R. Urbano - Academia.edu (original) (raw)

Papers by R. Urbano

Journal of Physics: Condensed Matter, 2015

β-YbAlB 4 has become one of the most studied heavy fermion systems since its discovery due to its... more β-YbAlB 4 has become one of the most studied heavy fermion systems since its discovery due to its remarkable physical properties. This system is the first reported Yb-based heavy-fermion superconductor (HFS) for which the low-T superconducting state emerges from a non-fermi-liquid (NFL) normal state associated with quantum criticality Nakatsuji et al 2008 Nature 4 603. Additionally, it presents a striking and unprecedented electron spin resonance (ESR) signal which behaves as a conduction electron spin resonance (CESR) at high temperatures and acquires features of the Yb 3+ local moment ESR at low temperatures. The latter, also named Kondo quasiparticles spin resonance (KQSR), has been defined as a 4f-ce strongly coupled ESR mode that behaves as a local probe of the Kondo quasiparticles in a quantum critical regime, Holanda et al 2011 Phys. Rev. Lett. 107 026402. Interestingly, β-YbAlB 4 possesses a previously known structural variant, namely the α-YbAlB 4 , phase which is a paramagnetic Fermi liquid (FL) at low temperatures Macaluso et al 2007 Chem. Mater. 19 1918. However, it has been recently suggested that the α-YbAlB 4 phase may be tuned to NFL behavior and/or magnetic ordering as the compound is doped with Fe. Here we report ESR studies on the α-Yb 1−x Fe x AlB 4 (0 x 0.50) series as well as on the reference compound α-LuAlB 4. For all measured samples, the observed ESR signal behaves as a CESR in the entire temperature range (10 K T 300 K) in clear contrast with what has been observed for β-YbAlB 4. This striking result indicates that the proximity to a quantum critical point is crucial to the occurrence of a KQSR signal.

Journal of Physics: Condensed Matter, Mar 31, 2016

Electron spin resonance (ESR) of diluted Nd 3+ ions in the topologically nontrivial semimetallic ... more Electron spin resonance (ESR) of diluted Nd 3+ ions in the topologically nontrivial semimetallic (TNSM) YBiPt compound is reported. The cubic YBiPt compound is a noncentrosymmetric half Heusler material which crystallizes in the F43m space group. The low temperature Nd 3+ ESR spectra showed a g-value of 2.66(4) corresponding to a Γ 6 cubic crystal field Kramers' doublet ground state. Remarkably, the observed metallic and diffusive (Dysonian) Nd 3+ lineshape presented an unusual dependence with grain size, microwave power, Nd 3+ concentration and temperature. Moreover, the spin dynamic of the localized Nd 3+ ions in YBiPt was found to be characteristic of a phonon-bottleneck regime. It is claimed that, in this regime for YBiPt, phonons are responsible for mediating the diffusion of the microwave energy absorbed at resonance by the Nd 3+ ions to the thermal bath throughout the skin depth (δ 15 μm). We argue that this is only possible because of the existence of highly mobile conduction electrons inside the skin depth of YBiPt that are strongly coupled to the phonons by spin-orbit coupling. Therefore, our unexpected ESR results point to a coexistence of metallic and insulating behaviors within the skin depth of YBiPt. This scenario is discussed in the light of the TNSM properties of this compound.

Journal of physics. Condensed matter : an Institute of Physics journal, Jan 3, 2018

We report in-plane electrical resistivity studies of CeCuBi2 and LaCuBi&lt... more We report in-plane electrical resistivity studies of CeCuBi2 and LaCuBi2 single crystals under applied pressure. At ambient pressure, CeCuBi2 is a c-axis Ising antiferromagnet with a transition temperature TN ≈ 16 K. In a magnetic field applied along the c-axis at T ≈ 2 K a spin-flop transition takes place Hflip ≈ 5.7 T. Applying pressure on CeCuBi2 suppresses TN at a slow rate. TN(P) extrapolates to zero temperature at Pc ≈ 7 GPa. The critical field of the spin-flop transition Hflip (P) displays a maximum of 6.8 T at P ≈ 4 GPa. At low temperatures, a zero-resistance sup...

Journal of Physics: Conference Series, 2015

In this work we report the effects of hydrostatic pressure and magnetic field on the electronic t... more In this work we report the effects of hydrostatic pressure and magnetic field on the electronic transport properties of the antiferromagnetic compound Eu3Ir4Sn13 (TN ∼ 10 K). Single crystals of Eu3Ir4Sn13 were synthesized using the Sn self-flux technique. DC electrical resistivity measurements as a function of temperature were performed by means of the fourprobe technique. The high-temperature anomaly at T * ∼ 57 K attributed to a structural distortion of the Sn1Sn212 cages in Eu3Ir4Sn13 is rapidly decreased to lower temperatures at a rate dT * /dP = 2 K/kbar, while the antiferromagnetic transition due to the Eu 2+ ions is only weakly affected. Our data do not indicate any magnetoelastic effect associated with the structural instability at T *. Furthermore, the suppression of the lattice distortion by application of external pressure is not accompanied by the emergence of superconductivity, possibly due to strong magnetic correlations between the Eu 2+ localized magnetic moments.

Journal of Physics: Condensed Matter, 2015

We report a combined study of external pressure and Cu-substitution on BaFe 2 As 2 single crystal... more We report a combined study of external pressure and Cu-substitution on BaFe 2 As 2 single crystals grown by the influx technique. At ambient pressure, the Cu-substitution is known to suppress the spin density wave (SDW) phase in pure BaFe 2 As 2 (T SDW ≈ 140 K) and to induce a superconducting (SC) dome with a maximum transition temperature T max c 4.2 K. This T max c is much lower than the T c ∼ 15-28 K achieved in the case of Ru, Ni and Co substitutions. Such a lower T c is attributed to a Cu 2+ magnetic pair-breaking effect. The latter is strongly suppressed by applied pressure, as shown herein, T c can be significantly enhanced by applying high pressures. In this work, we investigated the pressure effects on Cu 2+ magnetic pair-breaking in the BaFe 2−x Cu x As 2 series. Around the optimal concentration (x opd = 0.11), all samples showed a substantial increase of T c as a function of pressure. Yet for those samples with a slightly higher doping level (over-doped regime), T c presented a dome-like shape with maximum T c 8 K. Remarkably interesting, the under-doped samples, e.g. x = 0.02 display a maximum pressure induced T c 30 K which is comparable to the maximum T c 's found for the pure compound under external pressures. Furthermore, the magnetoresistance effect as a function of pressure in the normal state of the x = 0.02 sample also presented an evolution consistent with the screening of the Cu 2+ local moments. These findings demonstrate that the Cu 2+ magnetic pair-breaking effect is completely suppressed by applying pressure in the low concentration regime of Cu 2+ substituted BaFe 2 As 2 .

The long-term maintenance of specialized mutualisms remains an evolutionary puzzle. Recent focus ... more The long-term maintenance of specialized mutualisms remains an evolutionary puzzle. Recent focus has been on factors governing the stability of these mutualisms, including sanctions by the host, partner choice, and coevolutionary constraint, that is, the genetic correlation (r G) between fitness of both partners. So far these studies have been typically carried out in a single environment. Here, we ask if the genetic correlation between fitness of the host plant Medicago truncatula (Fabaceae) and its bacterial symbiont Sinorhizobium meliloti is affected by the presence/absence of a monoterpene (carvacrol) leached into the soil by Thymus vulgaris-a common plant of the Mediterranean vegetation, often co-occuring with Medicago. We show that the presence of carvacrol in the soil dramatically affects fitness of the rhizobial partner and increases the magnitude of r G between plant and rhizobia fitness (r G = 0.02 ± 0.05 vs. r G = 0.57 ± 0.02). This finding emphasizes the importance of heterogeneity in the biotic environment for understanding the evolution of species interactions.

Physical Review Letters, 2009

We report Knight shift experiments on the superconducting heavy electron material CeCoIn5 that al... more We report Knight shift experiments on the superconducting heavy electron material CeCoIn5 that allow one to track with some precision the behavior of the heavy electron Kondo liquid in the superconducting state with results in agreement with BCS theory. An analysis of the 115 In nuclear quadrupole resonance (NQR) spin-lattice relaxation rate T −1 1 measurements under pressure reveals the presence of 2d magnetic quantum critical fluctuations in the heavy electron component that are a promising candidate for the pairing mechanism in this material. Our results are consistent with an antiferromagnetic quantum critical point (QCP) located at slightly negative pressure in CeCoIn5 and provide additional evidence for significant similarities between the heavy electron materials and the high Tc cuprates.

Physical Review Letters, 2002

We report the first observation of the field distribution and flux-line lattice (FLL) depinning i... more We report the first observation of the field distribution and flux-line lattice (FLL) depinning in the vortex-state (VS) of a type-II superconductor probed by conduction electron spin resonance (CESR). CESR was performed in MgB 2 (T c ' 39 K) at 4:1 GHz (1455 Oe) and 9:5 GHz (3390 Oe). The field distribution, nH, and a standard deviation of 14 Oe (at 28 K=4:1 GHz and at 7 K=9:5 GHz) were inferred, respectively, from the distortion and broadening of the CESR in the VS. For both frequencies, the FLL depinning temperature was determined.

Physical Review B, 2004

We outline a theory for conduction electron-spin resonance (CESR) in highly oriented pyrolytic gr... more We outline a theory for conduction electron-spin resonance (CESR) in highly oriented pyrolytic graphite. The fundamental approximation is to treat the spin-orbit interaction as an effective field. In this approach, the shift in the g factor, which is associated with the mean value of the field, is related to the orbital susceptibility of the electrons. The linewidth comes from fluctuations in the effective field caused by the scattering of the electrons. The theory is used to interpret our CESR measurements.

$Research paper thumbnail of Metamagnetic transition in single-crystal Bi_{4}Cu_{3}V_{2}O_{14}$

Physical Review B, 2010

We report on high magnetic field magnetization, magnetic torque, specific-heat, and V51 NMR measu... more We report on high magnetic field magnetization, magnetic torque, specific-heat, and V51 NMR measurements on Bi4Cu3V2O14 single crystals, which display a characteristic chain structure along the crystallographic a axis. The system undergoes a magnetic phase transition below 5.5 K. For the magnetic field H||a , the data reveal a metamagnetic transition from an antiferromagnetic (AFM) to a weak-ferromagnetic (WFM) state

Physical Review B, 2006

We report experiments of electron spin resonance ͑ESR͒ of Cu 2+ in polycrystalline samples of CaC... more We report experiments of electron spin resonance ͑ESR͒ of Cu 2+ in polycrystalline samples of CaCu 3 Ti 4 O 12 post-annealed in different atmospheres. After being synthesized by solid state reaction, pellets of CaCu 3 Ti 4 O 12 were annealed for 24 h at 1000°C under air, Ar or O 2. Our temperature dependent ESR data revealed for all samples nearly temperature independent g value ͑2.15͑1͒͒ and linewidth for T ӷ T N Ϸ 25 K. However, the values of ESR linewidth are strongly affected by the oxygen content in the sample. For instance, argon post-annealed samples show a much larger linewidth than the O 2 or air post-annealed samples. We attribute this broadening to an increase of the dipolar homogeneous broadening of the Cu 2+ ESR lines due to the presence of oxygen vacancies which induce an S =1/2 spin inside the TiO 6 octahedra. Correlation between a systematic dependence of the ESR linewidth on the oxygen content and the high dielectric constant of these materials is addressed. Also, ESR, magnetic susceptibility, and specific heat data for a single crystal of CaCu 3 Ti 4 O 12 and for polycrystals of CdCu 3 Ti 4 O 12 are reported.

Physical Review B, 2006

The angular, magnetic field, and temperature dependence of the Eu 2+ ͑4f 7 , S =7/2͒ g-value in C... more The angular, magnetic field, and temperature dependence of the Eu 2+ ͑4f 7 , S =7/2͒ g-value in Ca 1−x Eu x B 6 ͑0.15Շ x ഛ 1.00͒ is measured by means of electron spin resonance at two microwave frequencies, 9.4 and 34.4 GHz. The g-value is found to be anisotropic and magnetic-field-dependent. The reduction with field of the positive g-shift is interpreted in terms of the exchange interaction between the localized Eu 2+ 4f 7 electrons with the conduction Eu 2+ 5d-like electrons and B 2p-like holes. The angular, temperature, and x dependence of the anisotropy of the g-value can be attributed to demagnetization effects due to the platelet-like shape of the samples. High-resolution synchrotron x-ray diffraction experiments show the absence of distortions larger than ␦a / a ϳ 10 −4. However, a mechanism for the anisotropy based on a weak tetragonal lattice distortion arising from the crystal surface cannot be excluded.

Physical Review B, 2004

The spin-flip scattering between conduction and 4f 7 Eu 2+ (8 S 7/2) electrons in the paramagneti... more The spin-flip scattering between conduction and 4f 7 Eu 2+ (8 S 7/2) electrons in the paramagnetic phase of EuB 6 (T > 2T c ≈ 30 K) is studied by means of electron spin resonance (ESR) at three frequencies. The single Dysonian resonance observed in all cases suggests a metallic environment for the Eu 2+ ions. The ESR at high field, H ≈ 12.05 kG (ν ≈ 33.9 GHz), has isotropic gvalue and an anisotropic linewidth with cubic symmetry. The low-field, 1.46 kG (4.1 GHz) and 3.35 kG (9.5 GHz), ESR linewidths are unexpectedly broader and have a smaller anisotropy than at the higher field. The unconventional narrowing and anisotropy of the linewidth at higher fields are indicative of a homogeneous resonance and microscopic evidence for a strong reduction in spin-flip scattering between the spins of Eu 2+ and the states in the electron and hole pockets at the Xpoints of the Brillouin zone by magnetic polarons.

Physical Review B, 2005

The compound La 0.66 Ba 0.40 Mn 0.61 Fe 0.33 O 3 shows anisotropic magnetic correlations with no ... more The compound La 0.66 Ba 0.40 Mn 0.61 Fe 0.33 O 3 shows anisotropic magnetic correlations with no long-range order. Specific heat measurements suggest these correlations represent the bulk. Orbital correlations of Mn 3+ e g electrons, surviving in an environment of largely disordered exchange interactions, are invoked to account for this magnetic state. These results argue in favor of a strain-field mechanism for orbital ordering in manganites.

Physical Review B, 2007

Electron spin resonance of Eu 2+ ͑4f 7 , S =7/2͒ in a La hexaboride ͑LaB 6 ͒ single crystal shows... more Electron spin resonance of Eu 2+ ͑4f 7 , S =7/2͒ in a La hexaboride ͑LaB 6 ͒ single crystal shows a single anisotropic Dysonian resonance. From the observed negative g shift of the resonance, it is inferred that the Eu 2+ ions are covalent exchange coupled to the B 2p-like host conduction electrons. From the anisotropy of the spectra ͑linewidth and field for resonance͒, we found that the S ground state of Eu 2+ ions experience a cubic crystal field of a negative fourth order crystal field parameter ͑CFP͒, b 4 = −11.5͑2.0͒ Oe, in agreement with the negative fourth order CFP, A 4 , found for the non-S ground state R hexaborides. These results support covalency as the dominant contribution to the fourth order CFP for the whole R hexaboride family.

Physical Review B, 2007

Electron spin resonance ͑ESR͒ results of Gd 3+ in YbAl 3 and LuAl 3 are analyzed using a multiban... more Electron spin resonance ͑ESR͒ results of Gd 3+ in YbAl 3 and LuAl 3 are analyzed using a multiband ͑f-, d-, and p-type͒ model of correlated conduction electrons. The need for a multiband analysis of our results is based on the following observations: ͑i͒ the Korringa rates b Ӎ 14 Oe/ K and b Ӎ 9 Oe/ K for Gd 3+ for YbAl 3 and LuAl 3 , respectively, are larger than those expected from the respective measured g shifts ͑Knight shift͒, and ͑ii͒ negative and positive g shifts ⌬g Ӎ −0.004 and ⌬g Ӎ + 0.003 were observed for Gd 3+ in YbAl 3 and LuAl 3 , respectively. Specific heat and magnetic susceptibility measurements, in the samples studied by ESR, show that electron-electron correlations are present in both compounds.

Physica B: Condensed Matter, 2007

The electron spin resonance (ESR) spectra of Eu 2þ ð4f 7 ; S ¼ 7 2 Þ in LaB 6 single crystal show... more The electron spin resonance (ESR) spectra of Eu 2þ ð4f 7 ; S ¼ 7 2 Þ in LaB 6 single crystal show a single Dysonian resonance for the localized Eu 2þ magnetic moments. It is shown that the Eu 2þ ions are covalent exchange coupled to the (B) 2p-like host conduction electrons.

Journal of Applied Physics, 2014

We report high field 75 As nuclear magnetic resonance (NMR) measurements on Co and Cu substituted... more We report high field 75 As nuclear magnetic resonance (NMR) measurements on Co and Cu substituted BaFe 2 As 2 single crystals displaying same structural/magnetic transition T 0 ' 128 K. From our anisotropy studies in the paramagnetic state, we strikingly found virtually identical quadrupolar splitting and consequently the quadrupole frequency Q ' 2:57ð1Þ MHz for both compounds, despite the claim that each Cu delivers 2 extra 3d electrons in BaFe 2 As 2 compared to Co substitution. These results allow us to conclude that a subtle change in the crystallographic structure, particularly in the Fe-As tetrahedra, must be the most probable tuning parameter to determine T 0 in this class of superconductors rather than electronic doping. Furthermore, our NMR data around T 0 suggest coexistence of tetragonal/paramagnetic and orthorhombic/antiferromagnetic phases between the structural and the spin density wave magnetic phase transitions, similarly to what was reported for K-doped BaFe 2 As 2 [Urbano et al.,

Journal of Applied Physics, 2008

Articles you may be interested in Crystal structure and physical properties of new transition met... more Articles you may be interested in Crystal structure and physical properties of new transition metal based pnictide compounds: LaTM 2AsN (TM = Fe, Co, and Ni)

IEEE Transactions on Magnetics, 2006

Amorphous and crystalline thin films of Mn-doped (0.5%-10%) GaAs and crystalline thin films of Zn... more Amorphous and crystalline thin films of Mn-doped (0.5%-10%) GaAs and crystalline thin films of Zn 1 Co O(= 3%-20%) were investigated by means of magnetic susceptibility and electron spin resonance (ESR). For the Mn-doped GaAs samples, our results show the absence of ferromagnetic ordering for the amorphous films in the 300 2 K temperature range, in contrast to the ferromagnetism found in crystalline films for 110 K. A single ESR line with a temperature independent-value (2) is observed for the amorphous films, and the behavior of this ESR linewidth depends on the level of crystallinity of the film. For the Mn-doped GaAs crystalline films, only a ferromagnetic mode is observed for when the film is ferromagnetic. Turning now the Zn 1 Co O films, ferromagnetic loops were observed at room temperature for these films. The magnetization data show an increasing of the saturation magnetization as a function of reaching a maximum value for 10%. ESR experiments at = 300 K in the same films show a strong anisotropic ferromagnetic mode (FMR) for = 0 10.

Journal of Physics: Condensed Matter, 2015

Journal of Physics: Condensed Matter, Mar 31, 2016

Journal of physics. Condensed matter : an Institute of Physics journal, Jan 3, 2018

Journal of Physics: Conference Series, 2015

Journal of Physics: Condensed Matter, 2015

Physical Review Letters, 2009

Physical Review Letters, 2002

Physical Review B, 2004

$Research paper thumbnail of Metamagnetic transition in single-crystal Bi_{4}Cu_{3}V_{2}O_{14}$

Physical Review B, 2010

Physical Review B, 2006

Physical Review B, 2004

Physical Review B, 2005

Physical Review B, 2007

Physica B: Condensed Matter, 2007

Journal of Applied Physics, 2014

Journal of Applied Physics, 2008

IEEE Transactions on Magnetics, 2006