Charge Carrier Doping Research Articles

Direct search for a ferromagnetic phase in a heavily overdoped nonsuperconducting copper oxide.

The doping of charge carriers into the CuO(2) planes of copper oxide Mott insulators causes a gradual destruction of antiferromagnetism and the emergence of high-temperature superconductivity. Optimal superconductivity is achieved at a doping concentration p beyond which further increases in doping cause a weakening and eventual disappearance of superconductivity. A potential explanation for this demise is that ferromagnetic fluctuations compete with superconductivity in the overdoped regime. In this case, a ferromagnetic phase at very low temperatures is predicted to exist beyond the doping concentration at which superconductivity disappears. Here we report on a direct examination of this scenario in overdoped La(2-x)Sr(x)CuO(4) using the technique of muon spin relaxation. We detect the onset of static magnetic moments of electronic origin at low temperature in the heavily overdoped nonsuperconducting region. However, the magnetism does not exist in a commensurate long-range ordered state. Instead it appears as a dilute concentration of static magnetic moments. This finding places severe restrictions on the form of ferromagnetism that may exist in the overdoped regime. Although an extrinsic impurity cannot be absolutely ruled out as the source of the magnetism that does occur, the results presented here lend support to electronic band calculations that predict the occurrence of weak localized ferromagnetism at high doping.

Effect of a Zn impurity on Tc and its implications for pairing symmetry in LaFeAsO1−xFx

The effect of a non-magnetic Zn impurity on superconductivity in the LaFe1−yZnyAsO1−xFx system is studied systematically. In the presence of a Zn impurity, the superconducting (SC) transition temperature increases in the under-doped regime, remains unchanged in the optimally doped regime and is severely suppressed in the over-doped regime. Our results suggest a switch in the symmetry of the SC order parameters from an s-wave to s±- or d-wave states as the charge carrier doping increases in FeAs-based superconductors.

Zero-doping state and electron–hole asymmetry in an ambipolar cuprate

High-temperature copper oxide superconductors are usually doped with holes or electrons, not both. The discovery of a material that can be doped with both electrons and holes finally clarifies the region of low doping and the effect of the added dopants. A Mott insulator is a material that is insulating because of strong Coulomb repulsions between electrons. Doping charge carriers, electrons or holes into a Mott insulator can induce high-temperature superconductivity. Thus, what exactly happens when a charge carrier is doped into a Mott insulator is a key question in many-body physics1,2,3,4. To address this issue, ideally one should start from a zero-doping state5,6,7 and be able to introduce both holes and electrons in the dilute limit. However, such an idealized experiment has been impossible because of the lack of suitable materials. Here we show that a new ‘ambipolar’ cuprate makes it possible for the first time to cross the zero-doping state in the same material, which in turn allows us to address the physics of the extremely low-doping region. Surprisingly, we found that the antiferromagnetic ground state sharply changes between electron- and hole-doped sides, and this change is dictated by the existence of only 0.1 ppm of charge carriers. Moreover, we observed that the Neel temperature TN shows an unexpected reduction in a narrow range centred at the zero-doping state, across which the system exhibits asymmetric behaviours in transport measurements. Our findings reveal the inherently different nature of electron and hole doping in the dilute limit of a Mott-insulating cuprate.

Spatially Resolved Spontaneous Reactivity of Diazonium Salt on Edge and Basal Plane of Graphene without Surfactant and Its Doping Effect

The site-dependent and spontaneous functionalization of 4-bromobenzene diazonium tetrafluoroborate (4-BBDT) and its doping effect on a mechanically exfoliated graphene (MEG) were investigated. The spatially resolved Raman spectra obtained from both edge and basal region of MEG revealed that 4-BBDT molecules were noncovalently functionalized on the basal region of MEG, while they were covalently bonded to the edge of MEG. The chemical doping effect induced by noncovalently functionalized 4-BBDT molecules on a basal plane region of MEG was successfully explicated by Raman spectroscopy. The position of Fermi level of MEG and the type of doping charge carrier induced by the noncovalently adsorbed 4-BBDT molecules were determined from systematic G band and 2D band changes. The successful spectroscopic elucidation of the different bonding characters of 4-BBDT depending on the site of graphene is beneficial for the fundamental studies about the charge transfer phenomena of graphene as well as for the potential applications, such as electronic devices, hybridized composite structures, etc.

Charge carrier self-organization in ferroelectromagnetic semiconductors Eu0.8Ce0.2Mn2O5

Giant permittivity (ε′∼104) and ferromagnetism have been observed above 185K (including room temperature) in single crystals of the dilute semiconductor manganite-ferroelectromagnetic Eu0.8Ce0.2Mn2O5 in the investigations of x-ray diffraction, dielectric and magnetic properties, and conductivity. X-ray diffraction study has revealed a layered superstructure along the c axis at room temperature. A model of the state with giant ε′ including as-grown 2D layers with doping impurities, charge carriers, and double-exchange coupled Mn3+–Mn4+ ion pairs is suggested. At low temperatures these layers form isolated electrically neutral small-size 1D superlattices, in which de Haas van Alphen oscillations were observed. As temperature and hopping conductivity increase, charge-carrier self-organization in the crystal cause the formation of a layered superstructure consisting of charged layers (with an excess Mn3+) alternating with dielectric layers of the initial crystal—the ferroelectricity state due to charge ordering. Ferromagnetism results from double exchange between Mn3+ and Mn4+ ions through charge carriers in the charged layers.

BiOCuS: A new superconducting compound with oxypnictide-related structure

The discovery of about 50K superconductivity in the tetragonal Fe-based pnictides has stimulated the search for superconductivity in a wide class of materials with similar structure. Copper forms compounds isostructural to LaOFeAs. Single phase BiOCuS can be prepared by a solid state reaction at temperature lower than 500°C from a mixture of Bi2O3, Bi2S3 and Cu2S. The samples have been characterized by means of EDX analysis, X-ray diffraction, magnetic and electrical measurements. The cell parameters are a=3.8708Å, c=8.565Å. Charge carrier doping can be realized either by F substitutions for O, or by Cu off-stoichiometry. The latter doping route leads to the occurrence of superconductivity below Tc=5.8K.

High-Temperature Transport Properties of the Zintl Phases Yb11GaSb9 and Yb11InSb9

Two rare-earth Zintl phases, Yb_(11)GaSb_9 and Yb_(11)InSb_9, were synthesized in high-temperature self-fluxes of molten Ga and In, respectively. Structures were characterized by both single-crystal X-ray diffraction and powder X-ray diffraction and are consistent with the published orthorhombic structure, with the space group Iba2. High-temperature differential scanning calorimetry (DSC) and thermal gravimetry (TG) measurements reveal thermal stability to 1300 K. Seebeck coefficient and resistivity measurements to 1000 K are consistent with the hypothesis that Yb_(11)GaSb_9 and Yb_(11)InSb_9 are small band gap semiconductors or semimetals. Low doping levels lead to bipolar conduction at high temperature, preventing a detailed analysis of the transport properties. Thermal diffusivity measurements yield particularly low lattice thermal conductivity values, less than 0.6 W/m K for both compounds. The low lattice thermal conductivity suggests that Yb_(11)MSb_9 (M = Ga, In) has the potential for high thermoelectric efficiency at high temperature if charge-carrier doping can be controlled.

Superconductivity around quantum critical point in P-doped iron arsenides

We demonstrate that, by the P/As substitution—without doping of charge carriers—in a FeAs-layer-based parent compound, superconductivity can be universally introduced. The maximum superconducting critical temperature (Tc) of BaFe2(As1-xPx)2 achieves 30K. The P doping in LnFeAsO system (Ln=La and Sm) produces superconductivity below 11K. The normal-state resistivity obeys linear temperature dependence and the normal-state Hall coefficient shows strong temperature dependence. These non-Fermi liquid behaviors suggest magnetic quantum criticality. The maximum Tc values in different systems correlates strongly with the diagonal bondangle of Fe–As–Fe, implying the important role of the next-nearest-neighbor magnetic exchange coupling in iron pnictide superconductors.

Tip-enhanced near-field optical microscopy of carbon nanotubes

We review recent experimental studies on single-walled carbon nanotubes on substrates using tip-enhanced near-field optical microscopy (TENOM). High-resolution optical and topographic imaging with sub 15 nm spatial resolution is shown to provide novel insights into the spectroscopic properties of these nanoscale materials. In the case of semiconducting nanotubes, the simultaneous observation of Raman scattering and photoluminescence (PL) is possible, enabling a direct correlation between vibrational and electronic properties on the nanoscale. So far, applications of TENOM have focused on the spectroscopy of localized phonon modes, local band energy renormalizations induced by charge carrier doping, the environmental sensitivity of nanotube PL, and inter-nanotube energy transfer. At the end of this review we discuss the remaining limitations and challenges in this field.

Thermoelectric properties of p-type PbSe nanowires

The thermoelectric properties of individual solution-phase synthesized p-type PbSe nanowires have been examined. The nanowires showed near degenerately doped charge carrier concentrations. Compared to the bulk, the PbSe nanowires exhibited a similar Seebeck coefficient and a significant reduction in thermal conductivity in the temperature range 20 K to 300 K. Thermal annealing of the PbSe nanowires allowed their thermoelectric properties to be controllably tuned by increasing their carrier concentration or hole mobility. After optimal annealing, single PbSe nanowires exhibited a thermoelectric figure of merit (ZT) of 0.12 at room temperature.

Optical conductivity in thet−JHolstein model

Using a recently developed numerical method we compute charge stiffness and optical conductivity of the $t\text{\ensuremath{-}}J$ model coupled to optical phonons. Coherent hole motion is most strongly influenced by the electron-phonon coupling within the physically relevant regime of the exchange interaction. We find unusual nonmonotonous dependence of the charge stiffness as a function of the exchange coupling near the crossover to the strong electron-phonon coupling regime. Optical conductivity in this regime shows a two-peak structure. The low-frequency peak represents local magnetic excitation, attached to the hole, while the higher-frequency peak corresponds to the mid-infrared band that originates from coupling to spin-wave excitations, broadened and renormalized by phonon excitations. We observe no separate peak at or slightly above the phonon frequency. This finding suggests that in the framework of the $t\text{\ensuremath{-}}J$ Holstein model the two-peak structure seen in recent optical measurements is due to magnetic excitations coupled to lattice degrees of freedom via doped charge carriers.

The electronic phase diagram of the LaO1−xFxFeAs superconductor

The competition of magnetic order and superconductivity is a key element in the physics of all unconventional superconductors, for example in high-transition-temperature cuprates, heavy fermions and organic superconductors. Here superconductivity is often found close to a quantum critical point where long-range antiferromagnetic order is gradually suppressed as a function of a control parameter, for example charge-carrier doping or pressure. It is believed that dynamic spin fluctuations associated with this quantum critical behaviour are crucial for the mechanism of superconductivity. Recently, high-temperature superconductivity has been discovered in iron pnictides, providing a new class of unconventional superconductors. Similar to other unconventional superconductors, the parent compounds of the pnictides show a magnetic ground state and superconductivity is induced on charge-carrier doping. In this Letter the structural and electronic phase diagram is investigated by means of X-ray scattering, muon spin relaxation and Mössbauer spectroscopy on the series LaO(1-x)F(x)FeAs. We find a discontinuous first-order-like change of the Néel temperature, the superconducting transition temperature and the respective order parameters. Our results strongly question the relevance of quantum critical behaviour in iron pnictides and prove a strong coupling of the structural orthorhombic distortion and the magnetic order both disappearing at the phase boundary to the superconducting state.

Carrier-Mediated Antiferromagnetic Interlayer Exchange Coupling in Diluted Magnetic Semiconductor MultilayersGa1−xMnxAs/GaAs:Be

We report the antiferromagnetic (AFM) interlayer exchange coupling between Ga0.97Mn0.03As ferromagnetic semiconductor layers separated by Be-doped GaAs spacers. Polarized neutron reflectivity reveals a characteristic splitting at the wave vector corresponding to twice the multilayer period, indicating that the coupling between the ferromagnetic layers is AFM. When the applied field is increased to above the saturation field, this AFM coupling is suppressed. This behavior is not observed when the spacers are undoped, suggesting that the observed AFM coupling is mediated by doped charge carriers.

Superconductivity in heavily boron-doped silicon carbide

The discoveries of superconductivity in heavily boron-doped diamond in 2004 and silicon in 2006 have renewed the interest in the superconducting state of semiconductors. Charge-carrier doping of wide-gap semiconductors leads to a metallic phase from which upon further doping superconductivity can emerge. Recently, we discovered superconductivity in a closely related system: heavily boron-doped silicon carbide. The sample used for that study consisted of cubic and hexagonal SiC phase fractions and hence this led to the question which of them participated in the superconductivity. Here we studied a hexagonal SiC sample, free from cubic SiC phase by means of x-ray diffraction, resistivity, and ac susceptibility.

Electrical transport and 1/f noise properties of LaFe1−xNixO3 (x = 0.3, 0.4 and 0.5) thin films

The electrical transport and 1/f conduction noise properties of LaFe1−xNixO3 (x = 0.3, 0.4 and 0.5) thin films have been studied. The temperature dependent resistivity data have been fitted with Mott's variable-range hopping model. From the resistivity data we have estimated various parameters, namely, the hopping distance (Rh), the hopping energy (Eh) and the density of states at the Fermi level (N(EF)). The decrease in the hopping energy and the increase in the density of states at room temperature have been observed with the increase in Ni doping for all samples. The Ni doping increases the density of charge carriers and enhances delocalization induced phenomena in the system. The magnitude of the normalized noise SV/V2 increases with the decrease in the Ni concentration over the whole temperature range. The conduction noise is also proportional to the square of the bias current, which confirms that the noise arises from the conduction fluctuations. The Hooge parameter (γ) calculated at room temperature is compared with its value in semiconductors and manganite/oxide materials. The observed features have been explained on the basis of charge-carrier doping in LaFeO3. The noise measurements in conjugation with the electrical properties of the film propose the propitious characteristic of the system.

Structural, electrical, magnetic, and electronic structure studies of PrFe1−xNixO3 (x⩽0.5)

We report the x-ray absorption studies on O K, Fe L3,2, Ni L3,2, and Pr M5,4 edges in PrFe1−xNixO3 along with their structural, electrical transport, and magnetization characterizations. All the samples are in single phase having orthorhombic structure with space group Pnma for x⩽0.4. Ni doping at Fe site brings the system in the conducting regime, resistivity decreases from GΩcmto260mΩcm at room temperature, and the magnetic ordering is stabilized. The temperature dependent resistivity follows the semiconducting behavior and fits well with Greaves’ variable range hopping model. The gap parameter is reduced from 2to0.118eV. The materials are in weak ferromagnetic state and magnetization is gradually decreasing with the enhancement of Ni substitution, whereas magnetic anisotropy is reduced substantially. A new feature about 2.0eV lower than the pre-edge of PrFeO3 in O K edge is observed with Ni substitution at Fe site due to the 3d contraction effect and is growing with the increase of Ni substitution. From 2p edges (L3,2 edges) of Fe and Ni, it is confirmed that both are in trivalent state and Ni is in mixed spin state with 3d7 (t2g5,eg2 and t2g6,eg1) configuration in ground state. The trivalent state of Pr is confirmed by x-ray absorption spectroscopy of Pr at M5,4 edges. These observations have been explained on the basis of charge carrier doping in PrFeO3 with the Ni substitution. The disorder induced localization is found to control the conductivity and magnetism in the present materials. The controlled doping of carriers in semiconducting regime and ferromagnetism propose these materials as a promising candidate for the spintronic applications.

Superconductivity and superconducting order parameter phase fluctuations in a weakly doped antiferromagnet

The superconducting properties of a recently proposed phenomenological model for a weakly doped antiferromagnet are analyzed, taking into account fluctuations of the phase of the order parameter. In this model, we assume that the doped charge carriers cannot move out of the antiferromagnetic sublattice in which they were introduced. This case corresponds to free-carrier spectra with the maximum at $\mathbf{k}=(\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}∕2,\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}∕2)$, as it was observed in angle-resolved photoemission spectroscopy experiments for some of the cuprates in the insulating state [A. Damascelli et al., Rev. Mod. Phys. 75, 473 (2003)]. The doping dependence of the superconducting gap and the temperature--carrier density phase diagram of the model are studied in the case of the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ pairing symmetry of the order parameter and different values of the model parameters. A possible relevance of the results to experiments on high-temperature superconductors is discussed.

Magnetic heat transport inR2CuO4(R=La, Pr, Nd, Sm, Eu, and Gd)

We have studied the thermal conductivity $\kappa$ on single crystalline samples of the antiferromagnetic monolayer cuprates R$_2$CuO$_4$ with R = La, Pr, Nd, Sm, Eu, and Gd. For a heat current within the CuO$_2$ planes, i.e. for $\kappa_{ab}$ we find high-temperature anomalies around 250 K in all samples. In contrast, the thermal conductivity $\kappa_c$ perpendicular to the CuO$_2$ planes, which we measured for R = La, Pr, and Gd, shows a conventional temperature dependence as expected for a purely phononic thermal conductivity. This qualitative anisotropy of $\kappa_i$ and the anomalous temperature dependence of $\kappa_{ab}$ give evidence for a significant magnetic contribution $\kappa_{mag}$ to the heat transport within the CuO$_2$ planes. Our results suggest, that a large magnetic contribution to the heat current is a common feature of single-layer cuprates. We find that $\kappa_{mag}$ is hardly affected by structural instabilities, whereas already weak charge carrier doping causes a strong suppression of $\kappa_{mag}$.

Charge carrier and spin doping in ZnO thin films

Recent efforts on doping ZnO films for charge and spin functionality are reviewed, focusing on chemical doping for charge and spin device formation. Discussion includes the behavior of phosphorus as an acceptor and magnetism in transition metal-doped ZnO. Evidence for p-type behavior in phosphorus-doped (Zn,Mg)O grown by pulsed laser deposition is presented. The magnetic properties of ZnO co-doped with Mn and Sn are also discussed.

M-Center in Low-Dose Proton Implanted 4H-SiC; Bistability and Change in Emission Rate

Electrically active point defects in semiconductor materials are important because they strongly affect material properties like effective doping concentration and charge carrier lifetimes. This thesis presents results on point defects introduced by ion implantation in silicon and silicon carbide. The defects have mainly been studied by deep level transient spectroscopy (DLTS) which is a quantitative, electrical characterization method highly suitable for point defect studies. The method is based on measurements of capacitance transients and both standard DLTS and new applications of the technique have been used. In silicon, a fundamental understanding of diffusion phenomena, like room-temperature migration of point defects and transient enhanced diffusion (TED), is still incomplete. This thesis presents new results which brings this understanding a step closer. In the implantation-based experimental method used to measure point defect migration at room temperature, it has been difficult to separate the effects of defect migration and ion channeling. For various reasons, the effect of channeling has so far been disregarded in this type of experiments. Here, a very simple method to assess the amount of channeling is presented, and it is shown that channeling dominates in our experiments. It is therefore recommended that this simple test for channeling is included in all such experiments. This thesis also contains a detailed experimental study on the defect distributions of vacancy and interstitial related damage in ion implanted silicon. Experiments show that interstitial related damage is positioned deeper (0.4 um or more) than vacancy related damage. A physical model to explain this is presented. This study is important to the future modeling of transient enhanced diffusion. Furthermore, the point defect evolution in low-fluence implanted 4H-SiC is investigated, and a large number of new defect levels has been observed. Many of these levels change or anneal out at temperatures below 300 C, which is not in accordance with the general belief that point defect diffusion in SiC requires high temperatures. This thesis also includes an extensive study on a metastable defect which we have observed for the first time and labeled the M-center. The defect is characterized with respect to DLTS signatures, reconfiguration barriers, kinetics and temperature interval for annealing, carrier capture cross sections, and charge state identification. A detailed configuration diagram for the M-center is presented.