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S. V. Kravchenko

Publications and source records attributed to S. V. Kravchenko.

At least 37 records · Page 2Linked to original sources
arXiv2017

Indication of band flattening at the Fermi level in a strongly correlated electron system

Using ultra-high quality SiGe/Si/SiGe quantum wells at millikelvin temperatures, we experimentally compare the energy-averaged effective mass, $m$, with that at the Fermi level, $m_F$, and verify that the behaviours of these measured values are qualitatively different. With decreasing electron density (or increasing interaction strength), the mass at the Fermi level monotonically increases in the entire range of electron densities, while the energy-averaged mass saturates at low densities. The qualitatively different behaviour reveals a precursor to the interaction-induced single-particle spectrum flattening at the Fermi level in this electron system.

cond-mat.str-el↗
arXiv2017

Spin polarization and exchange-correlation effects in transport properties of two-dimensional electron systems in silicon

We show that the parallel magnetic field-induced increase in the critical electron density for the Anderson transition in a strongly interacting two-dimensional electron system is caused by the effects of exchange and correlations. If the transition occurs when electron spins are only partially polarized, additional increase in the magnetic field is necessary to achieve the full spin polarization in the insulating state due to the exchange effects.

cond-mat.str-el↗
arXiv2015

Ultra-high mobility two-dimensional electron gas in a SiGe/Si/SiGe quantum well

We report the observation of an electron gas in a SiGe/Si/SiGe quantum well with maximum mobility up to 240 m^2/Vs, which is noticeably higher than previously reported results in silicon-based structures. Using SiO, rather than Al_2O_3, as an insulator, we obtain strongly reduced threshold voltages close to zero. In addition to the predominantly small-angle scattering well known in the high-mobility heterostructures, the observed linear temperature dependence of the conductivity reveals the presence of a short-range random potential.

cond-mat.mes-hall↗
arXiv2014

The effective electron mass in high-mobility SiGe/Si/SiGe quantum wells

The effective mass, m*, of the electrons confined in high-mobility SiGe/Si/SiGe quantum wells has been measured by the analysis of the temperature dependence of the Shubnikov-de Haas oscillations. In the accessible range of electron densities, n_s, the effective mass has been found to grow with decreasing n_s, obeying the relation m*/m_b=n_s/(n_s-n_c), where m_b is the electron band mass and n_c~0.54*10^11 cm^-2. In samples with maximum mobilities ranging between 90 and 220 m^2/Vs, the dependence of the effective mass on the electron density has been found to be identical suggesting that the effective mass is disorder-independent, at least in the most perfect samples.

cond-mat.str-el↗
arXiv2013

Resistance asymmetry of a two-dimensional electron gas caused by an effective spin injection

We have performed conductivity measurements on a Si-MOSFET sample with a slot in the upper gate, allowing for different electron densities n_1 and n_2 across the slot. Dynamic longitudinal resistance was measured by a standard lock-in technique, while maintaining a large DC current through the source-drain channel. We find that in a parallel magnetic field, the resistance of the sample, R(I_DC), is asymmetric with respect to the direction of the DC current. The asymmetry becomes stronger with an increase of either the magnetic field or the difference between n_1 and n_2. These observations are interpreted in terms of the effective spin injection: the degree of spin polarisation is different in the two parts of the sample, implying different magnitudes of spin current away from the slot. The carriers thus leave the excess spin (of the appropriate sign) in the region around the slot, leading to spin accumulation (or depletion) and to the spin drift-diffusion phenomena. Due to the positive magnetoresistance of the two-dimensional electron gas, this change in a local magnetisation affects the resistivity near the slot and the measured net resistance, giving rise to an asymmetric contribution. We further observe that the value of R(I_DC) saturates at large I_DC; we suggest that this is due to electron tunnelling from the two-dimensional n-type layer into the p-type silicon (or into another "spin reservoir") at the slot.

cond-mat.mes-hall↗
arXiv2012

Critical Behavior of a Strongly Interacting 2D Electron System

With decreasing density $n_s$ the thermopower $S$ of a low-disorder 2D electron system in silicon is found to exhibit a sharp increase by more than an order of magnitude, tending to a divergence at a finite, disorder-independent density $n_t$ consistent with the critical form $(-T/S) \propto (n_s-n_t)^x$ with $x=1.0\pm 0.1$ ($T$ is the temperature). Our results provide clear evidence for an interaction-induced transition to a new phase at low density in a strongly-interacting 2D electron system.

cond-mat.str-el↗
arXiv2012

Influence of spin polarization on resistivity of a two-dimensional electron gas in Si MOSFET at metallic densities

Positive magnetoresistance (PMR) of a silicon MOSFET in parallel magnetic fields B has been measured at high electron densities n >> n_c where n_c is the critical density of the metal-insulator transition (MIT). It turns out that the normalized PMR curves, R(B)/R(0), merge together when the field is scaled according to B/B_c(n) where B_c is the field in which electrons become fully spin polarized. The values of B_c have been calculated from the simple equality between the Zeeman splitting energy and the Fermi energy taking into account the experimentally measured dependence of the spin susceptibility on the electron density. This extends the range of validity of the scaling all the way to a deeply metallic regime far away from MIT. The subsequent analysis of PMR for low n >~ n_c demonstrated that the merging of the initial parts of curves can bee achieved only with taking into account the temperature dependence of B_c. It is also shown that the shape of the PMR curves at strong magnetic fields is affected by a crossover from a purely two-dimensional (2D) electron transport to a regime where out-of-plane carrier motion becomes important (quasi-three-dimensional regime).

cond-mat.str-el↗
arXiv2010

A metal-insulator transition in 2D: Established facts and open questions

The discovery of a metallic state and a metal-insulator transition (MIT) in two-dimensional (2D) electron systems challenges one of the most influential paradigms of modern mesoscopic physics, namely, that "there is no true metallic behavior in two dimensions". However, this conclusion was drawn for systems of noninteracting or weakly interacting carriers, while in all 2D systems exhibiting the metal-insulator transition, the interaction energy greatly exceeds all other energy scales. We review the main experimental findings and show that, although significant progress has been achieved in our understanding of the MIT in 2D, many open questions remain.

cond-mat.str-el↗
arXiv2009

Transport in strongly correlated two dimensional electron fluids

We present an overview of the measured transport properties of the two dimensional electron fluids in high mobility semiconductor devices with low electron densities, and of some of the theories that have been proposed to account for them. Many features of the observations are not easily reconciled with a description based on the well understood physics of weakly interacting quasiparticles in a disordered medium. Rather, they reflect new physics associated with strong correlation effects, which warrant further study.

cond-mat.dis-nn↗
arXiv2009

Conductance asymmetry of a slot gate Si-MOSFET in a strong parallel magnetic field

We report measurements on a Si-MOSFET sample with a slot in the upper gate, allowing for different electron densities n_{1,2} across the slot. The dynamic longitudinal resistance was measured by the standard lock-in technique, while maintaining a large DC current through the source-drain channel. We find that the conductance of the sample in a strong parallel magnetic field is asymmetric with respect to the DC current direction. This asymmetry increases with magnetic field. The results are interpreted in terms of electron spin accumulation or depletion near the slot.

cond-mat.mes-hall↗
arXiv2008

Electron transport in a slot-gate Si MOSFET

The transversal and longitudinal resistance in the quantum Hall effect regime was measured in a Si MOSFET sample in which a slot-gate allows one to vary the electron density and filling factor in different parts of the sample. In case of unequal gate voltages, the longitudinal resistances on the opposite sides of the sample differ from each other because the originated Hall voltage difference is added to the longitudinal voltage only on one side depending on the gradient of the gate voltages and the direction of the external magnetic field. After subtracting the Hall voltage difference, the increase in longitudinal resistance is observed when electrons on the opposite sides of the slot occupy Landau levels with different spin orientations.

cond-mat.mes-hall↗
arXiv2007

Flow diagram of the metal-insulator transition in two dimensions

The discovery of the metal-insulator transition (MIT) in two-dimensional (2D) electron systems challenged the veracity of one of the most influential conjectures in the physics of disordered electrons, which states that `in two dimensions, there is no true metallic behaviour'; no matter how weak the disorder, electrons would be trapped and unable to conduct a current. However, that theory did not account for interactions between the electrons. Here we investigate the interplay between the electron-electron interactions and disorder near the MIT using simultaneous measurements of electrical resistivity and magnetoconductance. We show that both the resistance and interaction amplitude exhibit a fan-like spread as the MIT is crossed. From these data we construct a resistance-interaction flow diagram of the MIT that clearly reveals a quantum critical point, as predicted by the two-parameter scaling theory (Punnoose and Finkel'stein, Science 310, 289 (2005)). The metallic side of this diagram is accurately described by the renormalization group theory without any fitting parameters. In particular, the metallic temperature dependence of the resistance sets in when the interaction amplitude reaches gamma_2 = 0.45 - a value in remarkable agreement with the one predicted by the theory.

cond-mat.str-el↗
arXiv2006

Thermodynamic magnetization of a strongly correlated two-dimensional electron system

We measure thermodynamic magnetization of a low-disordered, strongly correlated two-dimensional electron system in silicon. Pauli spin susceptibility is observed to grow critically at low electron densities - behavior that is characteristic of the existence of a phase transition. A new, parameter-free method is used to directly determine the spectrum characteristics (Lande g-factor and the cyclotron mass) when the Fermi level lies outside the spectral gaps and the inter-level interactions between quasiparticles are avoided. It turns out that, unlike in the Stoner scenario, the critical growth of the spin susceptibility originates from the dramatic enhancement of the effective mass, while the enhancement of the g-factor is weak and practically independent of the electron density.

cond-mat.str-el↗
arXiv2006

Conductivity of a spin-polarized two-dimensional electron liquid in the ballistic regime

In the ballistic regime, the metallic temperature dependence of the conductivity in a two-dimensional electron system in silicon is found to change non-monotonically with the degree of spin polarization. In particular, it fades away just before the onset of complete spin polarization but reappears again in the fully spin-polarized state, being, however, suppressed relative to the zero-field case. Analysis of the degree of the suppression allows one to distinguish between the screening and the interaction-based theories.

cond-mat.str-el↗
arXiv2006

Manifestation of the Exchange Enhancement of the Valley Splitting in the Quantum Hall Effect Regime

We report a new "dip" effect in the Hall resistance, R_{xy}, of a Si metal-oxide-semiconductor field-effect transistor in the quantum Hall effect regime. With increasing magnetic field, the Hall resistance moves from the plateau at Landau filling factor ν=6 directly to the plateau at ν=4, skipping the plateau at ν=5. However, when the filling factor approaches ν=5, the Hall resistance sharply "dives" to the value 1/5(h/e^2) characteristic of the ν=5 plateau, and then returns to 1/4(h/e^2). This is interpreted as a manifestation of the oscillating exchange enhancement of the valley splitting when the Fermi level is in the middle between two adjacent valley-split Landau bands with the asymmetric position of the extended states.

cond-mat.mes-hall↗