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Direct observation of hole carrier density profiles and their light induced manipulation at the surface of Ge
Authors:
T. Prokscha,
K. H. Chow,
Z. Salman,
E. Stilp,
A. Suter
Abstract:
We demonstrate that, by using low-energy positive muon ($μ^+$) spin spectroscopy as a local probe technique, the profiles of free charge carriers can be directly determined in the accumulation/depletion surface regions of p- or n-type Ge wafers. The detection of free holes is accomplished by measuring the effect of the interaction of the free carriers with the $μ^+$ probe spin on the observable mu…
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We demonstrate that, by using low-energy positive muon ($μ^+$) spin spectroscopy as a local probe technique, the profiles of free charge carriers can be directly determined in the accumulation/depletion surface regions of p- or n-type Ge wafers. The detection of free holes is accomplished by measuring the effect of the interaction of the free carriers with the $μ^+$ probe spin on the observable muon spin polarization. By tuning the energy of the low-energy $μ^+$ between 1 keV and 20 keV the near-surface region between 10 nm and 160 nm is probed. We find hole carrier depletion and electron accumulation in all samples with doping concentrations up to the $10^{17}$ cm$^{-3}$ range, which is opposite to the properties of cleaved Ge surfaces. By illumination with light the hole carrier density in the depletion zone can be manipulated in a controlled way. Depending on the used light wavelength $λ$ this change can be persistent ($λ= 405, 457$ nm) or non-persistent ($λ= 635$ nm) at temperatures $< 270$ K. This difference is attributed to the different kinetic energies of the photo-electrons. Photo-electrons generated by red light do not have sufficient energy to overcome a potential barrier at the surface to be trapped in empty surface acceptor states. Compared to standard macroscopic transport measurements our contact-less local probe technique offers the possibility of measuring carrier depth profiles and manipulation directly. Our approach may provide important microscopic information on a nanometer scale in semiconductor device studies.
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Submitted 21 July, 2020; v1 submitted 25 September, 2019;
originally announced September 2019.
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Superconductivity drives magnetism in delta-doped La2CuO4
Authors:
A. Suter,
G. Logvenov,
A. V. Boris,
F. Baiutti,
F. Wrobel,
L. Howald,
E. Stilp,
Z. Salman,
T. Prokscha,
B. Keimer
Abstract:
The understanding of the interplay between different orders in a solid is a key challenge in highly correlated electronic systems. In real systems this is even more difficult since disorder can have a strong influence on the subtle balance between these orders and thus can obscure the interpretation of the observed physical properties. Here we present a study on delta-doped La2CuO4 superlattices.…
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The understanding of the interplay between different orders in a solid is a key challenge in highly correlated electronic systems. In real systems this is even more difficult since disorder can have a strong influence on the subtle balance between these orders and thus can obscure the interpretation of the observed physical properties. Here we present a study on delta-doped La2CuO4 superlattices. By means of molecular beam epitaxy whole LaO-layers were periodically replaced through SrO-layers providing a charge reservoir, yet reducing the level of disorder typically present in doped cuprates to an absolute minimum. The induced superconductivity and its interplay with the antiferromagnetic order is studied by means of low-energy muSR. We find a quasi-2D superconducting state which couples to the antiferromagnetic order in a non-trivial way. Below the superconducting transition temperature, the magnetic volume fraction increases strongly. The reason could be a charge redistribution of the free carriers due to the opening of the superconducting gap which is possible due to the close proximity and low disorder between the different ordered regions.
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Submitted 23 June, 2017;
originally announced June 2017.
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Evidence for Coexistence of Bulk Superconductivity and Itinerant Antiferromagnetism in the Heavy Fermion System CeCo(In$_{1-x}$Cd$_x$)$_5$
Authors:
Ludovic Howald,
Evelyn Stilp,
Pierre Dalmas de Réotier,
Alain Yaouanc,
Stéphane Raymond,
Cinthia Piamonteze,
Gérard Lapertot,
Christopher Baines,
Hugo Keller
Abstract:
In the generic phase diagram of heavy fermion systems, tuning an external parameter such as hydrostatic or chemical pressure modifies the superconducting transition temperature. The superconducting phase forms a dome in the temperature-tuning parameter phase diagram, which is associated with a maximum of the superconducting pairing interaction. Proximity to antiferromagnetism suggests a relation b…
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In the generic phase diagram of heavy fermion systems, tuning an external parameter such as hydrostatic or chemical pressure modifies the superconducting transition temperature. The superconducting phase forms a dome in the temperature-tuning parameter phase diagram, which is associated with a maximum of the superconducting pairing interaction. Proximity to antiferromagnetism suggests a relation between the disappearance of antiferromagnetic order and superconductivity. We combine muon spin rotation, neutron scattering, and x-ray absorption spectroscopy techniques to gain access to the magnetic and electronic structure of CeCo(In$_{1-x}$Cd$_x$)$_5$ at different time scales. Different magnetic structures are obtained that indicate a magnetic order of itinerant character, coexisting with bulk superconductivity. The suppression of the antiferromagnetic order appears to be driven by a modification of the bandwidth/carrier concentration, implying that the electronic structure and consequently the interplay of superconductivity and magnetism is strongly affected by hydrostatic and chemical pressure.
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Submitted 2 September, 2015;
originally announced September 2015.
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Modifications of the Meissner screening profile in YBa$_2$Cu$_3$O$_{7-δ}$ thin films by gold nanoparticles
Authors:
E. Stilp,
A. Suter,
T. Prokscha,
Z. Salman,
E. Morenzoni,
H. Keller,
C. Katzer,
F. Schmidl,
M. Döbeli
Abstract:
Adding Au nanoparticles to YBa$_2$Cu$_3$O$_{7-δ}$ thin films leads to an increase of the superconducting transition temperature $T_{\rm c}$ and the critical current density $j_{\rm c}$. While the higher $j_{\rm c}$ can be understood in terms of a stronger pinning of the flux vortices at the Au nanoparticles, the enhanced $T_{\rm c}$ is still puzzling. In the present study, we determined the micros…
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Adding Au nanoparticles to YBa$_2$Cu$_3$O$_{7-δ}$ thin films leads to an increase of the superconducting transition temperature $T_{\rm c}$ and the critical current density $j_{\rm c}$. While the higher $j_{\rm c}$ can be understood in terms of a stronger pinning of the flux vortices at the Au nanoparticles, the enhanced $T_{\rm c}$ is still puzzling. In the present study, we determined the microscopic magnetic penetration profiles and the corresponding London penetration depths $λ_{\rm L}$ in the Meissner state of optimally doped YBa$_2$Cu$_3$O$_{7-δ}$ thin films with and without Au nanoparticles by low-energy muon spin rotation. By Rutherford backscattering spectrometry, we show that the Au nanoparitcles are distributed over the whole thickness of the thin-film samples. The superfluid density $n_{\rm s} \propto 1/λ_{\rm L}^2$ was found to increase in the films containing Au nanoparticles. We attribute this increase of $n_{\rm s}$ to a reduction of the defect density possibly due to defect condensation at the Au nanoparticles.
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Submitted 24 January, 2014;
originally announced January 2014.
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Photo-induced persistent inversion of germanium in a 200-nm-deep surface region
Authors:
T. Prokscha,
K. H. Chow,
E. Stilp,
A. Suter,
H. Luetkens,
E. Morenzoni,
G. J. Nieuwenhuys,
Z. Salman,
R. Scheuermann
Abstract:
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 wit…
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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.
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Submitted 16 December, 2013;
originally announced December 2013.
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Magnetic phase diagram of low-doped La{2-x}SrxCuO4 thin films studied by low-energy muon-spin rotation
Authors:
E. Stilp,
A. Suter,
T. Prokscha,
E. Morenzoni,
H. Keller,
B. M. Wojek,
H. Luetkens,
A. Gozar,
G. Logvenov,
I. Bozovic
Abstract:
The magnetic phase diagram of La2-xSrxCuO4 thin-films grown on single-crystal LaSrAlO4 substrates has been determined by low-energy muon-spin rotation. The diagram shows the same features as the one of bulk La2-xSrxCuO4, but the transition temperatures between distinct magnetic states are significantly different. In the antiferromagnetic phase the Neel temperature TN is strongly reduced, and no ho…
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The magnetic phase diagram of La2-xSrxCuO4 thin-films grown on single-crystal LaSrAlO4 substrates has been determined by low-energy muon-spin rotation. The diagram shows the same features as the one of bulk La2-xSrxCuO4, but the transition temperatures between distinct magnetic states are significantly different. In the antiferromagnetic phase the Neel temperature TN is strongly reduced, and no hole spin freezing is observed at low temperatures. In the disordered magnetic phase (x>0.02) the transition temperature to the cluster spin-glass state Tg is enhanced. Possible reasons for the pronounced differences between the magnetic phase diagrams of thin-film and bulk samples are discussed.
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Submitted 2 August, 2013;
originally announced August 2013.