Charge Reservoir Research Articles

Reversible pressure-induced polymerization of Fe(C5H5)2 doped C70

High pressure Raman, IR and X-ray diffraction (XRD) studies have been carried out on C70(Fe(C5H5)2)2 (hereafter, “C70(Fc)2”) sheets. Theoretical calculation is further used to analyze the Electron Localization Function (ELF) and charge transfer in the crystal and thus to understand the transformation of C70(Fc)2 under pressure. Our results show that even at room temperature dimeric phase and one dimensional (1D) polymer phase of C70 molecules can be formed at about 3 and 8GPa, respectively. The polymerization is found to be reversible upon decompression and the reversibility is related to the pressure-tuned charge transfer, as well as the overridden steric repulsion of counter ions. According to the layered structure of the intercalated ferrocene molecules formed in the crystal, we suggest that ferrocene acts as not only a spacer to restrict the polymerization of C70 molecules within a layer, but also as charge reservoir to tune the polymerization process. This supplies a possible way for us to design the polymerization of fullerenes at suitable conditions.

Superconductivity in Y-doped (Tl1-xYx)Ba2Ca2Cu3O10-δ

Superconducting Y-doped (Tl 1-x Y x )Ba 2 Ca 2 Cu 3 O 10-δ samples are synthesized by a solid-state reaction method and characterized by X-ray diffraction, DC resistivity (ρ), AC susceptibility (χ ac ) and Fourier transform infrared absorption spectroscopy. The suppression of zero resistivity critical temperature (T c (R = 0)) and the magnitude of diamagnetism is most likely due to the reduced efficiency with which (Tl 1-x Y x )Ba 2 O 4-δ charge reservoir layer supplies the carriers to the CuO 2 planes. The localization at Y 3+ reduces the number of carriers in the CuO 2 planes and thereby promotes antiferromagnetism, which is fatal for superconductivity.

Direct k-space mapping of the electronic structure in an oxide-oxide interface.

The interface between LaAlO(3) and SrTiO(3) hosts a two-dimensional electron system of itinerant carriers, although both oxides are band insulators. Interface ferromagnetism coexisting with superconductivity has been found and attributed to local moments. Experimentally, it has been established that Ti 3d electrons are confined to the interface. Using soft x-ray angle-resolved resonant photoelectron spectroscopy we have directly mapped the interface states in k space. Our data demonstrate a charge dichotomy. A mobile fraction contributes to Fermi surface sheets, whereas a localized portion at higher binding energies is tentatively attributed to electrons trapped by O vacancies in the SrTiO(3). While photovoltage effects in the polar LaAlO(3) layers cannot be excluded, the apparent absence of surface-related Fermi surface sheets could also be fully reconciled in a recently proposed electronic reconstruction picture where the built-in potential in the LaAlO(3) is compensated by surface O vacancies serving also as a charge reservoir.

What have we learnt from the highest-Tc superconducting Hg-based cuprates?

Hg-based high-temperature superconductors (Hg-HTSs) have a similar layered structure to that in most other superconducting cuprates. The weakly attached Hg cations can be swapped with Tl ones in a cation exchange process, which allows exploration of the physical properties associated closely to the charge reservoir block change as a consequence of the Hg–Tl cation exchange. This paper intends to provide an overview of over a decade long investigation of Hg(Tl)-HTSs thin films processed in forward and reverse cation exchange, and from which to elucidate the interesting aspects that are related to the superconductivity of these fascinating materials.

Modeling of charge injection and extraction in a metal/polymer interface through an exponential distribution of surface states

From a brief review of the literature, it is shown that surface states within dielectrics represent a critical step in charge injection whereas only scarce physical descriptions of these phenomena are available. It is confirmed from space charge measurements on low density polyethylene (LDPE) that the work function of metals used as electrodes does not correlate to injected charge quantities, thereby supporting the need for alternative descriptions of injection to the Schottky law. Hence, the influence of surface states on space charge formation in LDPE is investigated using a one-dimensional model of electronic charge injection and transport under dc voltage. Transport and trapping in the bulk of the insulation are accounted for using an exponential distribution of traps with a maximum limit in trap depth. We have modeled interface states considering the same form of distribution, hypothesizing an increasing density of states and trap depth when approaching the surface. We suppose that the interface region with modulated properties extends over 1μm from both surfaces of the 200 μm thick insulation. We also considered a reservoir of charge for providing carriers into the dielectric, instead of a classical injection law. After a sensitivity analysis of model parameters on space charge results, we present simulation results on charge injection for a unipolar and a bipolar model. It is shown that the presence of surface states and the reservoir of charges could explain the particular behavior of charges observed experimentally, as regards heterocharge accumulation, and field enhancement.

Disorder Effects on Competition between Antiferromagnetism and Superconductivity in Cuprate Superconductors through the Enhancement in Charge Susceptibility

The coexistence state of antiferromagnetism (AF) and superconductivity (SC) has been observed in five-layered cuprates. However, this coexistence state disappears, and the AF phase and SC phase lose contact in the doping phase diagram toward double- and single-layered cuprates. We investigate the mechanism of the disappearance of the coexistence of AF and SC in disordered cuprate superconductors in order to understand these doping phase diagrams. In single- and double-layered cuprates, electrons on the CuO$_2$ plane experience the disorder effect through inhomogeneity in the charge reservoir layer. These impurity potentials can be effectively enhanced toward the underdoped region by the effect of many-body corrections that involve an increase in charge susceptibility. As a result, strong disorder effects are expected particularly in the competing regions of AF and SC, where the coexistence phase of AF and SC is extremely suppressed. We show the validity of this suppression mechanism by considering the Aslamazov-Larkin-type vertex correction to the effective impurity potential in the effective mean-field phase diagram.

Temperature and Frequency Dependent Dielectric Properties of Cu0.5Tl0.5Ba2Ca3(Cu4−y Cd y )O12−δ Bulk Superconductor

The temperature and frequency dependent dielectric properties of polycrystalline Cd-doped Cu0.5Tl0.5Ba2Ca3(Cu4−yCdy)O12−δ (y=0,0.25,0.5,0.75) bulk superconductor samples are investigated. The zero resistivity critical temperature {Tc(R=0)} has decreased and normal state resistivity has increased with the increase of Cd-doping in Cu0.5Tl0.5Ba2Ca3(Cu4−yCdy)O12−δ samples. The dielectric properties such as dielectric constants (e′,e″), dielectric loss tangent (tanδ) and ac-conductivity (σac) are investigated by measuring the capacitance (C) and conductance (G) in the frequency range of 10 KHz to 10 MHz at different temperature from 80 K to 300 K. The negative capacitance (NC) is observed in all Cu0.5Tl0.5Ba2Ca3(Cu4−yCdy)O12−δ samples. The large values of NC observed at lower frequencies and temperatures may be due to reduced thermal vibrations and enhanced polarizability of the material. The effect of Cd-doping on bulk properties, dc-resistivity (ρ) and ac-electrical conductivity (σac) of these superconductor samples are investigated. The polarization in Cu0.5Tl0.5Ba2Ca3(Cu4−yCdy)O12−δ samples is most likely arising from the displacement of charges in CuO2/CdO2 planes relative to the static charges at Ba2+, Tl3+, and Cu2+ sites in Cu0.5Tl0.5Ba2O4−δ charge reservoir layers by external applied field.

Superconducting properties of Ba2Ca7Cu8O16(O0.8+δ F1.2) studied via reversible magnetization

The Ba2Ca7Cu8O16(O0.8+δ F1.2) (F-0278) superconductor with 8 CuO2 planes in the structural unit was investigated by analyzing the reversible magnetization based on the Hao-Clem model and fluctuation theories. In spite of the superconducting block (SCB) being thicker than the charge reservoir block (CRB), the analysis clearly revealed two-dimensional (2D) superconducting properties in F-0278, such as a strong temperature dependence of the Ginzburg parameter κ and a relatively short effective interlayer distance. The extreme 2D behavior can be understood in terms of the significant charge imbalance in the SCB, which was previously proposed to exist in F-0278, where the carriers are distributed quite unequally to crystallographically different CuO2 planes; i.e., the outer planes (OPs) adjacent to the CRB always contain more carriers than the inner planes (IPs). The 2D nature of F-0278 indicates a significant or complete suppression of the superconductivity in the IPs. The important superconducting parameters were also obtained from the analysis.

Phonon-affected steady-state transport through molecular quantum dots

We consider the transport through a vibrating molecular quantum dot contacted to macroscopic leads acting as charge reservoirs. In the equilibrium and nonequilibrium regimes, we study the formation of a polaron-like transient state at the quantum dot for all the ratios of the dot–lead coupling to the energy of the local phonon mode. We show that the polaronic renormalization of the dot–lead coupling is a possible mechanism for negative differential conductance. Moreover, the effective dot level follows one of the lead chemical potentials to enhance resonant transport, causing novel features in the inelastic tunneling signal. In the linear response regime, we investigate the impact of the electron–phonon interaction on the thermoelectrical properties of the quantum dot device.

Superconducting Mechanism Through Direct and Redox Layer Doping in Pnictides

The mechanism of superconductivity in pnictides is discussed through direct doping in superconducting FeAs and also in charge reservoir REO layers. The un-doped SmFeAsO is charge neutral SDW (Spin Density Wave) compound with magnetic ordering below 150 K. The Superconducting FeAs layers are doped with Co and Ni at Fe site, whereas REO layers are doped with F at O site. The electron doping in SmFeAsO through Co results in superconductivity with transition temperature (Tc) maximum up to 15 K, whereas F doping results in Tc upto 47 K in SmFeAsO. All these REFe/Co/NiAsO/F compounds are iso-structural to ZrCuSiAs structure. The samples are crystallized in a tetragonal structure with space group P4/nmm. Variation of Tc with different doping routes shows the versatility of the structure and mechanism of occurrence of superconductivity. It seems doping in redox layer is more effective than direct doping in superconducting FeAs layer.

Occurrence of a high-temperature superconducting phase in (CaCuO2)n/(SrTiO3)msuperlattices

We report the occurrence of superconductivity, with maximum T${}_{c}=40$ K, in superlattices (SLs) based on two insulating oxides, namely CaCuO${}_{2}$ and SrTiO${}_{3}$. In these (CaCuO${}_{2}$)${}_{n}$/(SrTiO${}_{3}$)${}_{m}$ SLs, the CuO${}_{2}$ planes belong only to CaCuO${}_{2}$ block, which is an antiferromagnetic insulator. Superconductivity, confined within a few unit cells at the CaCuO${}_{2}$/SrTiO${}_{3}$ interface, shows up only when the SLs are grown in a highly oxidizing atmosphere, because of extra oxygen ions entering at the interfaces. The hole doping is obtained by charge transfer from the interface layers, which thus act as charge reservoir.

Impact of graphene quantum capacitance on transport spectroscopy

We demonstrate experimentally that graphene quantum capacitance $C_{\mathrm{q}}$ can have a strong impact on transport spectroscopy through the interplay with nearby charge reservoirs. The effect is elucidated in a field-effect-gated epitaxial graphene device, in which interface states serve as charge reservoirs. The Fermi-level dependence of $C_{\mathrm{q}}$ is manifested as an unusual parabolic gate voltage ($V_{\mathrm{g}}$) dependence of the carrier density, centered on the Dirac point. Consequently, in high magnetic fields $B$, the spectroscopy of longitudinal resistance ($R_{xx}$) vs. $V_{\mathrm{g}}$ represents the structure of the unequally spaced relativistic graphene Landau levels (LLs). $R_{xx}$ mapping vs. $V_{\mathrm{g}}$ and $B$ thus reveals the vital role of the zero-energy LL on the development of the anomalously wide $\nu=2$ quantum Hall state.

OptimalTcof cuprates: The role of screening and reservoir layers

We explore the role of charge reservoir layers (CRLs) on the superconducting transition temperature of cuprate superconductors. Specifically, we study the effect of CRLs with efficient short distance dielectric screening coupled capacitively to copper oxide metallic layers. We argue that dielectric screening at short distances and at frequencies of the order of the superconducting gap, but small compared to the Fermi energy can significantly enhance T$_c$, the transition temperature of an unconventional superconductor. We discuss the relevance of our qualitative arguments to a broader class of unconventional superconductors.

Normal pressure synthesis of (Tl1−yCy)Ba2Ca3Cu4O12−δ superconductor

Single phase (Tl1−yCy)Ba2Ca3Cu4O12−δ (Tl1−yCy-1234) (y = 0, 0.25, 0.5 and 0.75) superconductor samples have been prepared by solid state reaction method. The FTIR absorption measurements have confirmed the substitution of carbon at thallium site in the charge reservoir layer, (Tl1−yCy)Ba2O4−δ. The electron micrographs of these samples have shown that the carbon substitution has improved the grain morphology of Tl0.75C0.25-1234 sample. The y = 0.25 was found to be the optimum carbon concentration to achieve higher superconducting transition temperature Tc[0] and improved grain morphology. The superconducting transition temperature of Tl0.75C0.25-1234 sample has been increased to 100 K whereas a decrease in the superconducting transition temperature of Tl1−yCy-1234 (y = 0.5 and 0.75) samples was observed. However, the magnitude of diamagnetism has been decreased in all the carbon substituted samples.

Enhancement in the Superconductivity of Tl(Ba2−x Mg x )(Ca1Be1)Cu3O10−δ System with the Decreased Thickness of Charge Reservoir Layer

Mg-doped Tl(Ba2−xMgx)(Ca1Be1)Cu3O10−δ (x=0, 0.25, 0.5, 0.75) superconductors are synthesized at the normal pressure and the possible mechanism of superconductivity in these compounds is studied. Tl(Ba2−xMgx)(Ca1Be1)Cu3O10−δ samples have shown an orthorhombic crystal structure and their c-axis length decreases with Mg doping. In these studies we have investigated the role of decreased thickness of charge reservoir layer in the mechanism of superconductivity. The Tc(R=0) in as-prepared Tl(Ba2−xMgx)(Ca1Be1)Cu3O10−δ (x=00, 0.25, 0.5, 0.75) samples was 97, 99, 100, 94 K and in oxygen post-annealed samples the Tc(R=0) is observed at 100, 98, 103, 101 K, respectively. The magnitude of the superconductivity after Mg doping is improved in Tl(Ba2−xMgx)(Ca1Be1)Cu3O10−δ (x=0, 0.25, 0.5, 0.75) samples. It was observed from the FTIR absorption measurements that the phonon modes related to CuO2 planar oxygen atoms are hardened with the doping of Mg in the charge reservoir layer. These studies have shown that the thickness of charge reservoir layer decreases with Mg doping which most likely makes charge transfer mechanism more efficient, promoting increase in the magnitude of superconductivity.

Origin of ferromagnetism enhancement in bi-layer chromium-doped indium zinc oxides

This work demonstrates that by controlling the rapid thermal annealing temperature, amorphous chromium-doped indium zinc oxide films develop an amorphous-crystalline bi-layer structure and show magnetization up to ∼30 emu/cm3. The crystalline layer arises from significant out-diffusion of Zn from surfaces, leading to a large difference in the Zn:In ratio in amorphous and crystalline layers. Doped Cr ions in amorphous and crystalline layers form different valence configurations, creating a charge reservoir which transfers electrons through amorphous-crystalline interfaces and in turn enhances ferromagnetism.

Reversible magnetization and superconducting properties of the four-layered [formula omitted] superconductor with [formula omitted

Reversible magnetization of grain-aligned Ba2Ca3Cu4O8+y(F,O)2 (F-0234) with Tc≃90.6K, which is a member with n=4 in the new homologous series Ba2Can−1CunO2n(F,O)2 was investigated using the Hao–Clem model and fluctuation theories. The almost temperature independence of the Ginzburg–Landau parameter (κ) up to 0.9 T/Tc and the crossover point in M(T) indicated less 2D character, suggesting no drastic decrease in interlayer coupling compared to other high-Tc cuprate superconductors. The relatively strong interlayer coupling results from the smallest thickness of the charge reservoir block among other multilayered systems. On the other hand, the λab(0) and Hc(0) values derived from the Hao–Clem analysis strongly suggest that the decrease in Tc is due to both the decrease in the carrier density and weakened pairing strength.

Unsynchronized resonance of covalent bonds in the superconducting state

DFT calculations performed on different cluster models of cuprates (LaBa2Cu3O6.7, La1.85Sr0.15CuO4, YBa2Cu3O7, TlBa2Ca2Cu3O8.78, HgBa2Ca2Cu3O8.27), metallic systems (Nb3Ge, MgB2) and the pnictide LaO0.92F0.08FeAs made evident the occurrence of unsynchronized resonance of covalent bonds in the superconducting state, as predicted by Pauling´s resonating valence bond (RVB) theory. For cuprates, the unsynchronized resonance involves electron transfer between Cu atoms accompanied by a decrease in the charge of the La, Sr, Y and Ca atoms. For MgB2, electron transfer occurs in the Mg layer, while the B layer behaves as charge reservoir. For Nb3Ge, unsynchronized resonance occurs among the Ge atoms, which should be responsible for charge transfer. For LaO0.92F0.08FeAs, the results suggest that both La-O and Fe-As layers are involved in the mechanism of superconductivity. The identification of unsynchronized resonances in these systems provides evidence which supports RVB as a suitable theory for high-temperature superconductivity (high-T C).

Spin Relaxation in Single-Layer Graphene with Tunable Mobility

Graphene is an attractive material for spintronics due to theoretical predictions of long spin lifetimes arising from low spin-orbit and hyperfine couplings. In experiments, however, spin lifetimes in single-layer graphene (SLG) measured via Hanle effects are much shorter than expected theoretically. Thus, the origin of spin relaxation in SLG is a major issue for graphene spintronics. Despite extensive theoretical and experimental work addressing this question, there is still little clarity on the microscopic origin of spin relaxation. By using organic ligand-bound nanoparticles as charge reservoirs to tune the mobility between 2700 and 12 000 cm(2)/(V s), we successfully isolate the effect of charged impurity scattering on spin relaxation in SLG. Our results demonstrate that, while charged impurities can greatly affect mobility, the spin lifetimes are not affected by charged impurity scattering.

Magnetic and ligand field properties of copper at the interfaces of (CaCuO2)n/(SrTiO3)nsuperlattices

Using high-resolution resonant inelastic x-ray scattering (RIXS) excited at the Cu ${L}_{3}$ edge we have investigated the ligand field ($dd$) and magnetic excitations in (CaCuO${}_{2}$)${}_{n}$/(SrTiO${}_{3}$)${}_{n}$ superlattices ($n=2,3$) and compared them to those of a 14-nm-thick CaCuO${}_{2}$ film. The $dd$ excitation spectrum reveals a pyramidal coordination of Cu ions at the CaCuO${}_{2}$/SrTiO${}_{3}$ interfaces. In the three samples, spin excitations are in the form of dispersing magnons, with similar spectral intensity but reduced dynamics in SLs with respect to pure CaCuO${}_{2}$. By fitting the dispersions within linear spin wave theory we have obtained the leading term of the in-plane superexchange parameters: $J=127$ meV, 138 meV, and 157 meV for $n=2$, 3 SLs, and CaCuO${}_{2}$, respectively. These results demonstrate that the antiferromagnetic order is preserved in these SLs down to very small cuprate layer thickness and despite the chemical and structural alterations at the interface. This finding opens the way to the production of artificial Cu-based high-temperature superconductors where the charge reservoir layer is constituted by the interface itself.