arXiv subjects

Martin Knupfer

Publications and source records attributed to Martin Knupfer.

At least 37 records · Page 2Linked to original sources

arXiv2013

Photoemission study of PDI8-CN2 /Au, PDI8-CN2 / Si and PDI8-CN2 - T6 interfaces

We report on the study of the electronic properties in PDI8-CN2 / Au, PDI8-CN2 / Si, PDI8-CN2 - T6 interfaces investigated by X-Ray (XPS) and Ultraviolet (UPS) Photoemission Spectroscopy. In order to verify the consistency with the results on field effect devices, heated substrates have been considered, while only in the case of PDI8-CN2 / Au interface also sputter-cleaned Au has been used as a reference. We found a small interface dipole between PDI8-CN2 and sputter-cleaned Au, which reverses its sign as a consequence of the 'pillow effect' due to the presence of the contamination layer in the heated substrates. This result is in agreement with the transport measurements on PDI8-CN2 field effect transistors, which show an 'always on' behavior. We also found evidences of the formation of a built-in potential in T6 / PDI8-CN2 and PDI8-CN2 / T6 interfaces, mainly from the T6 side. This also well agrees with previous results on T6 / PDI8-CN2 heterostructures devices, exhibiting transport properties explainable with the presence of an accumulation heterojunction at the interface.

cond-mat.mtrl-sci↗

arXiv2013

Doping Dependence of the Plasmon Dispersion in 2H-Tantalum-Diselenide

Inelastic electron scattering is applied to investigate the impact of potassium intercalation on the charge carrier plasmon energy and dispersion in the charge-density wave (CDW) bearing compound 2H-tantalum-diselenide. We observe an unususal doping dependence of the plasmon dispersion, which even changes sign upon alkali addition. In contrast to the continous energy shift of the plasmon position upon doping at lowest momentum transfer, its dispersion changes in a rather discontinuous manner. We argue that the observed dynamics can only be explained in a picture, where complex phenomena are taken into account including the suppression of the CDW upon doping as well as the interplay of the CDW and the plasma resonance.

cond-mat.str-el↗

arXiv2013

Challenging the nature of low energy plasmon excitations in CaC$_6$ using electron energy-loss spectroscopy

The nature of low energy plasmon excitations plays an important role in understanding the low energy electronic properties and coupling mechanism of different superconducting compounds such as CaC$_6$. Recent ab-initio studies predict a charge carrier intraband plasmon in keeping with a low energy acoustic plasmon. Here, we have studied the low-energy electronic excitations of CaC$_6$ using high-resolution electron energy-loss spectroscopy in transmission at low temperatures. The analysis of the core-level excitations leads to the conclusion that hybridization between graphite and calcium states plays an essential role in this graphite intercalated compound. Regarding the low energy plasmon excitation, we observe the formation of an intraband (charge carrier) plasmon with a negative dispersion at about 3.5 eV in sound agreement with the theory. Finally, a weak excitation around 1.2 eV with an almost linear dispersion relation can be observed as predicted for an acoustic plasmon that may mediate the superconducting coupling in CaC$_6$. However its optical limit at ~1 eV challenges the theoretical predictions and safely rules out an electronic superconducting coupling mechanism in CaC$_6$.

cond-mat.supr-con↗

arXiv2013

Exciton properties of selected aromatic hydrocarbon systems

We have examined the singlet excitons in two representatives of acene-type (tetracene and pentacene) and phenacene-type (chrysene and picene) molecular crystals, respectively, using electron energy-loss spectroscopy at low temperatures. We show that the excitation spectra of the two hydrocarbon families significantly differ. Moreover, close inspection of the data indicates that there is an increasing importance of charge-transfer excitons at lowest excitation energy with increasing length of the molecules.

cond-mat.mtrl-sci↗

arXiv2012

Evidence for phase formation in potassium intercalated 1,2;8,9-dibenzopentacene

We have prepared potassium intercalated 1,2;8,9-dibenzopentacene films under vacuum conditions. The evolution of the electronic excitation spectra upon potassium addition as measured using electron energy-loss spectroscopy clearly indicate the formation of particular doped phases with compositions K$_x$dibenzopentacene ($x$ = 1,2,3). Moreover, the stability of these phases as a function of temperature has been explored. Finally, the electronic excitation spectra also give insight into the electronic ground state of the potassium doped 1,2;8,9-dibenzopentacene films.

cond-mat.supr-con↗

arXiv2012

Momentum dependence of the excitons in pentacene

We have carried out electron energy-loss investigations of the lowest singlet excitons in pentacene at 20 K. Our studies allow to determine the full exciton band structure in the a*,b* reciprocal lattice plane. The lowest singlet exciton can move coherently within this plane, and the resulting exciton dispersion is highly anisotropic. The analysis of the energetically following (satellite) features indicates a strong admixture of charge transfer excitations to the exciton wave function.

cond-mat.mtrl-sci↗

arXiv2012

Absence of photoemission from the Fermi level in potassium intercalated picene and coronene films: structure, polaron or correlation physics?

The electronic structure of potassium intercalated picene and coronene films has been studied using photoemission spectroscopy. Picene has additionally been intercalated using sodium. Upon alkali metal addition core level as well as valence band photoemission data signal a filling of previously unoccupied states of the two molecular materials due to charge transfer from potassium. In contrast to the observation of superconductivity in K_xpicene and K_xcoronene (x ~ 3), none of the films studied shows emission from the Fermi level, i.e. we find no indication for a metallic ground state. Several reasons for this observation are discussed.

cond-mat.supr-con↗

arXiv2012

Electronic structure of undoped and potassium doped coronene investigated by electron energy-loss spectroscopy

We performed electron energy-loss spectroscopy studies in transmission in order to obtain insight into the electronic properties of potassium intercalated coronene, a recently discovered superconductor with a rather high transition temperature of about 15\,K. A comparison of the loss function of undoped and potassium intercalated coronene shows the appearance of several new peaks in the optical gap upon potassium addition. Furthermore, our core level excitation data clearly signal filling of the conduction bands with electrons.

cond-mat.supr-con↗

arXiv2011

Unoccupied electronic structure of TiOCl studied by x-ray absorption near-edge spectroscopy

We study the unoccupied electronic structure of the spin-1/2 quantum magnet TiOCl by x-ray absorption near-edge spectroscopy (XANES) at the Ti L and O K edges. Data is acquired both in total electron and fluorescence yield mode (TEY and FY, respectively). While only the latter allows to access the unconventional low-temperature spin-Peierls (SP) phase of TiOCl the signal is found to suffer from significant self-absorption in this case. Nevertheless, we conclude from FY data that effects of the SP distortion on the electronic structure are absent within experimental accuracy. The similarity of room-temperature FY and TEY data, the latter not being obscured by self-absorption, allows us to use TEY spectra for comparison with simulations. These are performed by cluster calculations in D$_{4h}$ and D$_{2h}$ symmetries using two different codes. We extract values of the crystal-field splitting (CFS) and parametrize our results in often found notations by Slater, Racah and Butler. In all cases, good agreement with published values from other studies is found.

cond-mat.str-el↗

arXiv2011

The Effect of Charge Order on the Plasmon Dispersion in Transition-Metal Dichalcogenides

We investigate the dispersion of the charge carrier plasmon in the three prototypical charge-density wave bearing transition-metal dichalcogenides 2H-TaSe2, 2H-TaS2 and 2H-NbSe2 employing electron energy-loss spectroscopy. For all three compounds the plasmon dispersion is found to be negative for small momentum transfers. This is in contrast to the generic behavior observed in simple metals as well as the related system 2H-NbS2, which does not exhibit charge order. We present a semiclassical Ginzburg-Landau model which accounts for these observations, and argue that the vicinity to a charge ordered state is thus reflected in the properties of the collective excitations.

cond-mat.str-el↗

arXiv2011

Electronic excitations of potassium intercalated manganese phthalocyanine investigated by electron energy-loss spectroscopy

The electronic excitations of manganese phthalocyanine (MnPc) films were studied as a function of potassium doping using electron energy-loss spectroscopy in transmission. Our data reveal doping induced changes in the excitation spectrum, and they provide evidence for the existence of three doped phases: K$_1$MnPc, K$_2$MnPc, and K$_4$MnPc. Furthermore, the addition of electrons first leads to a filling of orbitals with strong Mn 3d character, a situation which also affects the magnetic moment of the molecule.

cond-mat.mtrl-sci↗

arXiv2011

Exciton character in picene molecular solids

We have studied the low-energy electronic excitations of solid picene at 20 K using momentum dependent electron energy-loss spectroscopy. Our results demonstrate the presence of five excitonic features below the transport energy gap of picene, which all are characterized by a negligible dispersion. One of these excitons has not been observed in the optical absorption spectrum of picene molecules in solution and thus is assigned to a (solid-state induced) charge transfer exciton. This conclusion is supported by the momentum dependent intensity variation of this exciton which clearly signals a significant dipole forbidden contribution, in contrast to the other low energy excitations.

cond-mat.mtrl-sci↗

arXiv2011

The catalytic potential of high-k dielectrics for graphene formation

The growth of single and multilayer graphene nano-flakes on MgO and ZrO2 at low temperatures is shown through transmission electron microscopy. The graphene nano-flakes are ubiquitously anchored at step edges on MgO (100) surfaces. Density functional theory investigations on MgO (100) indicate C2H2 decomposition and carbon adsorption at step-edges. Hence, both the experimental and theoretical data highlight the importance of step sites for graphene growth on MgO.

cond-mat.mes-hall↗

arXiv2011

Dynamic response and electronic structure of potassium doped picene investigated by electron energy-loss spectroscopy

We performed electron energy-loss spectroscopy studies in order to get a deeper insight into the electronic properties of potassium intercalated picene, a recently discovered superconductor. A comparison of the loss function of the undoped and doped compound shows the appearance of a new peak in the optical gap, which we attribute to the charge carrier plasmon. We find a dramatic increase for the value of the background dielectric constant $ε_{\infty}$ upon doping. Our core level excitation data clearly signal filling of the conduction bands with electrons upon potassium addition.

cond-mat.supr-con↗

arXiv2010

Plasmons and Interband Transitions of Ca$_{11}$Sr$_3$Cu$_{24}$O$_{41}$ investigated by Electron Energy-Loss Spectroscopy

Electron energy-loss spectroscopy studies have been performed in order to get a deeper insight into the electronic structure and elementary excitations of the two-leg ladder system Ca$_{11}$Sr$_3$Cu$_{24}$O$_{41}$. We find a strong anisotropy of the loss function for momentum transfers along the a and c-crystallographic axis, and a remarkable linear plasmon dispersion for a momentum transfer parallel to the legs of the ladders. The investigated spectral features are attributed to localized and delocalized charge-transfer excitations and the charge carrier plasmon. The charge carrier plasmon position and dispersion in the long wave-length limit agree well with expectations based upon the band structure of the two-leg ladder, while the observed quasi-linear plasmon dispersion might be related to the peculiar properties of underdoped cuprates in general.

cond-mat.supr-con↗

arXiv2010

Electronic properties of molecular solids: the peculiar case of solid Picene

Recently, a new organic superconductor, K-intercalated Picene with high transition temperatures $T_c$ (up to 18\,K) has been discovered. We have investigated the electronic properties of the undoped relative, solid picene, using a combination of experimental and theoretical methods. Our results provide detailed insight into the occuopied and unoccupied electronic states.

cond-mat.supr-con↗

arXiv2006

Dressing of the charge carriers in high-Tc superconductors

In this contribution we review recent ARPES results on the dressing of the charge carriers in high-temperature superconductors. After an introduction into the cuprates and their electronic structure, ARPES, the spectral function and the self-energy function in the normal state and in the superconducting state is discussed.. Finally results on the dressing of the charge carriers in the system Bi2Sr2caCu2O7 at the nodal and the antinodal point are presented.

cond-mat.supr-con↗

arXiv2004

On the formation process of silicon carbide nanophases via hydrogenated thermally induced templated synthesis

A thermally induced templated synthesis for SiC nanotubes and nanofibers using ammonia or nitrogen as a carrier gas, single wall carbon nanotubes (SWCNT) as templates as well as gaseous Si is presented. The bundles of SWCNT act as both the carbon source and as a nanoframe from which SiC structuctures form. Depending on the duration of the thermally induced templated reaction, for a fixed temperature, carrier gas, and gas pressure, various SiC nanostructures are obtained. These structures include SiC nanorods coated in C, SiC nanorods, SiC nanotubes, and SiC nanocrytals. From our analysis using transmission electron microscopy (TEM) and scanning electron microscopy (SEM), electron energy-loss spectroscopy (EELS), electron diffraction (EDX), optical absorption spectroscopy and Raman spectroscopy as probes we prove that H has a key role on the morphology and stochiometry of the different SiC nanostructures.

cond-mat.mtrl-sci↗