arXiv subjects

G. J. Nieuwenhuys

Publications and source records attributed to G. J. Nieuwenhuys.

At least 19 recordsLinked to original sources

arXiv2014

Depth dependence of the ionization energy of shallow hydrogen states in ZnO and CdS

The characteristics of shallow hydrogen-like muonium (Mu) states in nominally undoped ZnO and CdS (0001) crystals have been studied close to the surface at depths in the range of 10 nm - 180 nm by using low-energy muons, and in the bulk using conventional muSR. The muon implantation depths are adjusted by tuning the energy of the low-energy muons between 2.5 keV and 30 keV. We find that the bulk ionization energy of the shallow donor-like Mu state is lowered by about 10 meV at a depth of 100 nm, and continuously decreasing on approaching the surface. At a depth of about 10 nm the ionization energy is further reduced by 25-30 meV compared to its bulk value. We attribute this change to the presence of electric fields due to band bending close to the surface, and we determine the depth profile of the electric field within a simple one-dimensional model.

cond-mat.mes-hall↗

arXiv2014

Magnetism of PdNi alloys near the critical concentration for ferromagnetism

We report results of a muon spin rotation and relaxation ($μ$SR) study of dilute Pd$_{1-x}$Ni$_x$ alloys, with emphasis on Ni concentrations $x =$ 0.0243 and 0.025. These are close to the critical value $x_\mathrm{cr}$ for the onset of ferromagnetic long-range order (LRO), which is a candidate for a quantum critical point. The 2.43 and 2.5 at.% Ni alloys exhibit similar $μ$SR properties. Both samples are fully magnetic, with average muon local fields $\langle B^\mathrm{loc}\rangle =$ 2.0 and 3.8 mT and Curie temperatures $T_C =$ 1.0 and 2.03 K for 2.43 and 2.5 at.% Ni, respectively, at $T = 0$. The temperature dependence of $\langle B^\mathrm{loc}\rangle$ suggests ordering of Ni spin clusters rather than isolated spins. Just above $T_C$ a two-phase region is found with separate volume fractions of quasistatic short-range order (SRO) and paramagnetism. The SRO fraction decreases to zero with increasing temperature a few kelvin above $T_C$. This mixture of SRO and paramagnetism is consistent with the notion of an inhomogeneous alloy with Ni clustering. The measured values of $T_C$ extrapolate to $x_\mathrm{cr}$ = 0.0236 $\pm$ 0.0027. The dynamic muon spin relaxation in the vicinity of $T_C$ differs for the two samples: a relaxation-rate maximum at $T_C$ is observed for $x$ = 0.0243, reminiscent of critical slowing down, whereas for $x =$ 0.025 no dynamic relaxation is observed within the $μ$SR time window. The data suggest a mean-field-like transition in this alloy.

cond-mat.str-el↗

arXiv2013

Photo-induced persistent inversion of germanium in a 200-nm-deep surface region

The controlled manipulation of the charge carrier concentration in nanometer thin layers is the basis of current semiconductor technology and of fundamental importance for device applications. Here we show that it is possible to induce a persistent inversion from n- to p-type in a 200-nm-thick surface layer of a germanium wafer by illumination with white and blue light. We induce the inversion with a half-life of ~12 hours at a temperature of 220 K which disappears above 280 K. The photo-induced inversion is absent for a sample with a 20-nm-thick gold capping layer providing a Schottky barrier at the interface. This indicates that charge accumulation at the surface is essential to explain the observed inversion. The contactless change of carrier concentration is potentially interesting for device applications in opto-electronics where the gate electrode and gate oxide could be replaced by the semiconductor surface.

cond-mat.mtrl-sci↗

arXiv2006

Charge Localization due to RKKY Interaction in the Spin Glass AuFe

Measurements of electrodynamic response of of spin glass AuFe films in comparison with pure gold films are performed at frequencies from 0.3 THz (10 cm-1) up to 1000 THz (33000 cm-1) using different spectroscopic methods. At room temperatures the spectra of pure gold and of AuFe are typically metallic with the scattering rate of carriers in AuFe being significantly enlarged due to scattering on localized magnetic moments of Fe ions. In the spin-glass phase of AuFe at T = 5 K a pseudogap in the conductivity spectrum is detected with the magnitude close to the Ruderman-Kittel-Kasuya-Yosida (RKKY) energy for AuFe: Delta(RKKY) = 2.2 meV. The origin of the pseudogap is associated with partial localization of electrons which mediate the RKKY interaction between localized magnetic Fe centers.

cond-mat.str-el↗

arXiv2006

Muon Spin Relaxation Studies of Superconductivity in a Crystalline Array of Weakly Coupled Metal Nanoparticles

We report Muon Spin Relaxation studies in weak transverse fields of the superconductivity in the metal cluster compound, Ga$\_{84}$[N(SiMe$\_{3}$)$\_{2}$]$\_{20}$-Li$\_{6}$Br$\_{2}$(thf)$\_{20}\cdot $2toluene. The temperature and field dependence of the muon spin relaxation rate and Knight shift clearly evidence type II bulk superconductivity below $T\_{\text{c}}\approx7.8$ K, with $B\_{\text{c1}}\approx 0.06$ T, $B\_{\text{c2}}\approx 0.26 $ T, $κ\sim 2$ and weak flux pinning. The data are well described by the s-wave BCS model with weak electron-phonon coupling in the clean limit. A qualitative explanation for the conduction mechanism in this novel type of narrow band superconductor is presented.

cond-mat.str-el↗

arXiv2005

Evidence for electronic phase separation between orbital orderings in SmVO3

We report evidence for phase coexistence of orbital orderings of different symmetry in SmVO$_3$ by high resolution X-Ray powder diffraction. The phase coexistence is triggered by an antiferromagnetic ordering of the vanadium spins near 130K, below an initial orbital ordering near 200K. The phase coexistence is the result of the intermediate ionic size of samarium coupled to exchange striction at the vanadium spin ordering.

cond-mat.str-el↗

arXiv2005

Susceptibility inhomogeneity and non-Fermi liquid behavior in UCu_{5-x}Pt_x

Transverse-field muSR shifts and relaxation rates have been measured in the non-Fermi liquid (NFL) alloy system UCu_{5-x}Pt_x, x = 1.0, 1.5, and 2.5. At low temperatures the fractional spread in Knight shifts delta K/K approx deltachi/chi is gtrsim 2 for x = 1, but is only half this value for x = 1.5 and 2.5. In a disorder-driven scenario where the NFL behavior is due to a broadly distributed (Kondo or Griffiths-phase cluster) characteristic energy E, our results indicate that delta E/E_{rm av} approx (delta K/K)_{T=0} is similar for UCu_{5-x}Pd_x (x = 1 and 1.5) and UCu_4Pt, but is reduced for UCu_{5-x}Pt_x, x = 1.5 and 2.5. This reduction is due to a marked increase of E with increasing x; the spread delta E is found to be roughly independent of x. Our results correlate with the observed suppression of other NFL anomalies for x > 1 in UCu_{5-x}Pt_x but not in UCu_{5-x}Pd_x, and are further evidence for the importance of disorder in the NFL behavior of both these alloy systems.

cond-mat.str-el↗

arXiv2005

Effect of annealing on glassy dynamics and non-Fermi liquid behavior in UCu_4Pd

Longitudinal-field muon spin relaxation (LF-muSR) experiments have been performed in unannealed and annealed samples of the heavy-fermion compound UCu_4Pd to study the effect of disorder on non-Fermi liquid behavior in this material. The muon spin relaxation functions G(t,H) obey the time-field scaling relation G(t,H) = G(t/H^gamma) previously observed in this compound. The observed scaling exponent gamma = 0.3 pm 0.1, independent of annealing. Fits of the stretched-exponential relaxation function G(t) = exp[-(Lambda t)^K] to the data yielded stretching exponentials K < 1 for all samples. Annealed samples exhibited a reduction of the relaxation rate at low temperatures, indicating that annealing shifts fluctuation noise power to higher frequencies. There was no tendency of the inhomogeneous spread in rates to decrease with annealing, which modifies but does not eliminate the glassy spin dynamics reported previously in this compound. The correlation with residual resistivity previously observed for a number of NFL heavy-electron materials is also found in the present work.

cond-mat.str-el↗

arXiv2004

Disorder, inhomogeneity and spin dynamics in f-electron non-Fermi liquid systems

Muon spin rotation and relaxation ($μ$SR) experiments have yielded evidence that structural disorder is an important factor in many f-electron-based non-Fermi-liquid (NFL) systems. Disorder-driven mechanisms for NFL behaviour are suggested by the observed broad and strongly temperature-dependent $μ$SR (and NMR) linewidths in several NFL compounds and alloys. Local disorder-driven theories (Kondo disorder, Griffiths-McCoy singularity) are, however, not capable of describing the time-field scaling seen in muon spin relaxation experiments, which suggest cooperative and critical spin fluctuations rather than a distribution of local fluctuation rates. A strong empirical correlation is established between electronic disorder and slow spin fluctuations in NFL materials

cond-mat.str-el↗

arXiv2003

Magnetism and superconductivity in CeRh_{1-x}Ir_xIn_5 heavy fermion materials

We report on zero-field muon spin relaxation studies of cerium based heavy-fermion materials CeRh_{1-x}Ir_xIn_5. In the superconducting x=0.75 and 1 compositions muon spin relaxation functions were found to be temperature independent across T_c; no evidence for the presence of electronic magnetic moments was observed. The x=0.5 material is antiferromagnetic below T_N=3.75 K and superconducting below T_c=0.8 K. Muon spin realxation spectra show the gradual onset of damped coherent oscillations characteristic of magnetic order below T_N. At 1.65 K the total oscillating amplitude accounts for at least 85% of the sample volume. No change in muon precession frequency or amplitude is detected on cooling below T_c, indicating the microscopic coexistence of magnetism and superconductivity in this material.

cond-mat.supr-con↗

arXiv2002

muSR and NMR in f-electron non-Fermi liquid materials

Magnetic resonance (muSR and NMR) studies of f-electron non-Fermi-liquid (NFL) materials give clear evidence that structural disorder is a major factor in NFL behavior. Longitudinal-field muSR relaxation measurements at low fields reveal a wide distribution of muon relaxation rates and divergences in the frequency dependence of spin correlation functions in the NFL systems UCu_{5-x}Pd_x and CePtSi_{1-x}Ge_x. These divergences seem to be due to slow dynamics associated with quantum spin-glass behavior, rather than quantum criticality as in a uniform system, for two reasons: the observed strong inhomogeneity in the muon relaxation rate, and the strong and frequency-dependent low-frequency fluctuation observed in U(Cu,Pd)_5 and CePt(Si,Ge). In the NFL materials CeCu_{5.9}Au_{0.1}, Ce(Ru_{0.5}Rh_{0.5})_2Si_2, CeNi_2Ge_2, and YbRh_2Si_2 the low-frequency weight of the spin fluctuation spectrum is much weaker than in the disordered NFL systems.

cond-mat.str-el↗

arXiv2002

muSR in Ce_{1-x}La_xAl_3: anisotropic Kondo effect?

Zero-field muSR experiments in the heavy-fermion alloys Ce_{1-x}La_xAl_3, x = 0 and 0.2, examine a recent proposal that the system exhibits a strong anisotropic Kondo effect. We resolve a damped oscillatory component for both La concentrations, indicative of disordered antiferromagnetism. For x = 0.2 the oscillation frequency decreases smoothly with increasing temperature, and vanishes at the specific heat anomaly temperature T* \approx 2.2 K. Our results are consistent with the view that T* is due to a magnetic transition rather than anisotropic Kondo behavior.

cond-mat.str-el↗

arXiv2002

Spin dynamics in a structurally ordered non-Fermi liquid compound: YbRh_2Si_2

Muon spin relaxation (muSR) experiments have been carried out at low temperatures in the non-Fermi-liquid heavy-fermion compound YbRh_2Si_2. The longitudinal-field muSR relaxation function is exponential, indicative that the dynamic spin fluctuations are homogeneous. The relaxation rate 1/T_1 varies with applied field as H^{-y}, y = 1.0 \pm 0.1, which implies a scaling law of the form χ''(ω) \propto ω^{-y} f(ω/T), \lim_{x\to0} f(x) = x for the dynamic spin susceptibility.

cond-mat.str-el↗

arXiv2001

Slow Spin Dynamics in Non-Fermi-Liquid UCu_{5-x}Pd_x, x = 1.0 and 1.5

Low-temperature muon spin-lattice relaxation measurements in the non-Fermi-liquid heavy-fermion alloys UCu_{5-x}Pd_x, x = 1.0 and 1.5, indicate inhomogeneously distributed f-electron spin fluctuation rates, and exhibit a time-field scaling of the muon relaxation function indicative of long-lived spin correlations. In UCu_4Pd the scaling exponent γis small and temperature independent. In UCu_{3.5}Pd_{1.5} γvaries with temperature, increasing with decreasing temperature similar to spin-glass AgMn. This suggests that the spin-glass state found for x \gtrsim 2 in UCu_{5-x}Pd_x modifies the low-frequency spin dynamics in UCu_{3.5}Pd_{1.5}.

cond-mat.str-el↗

arXiv2001

Time-Reversal-Symmetry Violation and Coexistence of Superconducting and Magnetic Order in CeRh(1-x)Ir(x)In(5)

This paper has been withdrawn by the authors. We performed additional zero-field muon spin relaxation measurements in the superconducting state of CeIrIn$_5$ and found that the spontaneous fields reported previously below $T_c$ are not present. Thus, there is no evidence for a time-reversal-symmetry-violating superconducting order parameter. These new zero-field measurements, as well as new measurements of the penetration depth in this system, will be reported elsewhere. Our zero-field measurements in CeIr$_{0.5}$Rh$_{0.5}$In$_5$, reporting coexistence of superconductivity and magnetic order, are still valid.

cond-mat.str-el↗

arXiv2001

Thickness dependence of the ground-state properties in thin films of the heavy-fermion compound CeCu_6

High-quality thin polycrystalline films of the heavy-fermion compound CeCu_6 were prepared by sputter deposition. The thicker of these films (with thickness up to around 200 nm) reproduce the properties of the bulk compound CeCu_6. As the thickness of the films is decreased, our measurements display strong deviations from the bulk properties, namely, a suppression of the heavy-fermion state. We show that possible `external' effects, like disorder, oxidation and morphology can be excluded and that this size effect is therefore an intrinsic property of CeCu_6. In addition, we investigate possible scenarios explaining the size effect, and find that the proximity of CeCu_6 to a quantum phase transition can account for this striking result.

cond-mat.str-el↗

arXiv2001

Glassy Spin Dynamics in Non-Fermi-Liquid UCu_{5-x}Pd_x, x = 1.0 and 1.5

Local f-electron spin dynamics in the non-Fermi-liquid heavy-fermion alloys UCu_{5-x}Pd_x, x = 1.0 and 1.5, have been studied using muon spin-lattice relaxation. The sample-averaged asymmetry function Gbar(t) indicates strongly inhomogeneous spin fluctuations, and exhibits the scaling Gbar(t,H) = Gbar(t/H^γ) expected from glassy dynamics. At 0.05 K γ(x=1.0) = 0.35 \pm 0.1, but γ(x=1.5) = 0.7 \pm 0.1. This is in contrast to inelastic neutron scattering results, which yield γ= 0.33 for both concentrations. There is no sign of static magnetism \gtrsim 10^{-3} μ_B/U ion in either material above 0.05 K. Our results strongy suggest that both alloys are quantum spin glasses.

cond-mat.str-el↗

arXiv2001

Magnetic properties of hexagonal YMnO$_3$, LuMnO$_3$ and ScMnO$_3$ Magnetic susceptibility, specific heat and dielectric constant of hexagonal YMnO_3, LuMnO_3 and ScMnO_3

We report the magnetic susceptibility, specific heat and dielectric constant on high purity polycrystalline samples of three hexagonal manganites: YMnO_3, LuMnO_3 and ScMnO_3. These materials can exhibit a ferroelectric transition at very high temperatures (T_{FE} > 700K). At lower temperatures there is magnetic ordering of the frustrated Mn^{3+} spins (S=2) on a triangular Mn lattice (YMnO_3:T_N=71K; LuMnO$_3:T_N=90K and ScMnO_3:T_N=130K). The transition is characterized by a sharp kink in the magnetic susceptibility at T_N below which it continues to increase due to the frustration on the triangular lattice. The specific heat shows one clear continuous phase transition at T_N, which is independent of external magnetic field up to 9T with an entropy content as expected for Mn^{3+} ions. The temperature dependent dielectric constant displays a distinct anomaly at T_N.

cond-mat.mtrl-sci↗