Browsing by Subject "Hall effect"

Now showing 1 - 20 of 21

Open Access
Direct magnetic imaging of ferromagnetic domain structures by room temperature scanning hall probe microscopy using a bismuth micro-Hall probe
(Japan Society of Applied Physics, 2001) Sandhu, A.; Masuda, H.; Oral, A.; Bending, S. J.
A bismuth micro-Hall probe sensor with an integrated scanning tunnelling microscope tip was incorporated into a room temperature scanning Hall probe microscope system and successfully used for the direct magnetic imaging of microscopic domains of low coercivity perpendicular garnet thin films and demagnetized strontium ferrite permanent magnets. At a driving current of 800 μA, the Hall coefficient, magnetic field sensitivity and spatial resolution of the Bi probe were 3.3 × 10-4 Ω/G, 0.38 G/√Hz and ∼ 2.8 μm, respectively. The room temperature magnetic field sensitivity of the Bi probe was comparable to that of a semiconducting 1.2μm GaAs/AlGaAs heterostructure micro-Hall probe, which exhibited a value of 0.41 G/√Hz at a maximum driving current of 2μA.
Open Access
Disorder and localization in the lowest Landau level in the presence of dilute point scatterers
(Pergamon Press, 1999) Gedik, Z.; Bayındır, Mehmet
We study the localization properties of a two-dimensional noninteracting electron gas in the presence of randomly distributed short-range scatterers in very high magnetic fields. We evaluate the participation number of the eigenstates obtained by exact diagonalization technique. At low impurity concentrations we obtain self-averaged values showing that all states, except those exactly at the Landau level, are localized with finite localization length. We conclude that in this dilute regime the localization length does not diverge. We also find that the maximum localization length increases exponentially with impurity concentration. Our calculations suggest that scaling behavior may be absent even for higher concentrations of scatterers.
Open Access
Double subband occupation of the two-dimensional electron gas in InxAl1-XN/AlN/GaN/AlN heterostructures with a low indium content (0.064 ≤ x ≤ 0.140) barrier
(Elsevier, 2010-05-08) Lisesivdin, S. B.; Tasli, P.; Kasap, M.; Ozturk, M.; Arslan, E.; Ozcelik, S.; Özbay, Ekmel
We present a carrier transport study on low indium content (0.064 ≤ x ≤ 0.140) InxAl1 - xN/AlN/GaN/AlN heterostructures. Experimental Hall data were carried out as a function of temperature (33-300 K) and a magnetic field (0-1.4 T). A two-dimensional electron gas (2DEG) with single or double subbands and a two-dimensional hole gas were extracted after implementing quantitative mobility spectrum analysis on the magnetic field dependent Hall data. The mobility of the lowest subband of 2DEG was found to be lower than the mobility of the second subband. This behavior is explained by way of interface related scattering mechanisms, and the results are supported with a one-dimensional self-consistent solution of non-linear Schrödinger- Poisson equations.
Open Access
DX-center energy calculation with quantitative mobility spectrum analysis in n-AlGaAs/GaAs structures with low Al content
(ELSEVIER, 2009-03-17) Lisesivdin, S. B.; Altuntas, H.; Yildiz, A.; Kasap, M.; Özbay, Ekmel; Ozcelik, S.
Experimental Hall data that were carried out as a function of temperature (60-350 K) and magnetic field (0-1.4 T) were presented for Si-doped low Al content (x=0.14) n-AlxGa1-xAs/GaAs heterostructures that were grown by molecular beam epitaxy (MBE). A 2-dimensional electron gas (2DEG) conduction channel and a bulk conduction channel were founded after implementing quantitative mobility spectrum analysis (QMSA) on the magnetic field dependent Hall data. An important decrease in 2DEG carrier density was observed with increasing temperature. The relationship between the bulk carriers and 2DEG carriers was investigated with 1D self consistent Schrödinger-Poisson simulations. The decrement in the 2DEG carrier density was related to the DX-center carrier trapping. With the simulation data that are not included in the effects of DX-centers, 17 meV of effective barrier height between AlGaAs/GaAs layers was found for high temperatures (T>300 K). With the QMSA extracted values that are influenced by DX-centers, 166 meV of the DX-center activation energy value were founded at the same temperatures.
Open Access
Electrical conduction properties of Si δ-doped GaAs grown by MBE
(2009) Yildiz, A.; Lisesivdin, S.B.; Altuntas H.; Kasap, M.; Ozcelik, S.
The temperature dependent Hall effect and resistivity measurements of Si δ-doped GaAs are performed in a temperature range of 25-300 K. The temperature dependence of carrier concentration shows a characteristic minimum at about 200 K, which indicates a transition from the conduction band conduction to the impurity band conduction. The temperature dependence of the conductivity results are in agreement with terms due to conduction band conduction and localized state hopping conduction in the impurity band. It is found that the transport properties of Si δ-doped GaAs are mainly governed by the dislocation scattering mechanism at high temperatures. On the other hand, the conductivity follows the Mott variable range hopping conduction (VRH) at low temperatures in the studied structures. © 2009 Elsevier B.V. All rights reserved.
Open Access
Experiments on strongly correlated materials: magneto-transport properties of VO2 AND V2O3
(2019-09) Sürmeli, Engin Can
Vanadium oxides provide unusual electrical and magnetic phenomena emerging from strong electronic correlations, which include, among other things, a thermally induced metal-insulator transition (MIT). Investigation of the changes in carrier concentration and mobility across the MIT in vanadium oxides, such as vanadium dioxide (VO2) and vanadium sesquioxide (V2O3), carries great importance for understanding the micromechanisms behind suchfirst-order phase transitions. A well-known approach to measuring such parameters in semiconductor materials is Hall effect measurement. So far, magnetotransport studies have only been conducted on polycrystalline thinfilms of VO2/V2O3. As a result, reports on the Hall mobility of these materials often contradict with each other due to the non-uniform stress building on the crystal by adhesion to the substrate. Thus, a thorough investigation of Hall effect measurements on single-crystalline, stress-free VO2 nanobeams and V2O3 nanoplates is required. However, achieving this task is not a straightforward process. First of all, the relatively small size of nanobeams compared to the epitaxialfilms creates the necessity to utilize a bridge-type Hall-bar shaping of the crystal. Additionally, in order to produce a stress-free environment, the crystals must be detached from the substrate and transferred to an atomically at surface, such as hexagonal boron nitride (h-BN). Therefore, the device fabrication method demands many steps despite that VO2 is a very fragile material. In this work, we provide a new fabrication method for shaping VO2 and V2O3 into Hall-bar structure via Gallium and Argon-ion milling while inducing minimal damage on the crystal. We also investigate the strain level of shaped crystals and provide methods to prevent cracking in the devices upon structural phase As a second objective, we investigate the resistivity behavior and magnetic response of VO2 nanobeams at low temperature ranges. We show that the high magnetoresistance of VO2 creates demand for very high magneticfields in the Hall effect measurements. Finally, we demonstrate a Hall effect measurement on an as-grown V2O3 nanoplatelet across its phase transition.
Open Access
Extraction and scattering analyses of 2D and bulk carriers in epitaxial graphene-on-SiC structure
(Elsevier BV, 2014-09) Lisesivdin, S. B.; Atmaca, G.; Arslan, E.; Çakmakyapan S.; Kazar, Ö.; Bütün, S.; Ul-Hassan, J.; Janzén, E.; Özbay, Ekmel
Hall effect measurements of a graphene-on-SiC system were carried out as a function of temperature (1.8-200 K) at a static magnetic field (0.51) With the analysis of temperature dependent single-field Hall data with the Simple Parallel Conduction Extraction Method (SPCEM), bulk and two-dimensional (2D) carrier densities and mobilities were extracted successfully. Bulk carrier is attributed to SIC substrate and 2D carrier is attributed to the graphene layer. For each SPCEM extracted carrier data, relevant three-dimensional or 2D scattering analyses were performed. Each SPCEM extracted carrier data were explained with the related scattering analyses. A temperature independent mobility component, which may related to an interaction between graphene and SIC, was observed for both scattering analyses with the same mobility limiting value. With the SPCEM, effective ionized impurity concentration of SiC substrate, extracted 2D-mobility, and sheet carrier density of the graphene layer are calculated with using temperature dependent static magnetic field Hall data. (c) 2014 Elsevier B.V. All rights reserved.
Open Access
Fabrication and characterization of Bismuth Hall sensors at room temperature
(2003) Bayer, Gözde
Small-scale Hall effect devices have attracted a great deal of research interest in recent years. It is well known that bulk single crystal of bismuth exhibit a large magnetoresistance effect and the recognition of this fact has stimulated a number of recent efforts to grow thin films of bismuth. Such films are useful in magnetic sensing applications. We fabricated Hall sensors having thickness of 30 nm and 50 nm of Bismuth using photolithography. Bismuth was deposited on to the surface of GaAs by evaporation teclinique. The properties of these sensors were then studied; dependences of the resistivity, and Hall coefficient on layer thickness were investigated at room temperature. Hall coefficients were calculated under the effect of a magnetic field. Results were then compared with the previously obtained values. Bismuth micro-Hall probes with dimensions as small as »0.25 pm x 0.25 pm produced by Focused Ion Beam (FIB) milling were also presented in this study. Hall coefficient was then calculated.
Open Access
Fractional quantum Hall states in the vicinity of Mott plateaus
(The American Physical Society, 2010) UmucalIlar, R. O.; Mueller, E. J.
We perform variational Monte Carlo calculations to show that bosons in a rotating optical lattice will form analogs of fractional quantum Hall states when the tunneling is sufficiently weak compared to the interactions, and the deviation of density from an integer is commensurate with the effective magnetic field. We compare the energies of superfluid and correlated states to one another and to the energies found in full configuration-interaction calculations on small systems. We look at overlaps between our variational states and the exact ground state, characterizing the ways in which fractional quantum Hall effect correlations manifest themselves near the Mott insulating state. We explore the experimental signatures of these states.
Open Access
Local current distribution at large quantum dots (QDs): A self-consistent screening model
(Elsevier B.V., 2008) Krishna, P. M.; Siddiki, A.; Güven, K.; Hakioǧlu T.
We report the implementation of the self-consistent Thomas-Fermi screening theory, together with the local Ohm's law to a quantum dot system in order to obtain local current distribution within the dot and at the leads. We consider a large dot (size > 700 nm) defined by split gates, and coupled to the leads. Numerical calculations show that the non-dissipative current is confined to the incompressible strips. Due to the non-linear screening properties of the 2DES at low temperatures, this distribution is highly sensitive to external magnetic field. Our findings support the phenomenological models provided by the experimental studies so far, where the formation of the (direct) edge channels dominate the transport.
Open Access
Micromachined III-V cantilevers for AFM-tracking scanning Hall probe microscopy
(Institute of Physics, 2003) Brook, A. J.; Bending, S. J.; Pinto, J.; Oral, A.; Ritchie, D.; Beere, H.; Springthorpe, A.; Henini, M.
In this paper we report the development of a new III-V cantilever-based atomic force sensor with piezoresistive detection and an integrated Hall probe for scanning Hall probe microscopy. We give detailed descriptions of the fabrication process and characterization of the new integrated sensor, which will allow the investigation of magnetic samples with no sample preparation at both room and cryogenic temperatures. We also introduce a novel piezoresistive material based on the ternary alloy n+-Al0.4Ga0.6As which allows us to achieve a cantilever deflection sensitivity ΔR/(RΔz) = 2 × 10-6 Å-1 at room temperature.
Open Access
Mobility limiting scattering mechanisms in nitride-based two-dimensional heterostructures with the InGaN channel
(IOP Publishing, 2010-03-16) Gökden, S.; Tülek, R.; Teke, A.; Leach, J. H.; Fan, Q.; Xie, J.; Özgür, Ü.; Morkoç, H.; Lisesivdin, S. B.; Özbay, Ekmel
The scattering mechanisms limiting the carrier mobility in AlInN/AlN/InGaN/GaN two-dimensional electron gas (2DEG) heterostructures were investigated and compared with devices without InGaN channel. Although it is expected that InGaN will lead to relatively higher electron mobilities than GaN, Hall mobilities were measured to be much lower for samples with InGaN channels as compared to GaN. To investigate these observations the major scattering processes including acoustic and optical phonons, ionized impurity, interface roughness, dislocation and alloy disorder were applied to the temperature-dependent mobility data. It was found that scattering due mainly to interface roughness limits the electron mobility at low and intermediate temperatures for samples having InGaN channels. The room temperature electron mobilities which were determined by a combination of both optical phonon and interface roughness scattering were measured between 630 and 910 cm2 (V s)-1 with corresponding sheet carrier densities of 2.3-1.3 × 1013 cm-2. On the other hand, electron mobilities were mainly limited by intrinsic scattering processes such as acoustic and optical phonons over the whole temperature range for Al0.82In 0.18N/AlN/GaN and Al0.3Ga0.7N/AlN/GaN heterostructures where the room temperature electron mobilities were found to be 1630 and 1573 cm2 (V s)-1 with corresponding sheet carrier densities of 1.3 and 1.1 × 1013 cm-2, respectively. By these analyses, it could be concluded that the interfaces of HEMT structures with the InGaN channel layer are not as good as that of a conventional GaN channel where either AlGaN or AlInN barriers are used. It could also be pointed out that as the In content in the AlInN barrier layer increases the interface becomes smoother resulted in higher electron mobility.
Open Access
Phase boundary of the boson Mott insulator in a rotating optical lattice
(The American Physical Society, 2007) Umucalilar, R. O.; Oktel, M. Ö.
We consider the Bose-Hubbard model in a two-dimensional rotating optical lattice and investigate the consequences of the effective magnetic field created by rotation. Using a Gutzwiller-type variational wave function, we find an analytical expression for the Mott insulator (MI)-superfluid (SF) transition boundary in terms of the maximum eigenvalue of the Hofstadter butterfly. The dependence of phase boundary on the effective magnetic field is complex, reflecting the self-similar properties of the single particle energy spectrum. Finally, we argue that fractional quantum Hall phases exist close to the MI-SF transition boundaries, including MI states with particle densities greater than one.
Open Access
Real-time imaging of vortex-antivortex annihilation in Bi 2Sr2CaCu2O8+δ single crystals by low temperature scanning hall probe microscopy
(IOP Institute of Physics Publishing, 2006) Dede, M.; Oral, A.; Yamamoto, T.; Kadowaki, K.; Shtrikman, H.
Vortices in superconductors play an important role in operating limits and applications of the superconductors. Scanning Hall probe microscopes have proven themselves to be quantitative and non-invasive tools for investigating magnetic samples down to 50 nm scale. Penetration of vortices in high quality single crystal Bi2Sr2CaCu2O8+δ superconductor has been studied in real-time with single vortex resolution at 77 K using a low temperature scanning Hall probe microscope (LT-SHPM). Vortices have been observed to be annihilated by the antivortices in small M-H loops.
Open Access
Room-temperature scanning Hall probe microscope (RT-SHPM) imaging of garnet films using new high-performance InSb sensors
(IEEE, 2002) Oral, Ahmet; Kaval, Murat; Dede, Münir; Masuda, H.; Okamoto, A.; Shibasaki, I.; Sandhu, A.
The room-temperature scanning Hall probe microscopy (RT-SHPM) imaging of garnet films using high-performance InSb sensors was discussed. The high-performance InSb micro-Hall sensors were fabricated by optical lithography. It was found that the room-temperature noise figure of the InSb sensors was 6-10 mG/√Hz, which was an order of magnitude better than GaAs-AlGaAs two-dimensional electron gas sensors.
Open Access
The self-consistent calculation of the edge states at quantum Hall effect (QHE) based Mach-Zehnder interferometers (MZI)
(Elsevier B.V., 2008) Siddiki, A.; Kavruk, A. E.; Öztürk, T.; Atav, U.; Şahin, M.; Hakioǧlu T.
The spatial distribution of the incompressible edge states (IES) is obtained for a geometry which is topologically equivalent to an electronic Mach-Zehnder interferometer, taking into account the electron-electron interactions within a Hartree type self-consistent model. The magnetic field dependence of these IES is investigated and it is found that an interference pattern may be observed if two IES merge or come very close, near the quantum point contacts. Our calculations demonstrate that, being in a quantized Hall plateau does not guarantee observing the interference behavior.
Open Access
A self-consistent microscopic model of Coulomb interaction in a bilayer system as an origin of Drag Effect Phenomenon
(Elsevier B.V., 2008) Güven, K.; Siddiki, A.; Krishna, P. M.; Hakioǧlu T.
In this work we implement the self-consistent Thomas-Fermi model that also incorporates a local conductivity model to an electron-electron bilayer system, in order to describe novel magneto-transport properties such as the Drag Phenomenon. The model can successfully account for the poor screening of the potential within the incompressible strips and its impact on the inter-layer Coulomb interaction. An externally applied current in the active layer results in the tilting of the Landau levels and built-up of a Hall potential across the layer, which, in turn, induces a tilted potential profile in the passive layer as well. We investigate the effect of the current intensity, temperature, magnetic field, and unequal density of layers on the self-consistent density and potential profiles of the bilayer system.
Open Access
Tkachenko modes and structural phase transitions of the vortex lattice of a two-component Bose-Einstein condensate
(American Physical Society, 2006) Keçeli, M.; Oktel, M. Ö.
We consider a rapidly rotating two-component Bose-Einstein condensate (BEC) containing a vortex lattice. We calculate the dispersion relation for small oscillations of vortex positions (Tkachenko modes) in the mean-field quantum Hall regime, taking into account the coupling of these modes with density excitations. Using an analytic form for the density of the vortex lattice, we numerically calculate the elastic constants for different lattice geometries. We also apply this method to calculate the elastic constant for the single-component triangular lattice. For a two-component BEC, there are two kinds of Tkachenko modes, which we call acoustic and optical in analogy with phonons. For all lattice types, acoustic Tkachenko mode frequencies have quadratic wave-number dependence at long wavelengths, while the optical Tkachenko modes have linear dependence. For triangular lattices the dispersion of the Tkachenko modes are isotropic, while for other lattice types the dispersion relations show directional dependence consistent with the symmetry of the lattice. Depending on the intercomponent interaction there are five distinct lattice types, and four structural phase transitions between them. Two of these transitions are second order and are accompanied by the softening of an acoustic Tkachenko mode. The remaining two transitions are first order and while one of them is accompanied by the softening of an optical mode, the other does not have any dramatic effect on the Tkachenko spectrum. We also find an instability of the vortex lattice when the intercomponent repulsion becomes stronger than the repulsion within components.
Open Access
Tkachenko modes of the square vortex lattice in a two-component Bose-Einstein condensate
(2006) Keçeli, Murat; Öktel, M. Özgür
We study Tkachenko modes of the square vortex lattice of a two-component Bose-Einstein condensate (BEC) in the mean-field quantum Hall regime, considering the coupling of these modes with density excitations. We derive the hydrodynamic equations and obtain the dispersion relations of the excitation modes. We find that there are two types of excitations, gapped inertial modes and gapless Tkachenko modes. These modes have two branches which we call acoustic and optical modes in analogy with phonons. The former has quadratic while the latter has linear wave-number dependence in both inertial and Tkachenko modes. Acoustic Tkachenko mode is found to be anisotropic while the other three modes are isotropic. The anisotropy of the acoustic Tkachenko mode reflects the four-fold symmetry of the square lattice.
Open Access
Transport properties of epitaxial graphene grown on SiC substrate
(National Institute of Optoelectronics, 2017) Ağızaçmak, S.; Tülek, R.; Gökden, S.; Teke, A.; Arslan, E.; Aygar, A. M.; Özbay, Ekmel
In this study, the Hall effect measurement of graphene on SiC substrate was carried out as a function of temperature (12-300 K). Hall data were first analyzed to extract the temperature dependent mobilities and carrier densities of the bulk (3D) and two dimensional (2D) channels using a Simple Parallel Conduction Extraction Method (SPCEM) successfully. High carrier mobility 2.296 cm2/V.s from the graphene layer and low carrier mobility 813 cm2/V.s from the SiC were obtained at room temperature. By using SPCEM extracted data, 3D and 2D scattering mechanisms were analyzed and the dominant scattering mechanisms in low and high temperature regimes were determined. It was found that the transport was mainly determined by scattering processes in 2D graphene.