germany (Received 30 June 2009; revised manuscript received 16 May 2011; published 28 July 2011)

We report evidence of two superconducting phases in the ${\text{Ce}}_{2}{\text{Rh}}_{1\ensuremath{-}x}{\text{Ir}}_{x}{\text{In}}_{8}$ heavy fermion systems. One of these phases is pressure induced and occurs for a range of compositions situated near the Rh-rich extreme, consistent with the behavior observed for the pure ${\text{Ce}}_{2}{\text{RhIn}}_{8}$ compound. This superconducting region is expanded to higher critical temperatures and lower Rh concentrations with increasing pressure. It has a clear interplay with the established antiferromagnetic state, ${T}_{N}\ensuremath{\sim}2.8\text{ }\text{K}$, suggesting that magnetic fluctuations are important for its realization. The second superconducting phase appears already at ambient pressure and it is characterized by a dome centered around $x\ensuremath{\sim}0.6$ which, in sharp contrast with the first transition, is progressively eliminated by the application of pressure. These strikingly opposite behaviors under the same tuning parameter indicate the two transitions may have different natures. We compare these findings in the ${\text{Ce}}_{2}{\text{Rh}}_{1\ensuremath{-}x}{\text{Ir}}_{x}{\text{In}}_{8}$ alloys to its related ${\text{CeRh}}_{1\ensuremath{-}x}{\text{Ir}}_{x}{\text{In}}_{5}$, arguing that the occurrence of the superconducting phases become unfavorable for the bilayers alloys due to higher dimensionality and stronger disorder. Further, we discuss whether the present results warrant similar claims with respect to the ${\text{CeRh}}_{1\ensuremath{-}x}{\text{Ir}}_{x}{\text{In}}_{5}$ phase diagram and the possible nature of the superconducting phases.

We report a combined pressure-doping study in the nearly two-dimensional heavy fermion superconductor CeCoIn5 as its superconducting phase is driven to the normal state by Sn doping and/or applied pressure. Temperature-pressure-dependent electrical resistivity measurements were performed at the vicinity of a superconducting quantum critical point where Tc→0. A universal plot of the concentration- and pressure-dependent phase diagram suggests that for the concentrations studied a single mechanism is responsible for reducing Tc and bringing the system to the superconducting quantum critical point. A two-band model with hybridization controlled by pressure and doping provides a consistent description of the phase diagram and the suppression of the d-wave superconductivity in this material.

The intermetallic compound CeCo(Ge1−xSix)3 for x=0, is an antiferromagnetic Kondo lattice system with a double magnetic transition at Tc=21.2 K and TN=19 K. Upon doping with Si, these transitions merge into a single antiferromagnetic transition, as for the x=0.75 composition studied here (TN=5.5 K). We have applied hydrostatic pressure to this material and observed the vanishing of the antiferromagnetic transition as pressure increases, using electrical resistance and AC susceptibility measurements. For T⪡TN, we show that the resistance is dominated by electron–magnon scattering. The magnon spectrum is characteristic of anisotropic antiferromagnets. The gap and stiffness of the spin waves, obtained from the fits of the resistance curves, vary with pressure. We discuss the behavior of these quantities close to the quantum critical point.

Pressure- and temperature-dependent heat capacity and electrical resistivity experiments on Sn- and La-doped CeRhIn5 are reported for two samples with specific concentrations, Ce(0.90)La(0.10)RhIn5 and CeRhIn(4.84)Sn(0.16), which present the same TN=2.8 K. The obtained P-T phase diagrams for doped CeRhIn5 compared to that for the pure compound show that Sn doping shifts the diagram to lower pressures while La doping does exactly the opposite, indicating that the important energy scale to define the pressure range for superconductivity in CeRhIn5 is the strength of the on-site Kondo coupling.

We present the results of magnetoresistance ρ(H, T) measurements for CeFeGe3 and CeCoGe3 as a function of temperature in fixed applied fields and as a function of field at fixed temperatures. For CeCoGe3 the H–T phase diagram was constructed from ρ(H, T) results. Temperature dependent magnetoresistance of CeCoGe3 in applied field was fitted taking into account electron–magnon scattering in different regions of the H–T diagram.