arXiv subjects

G. Schmerber

Publications and source records attributed to G. Schmerber.

13 recordsLinked to original sources

arXiv2019

Larger photovoltaic effect and hysteretic photocarrier dynamics in Pb[(Mg1/3Nb2/3)0.70Ti0.30]O3 crystal

Following the recent discovery of a bulk photovoltaic effect in the Pb[(Mg1/3Nb2/3)0.68Ti0.32]O3 crystal, we report here more than one order of magnitude improvement of photovoltaicity as well as its poling dependence in the related composition of lead magnesium niobate-lead titanate noted Pb[(Mg1/3Nb2/3)0.7Ti0.30]O3. Photocurrent measurements versus light intensity reveal a fascinating hysteretic charge carriers dynamics clearly demonstrating charge generation, trapping and release processes.

cond-mat.mtrl-sci↗

arXiv2019

Thickness-dependent electron momentum relaxation times in thin iron films

Terahertz time-domain conductivity measurements in 2 to 100 nm thick iron films resolve the femtosecond time delay between applied electric fields and resulting currents. This response time decreases for thinner metal films. The macroscopic response time depends on the mean and the variance of the distribution of microscopic momentum relaxation times of the conducting electrons. Comparing the recorded response times with DC-conductivities demonstrates increasing variance of the microscopic relaxation times with increasing film thickness. At least two electron species contribute to conduction in bulk with substantially differing relaxation times. The different electron species are affected differently by the confinement because they have different mean free paths.

cond-mat.mtrl-sci↗

arXiv2018

Tuning photovoltaic response in Bi2FeCrO6 films by ferroelectric poling

Ferroelectric materials are interesting candidates for future photovoltaic applications due to their potential to overcome the fundamental limits of conventional single bandgap semiconductor-based solar cells. Although a more efficient charge separation and above bandgap photovoltages are advantageous in these materials, tailoring their photovoltaic response using ferroelectric functionalities remains puzzling. Here we address this issue by reporting a clear hysteretic character of the photovoltaic effect as a function of electric field and its dependence on the poling history. Furthermore, we obtain insight into light induced nonequilibrium charge carrier dynamics in Bi2FeCrO6 films involving not only charge generation, but also recombination processes. At the ferroelectric remanence, light is able to electrically depolarize the films with remanent and transient effects as evidenced by electrical and piezoresponse force microscopy (PFM) measurements. The hysteretic nature of the photovoltaic response and its nonlinear character at larger light intensities can be used to optimize the photovoltaic performance of future ferro-electric-based solar cells.

cond-mat.mtrl-sci↗

arXiv2016

Optical Writing of Magnetic Properties by Remanent Photostriction

We present an optically induced remanent photostriction in BiFeO3, resulting from the photovoltaic effect, which is used to modify the ferromagnetism of Ni film in a hybrid BiFeO3/Ni structure. The 75% change in coercivity in the Ni film is achieved via optical and nonvolatile control. This photoferromagnetic effect can be reversed by static or ac electric depolarization of BiFeO3. Hence, the strain dependent changes in magnetic properties are written optically, and erased electrically. Light-mediated straintronics is therefore a possible approach for low-power multistate control of magnetic elements relevant for memory and spintronic applications.

cond-mat.mtrl-sci↗

arXiv2014

From itinerant to local transformation and critical point in Ni-rich Ce_2(Ni_{1-y}Pd_y)_2Sn

Structural, magnetic (M) and thermal (C_m) studies on Ce_2(Ni_{1-y}Pd_y)_2Sn alloys are presented within the 0<y<0.55 range of concentration, showing evidences for itinerant to local electronic transformation. At variance with RKKY type interactions between localized moments μ_{eff}, the substitution of Ni by isoelectronic Pd leads the antiferromagnetic transition to decrease from T_N~3.8K to ~1.2K between y=0 and 0.48, while M(H) measured at H=5Tesla and 1.8K rises from 0.12 up to 0.75μ_B/Ce-at. Furthermore, the C_m(T_N) jump increases with concentration whereas |θ_P| decreases. The magnetic entropy S_m(T) grows moderately with temperature for y=0 due to a significant contribution of excited levels at low energy, while at y=0.5 it shows a incipient plateau around S_m=Rln2. All these features reflect the progressive ground state transformation of from itinerant to a local character. Another peculiarity of this system is the nearly constant value of C_m(T_N) that ends in an "entropy bottleneck" as T_N decreases. Consequently, the system shows a critical point at y_{cr}~0.48 with signs of ferromagnetic behavior above H_{cr}~0.3T. A splitting of the C_m(T_N) maximum, tuned by field and concentration, indicates a competition between two magnetic phases, with respective peaks at T_N~1.2K and T_I~1.45K.

cond-mat.str-el↗

arXiv2013

Reduction of conductivity and ferromagnetism induced by Ag doping in ZnO:Co

Cobalt and silver co-doping has been undertaken in ZnO thin films grown by pulsed laser deposition in order to investigate the ferromagnetic properties in ZnO-based diluted magnetic materials and to understand the eventual relation between ferromagnetism and charge carriers. Hall transport measurements reveal that Ag doping up to 5% leads to a progressive compensation of the native n-type carriers. The magnetization curves show ferromagnetic contributions for all samples at both 5 K and room temperature, decreasing with increasing the Ag concentration. First principles modeling of the possible configurations of Co-Ag defects suggest the formation of nano-clusters around interstitial Co impurity as the origin of the ferromagnetism. The Ag co-doping results in a decrease of the total spin of these clusters and of the Curie temperature.

cond-mat.mtrl-sci↗

arXiv2011

Tri-critical point and suppression of the Shastry-Sutherland phase in Ce$_{2}$Pd$_{2}$Sn by Ni doping

Structural, magnetization and heat capacity measurements were performed on Ce$_2$(Pd$_{1-x}$Ni$_x$)$_2$Sn ($0 \leq x \leq 0.25$) alloys, covering the full range of the Mo$_2$FeB$_2$ structure stability. In this system, the two transitions observed in Ce$_2$Pd$_2$Sn (at $T_N=4.8$\,K and $T_C=2.1$\,K respectively) converge into a tri-critical point at $T_{cr}\approx 3.4$\,K for $x\approx 0.3$, where the intermediate antiferromagnetic AF phase is suppressed. The $T_N(x)$ phase boundary decrease is due to an incipient Kondo screening of the Ce-4f moments and local atomic disorder in the alloy. Both mechanisms affect the formation of Ce-magnetic dimers on which the Shastry-Sutherland lattice (SSL) builds up. On the contrary, the $T_C(x)$ transition to the ferromagnetic ground state increases as a consequence of the weakening of the AF-SSL phase. Applied magnetic field also suppresses the AF phase like in the stoichiometric compound.

cond-mat.str-el↗

arXiv2011

Structural, magnetic and thermal properties of the substitution series Ce$_2$(Pd$_{1-x}$Ni$_x$)$_2$Sn

Structural, magnetization and heat capacity studies were performed on Ce$_2$(Pd$_{1-x}$Ni$_x$)$_2$Sn ($0 \leq x \leq 1$) alloys. The substitution of Pd atoms by isoelectronic Ni leads to a change in the crystallographic structure from tetragonal (for $x \leq 0.3$) to centered orthorhombic lattice (for $x \geq 0.4$). The volume contraction thorough the series is comparable to the expected from the atomic size ratio between transition metal components. The consequent weak increase of the Kondo temperature drives the two transitions observed in Ce$_2$Pd$_2$Sn to merge at $x = 0.25$. After about a 1% of volume collapse at the structural modification, the system behaves as a weakly magnetic heavy fermion with an enhanced degenerate ground state. Notably, an incipient magnetic transition arises on the Ni-rich size. This unexpected behavior is discussed in terms of an enhancement of the density of states driven by the increase of the $4f$-conduction band hybridization and the incipient contribution of the first excited crystal field doublet on the ground state properties.

cond-mat.str-el↗

arXiv2010

Control of defect-mediated tunneling barrier heights in ultrathin MgO films

The impact of oxygen vacancies on local tunneling properties across rf-sputtered MgO thin films was investigated by optical absorption spectroscopy and conducting atomic force microscopy. Adding O$_2$ to the Ar plasma during MgO growth alters the oxygen defect populations, leading to improved local tunneling characteristics such as a lower density of current hotspots and a lower tunnel current amplitude. We discuss a defect-based potential landscape across ultrathin MgO barriers.

cond-mat.mtrl-sci↗

arXiv2010

Suppression of the Shastry-Sutherland phase in Ce$_{2}$Pd$_{2}$Sn at a field induced critical point

The magnetic phase diagram of Ce$_2$Pd$_2$Sn is investigated through the field dependence of thermal, transport and magnetic measurements performed at low temperature. The upper transition, $T_M=4.8$ K is practically not affected by magnetic field up to B=1 T, whereas the lower one $T_C(B)$ rapidly increases from 2.1 K joining $T_M$ in a critical point at $T_{cr}=(4.2\pm 0.3$)K for $B_{cr}=(0.12\pm 0.03)$ T. At that point the intermediate phase, previously described as an unstable Shastry-Sutherland phase, is suppressed. A detailed analysis around the critical point reveals a structure in the maximum of the $\partial M/\partial B(B)$ derivative which could be related to the formation of a novel phase in that critical region.

cond-mat.str-el↗

arXiv2009

Study of molecular spin-crossover complex Fe(phen)2(NCS)2 thin films

We report on the growth by evaporation under high vacuum of high-quality thin films of Fe(phen)2(NCS)2 (phen=1,10-phenanthroline) that maintain the expected electronic structure down to a thickness of 10 nm and that exhibit a temperature-driven spin transition. We have investigated the current-voltage characteristics of a device based on such films. From the space charge-limited current regime, we deduce a mobility of 6.5x10-6 cm2/V?s that is similar to the low-range mobility measured on the widely studied tris(8-hydroxyquinoline)aluminium organic semiconductor. This work paves the way for multifunctional molecular devices based on spin-crossover complexes.

cond-mat.mtrl-sci↗

arXiv2009

Unstable Shastry-Sutherland phase in Ce$_{2}$Pd$_{2}$Sn

Thermal ($C_P$), magnetic ($M$ and $χ_{ac}$) and transport ($ρ$) measurements on Ce$_{2}$Pd$_{2}$Sn are reported. High temperature properties are well described by the presence of two excited crystal field levels at $(65\pm 5)$K and $(230\pm 20)$K, with negligible hybridization (Kondo) effects. According to literature, two transitions were observed at $T_M=4.8$K and $T_C =2.1$K respectively. The upper transition cannot be considered as a standard anti-ferromagnetic because of frustration effects in a triangular network of Ce-atoms and the positive sign of the paramagnetic temperature $θ_P^{LT} = 4.4$K. The nature of the this intermediated phase is described accounting for the formation of ferromagnetic (F) Ce-dimers disposed in a quasi-2D square lattice, resembling a Shastry-Sutherland pattern. According to hysteretic features in $ρ(T)$ and $χ_{ac}(T)$, the lower F-transition is of first order, with $C_P(T<T_C)$ revealing a gap of anisotropy $E_g\approx 7$K.

cond-mat.str-el↗

arXiv2006

Paramagnetism of the Co sublattice in ferromagnetic Zn$_{1-x}$Co$_{x}$O films

Using the spectroscopies based upon x-ray absorption, we have studied the structural and magnetic properties of Zn$_{1-x}$Co$_{x}$O films ($x$ = 0.1 and 0.25) produced by reactive magnetron sputtering. These films show ferromagnetism with a Curie temperature $T_{\mathrm{C}}$ above room temperature in bulk magnetization measurements. Our results show that the Co atoms are in a divalent state and in tetrahedral coordination, thus substituting Zn in the wurtzite-type structure of ZnO. However, x-ray magnetic circular dichroism at the Co \textit{L}$_{2,3}$ edges reveals that the Co 3\textit{d} sublattice is paramagnetic at all temperatures down to 2 K, both at the surface and in the bulk of the films. The Co 3\textit{d} magnetic moment at room temperature is considerably smaller than that inferred from bulk magnetisation measurements, suggesting that the Co 3\textit{d} electrons are not directly at the origin of the observed ferromagnetism.

cond-mat.mtrl-sci↗