Abstract
We report on absolute magneto-transmission experiments on highly doped quasi-free-standing epitaxial graphene targeting the classical-to-quantum crossover of the cyclotron resonance. This study allows us to directly extract the carrier density and also other relevant quantities such as the quasiparticle velocity and the Drude weight, which is precisely measured from the strength of the cyclotron resonance. We find that the Drude weight is renormalized with respect to its non-interacting (or random phase approximation) value and that the renormalization is tied to the quasiparticle velocity enhancement. This finding is in agreement with recent theoretical predictions, which attribute the renormalization of the Drude weight in graphene to the interplay between broken Galilean invariance and electron–electron interactions.
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1. Introduction
The low-frequency dynamical conductivity, , where vF stands for the (Fermi) velocity parameter and n is the free-carrier concentration. Such an effective single-particle (ESP) picture correctly reproduces the measured strength of interband-absorption processes with the characteristic onset at 2|EF| and the dispersionless universal amplitude
of intraband processes (the so-called Drude weight):
![Equation (1)](https://content.cld.iop.org/journals/1367-2630/14/9/095008/revision1/nj435878eqn1.gif)
in which mc = |EF|/v2F accounts for the effective cyclotron mass of carriers at the Fermi energy.
The experimental measurements available so far [12, 13] are in conflict with this simple prediction. They suggest a suppression of the Drude weight up to 40% (notably, in graphenes with high carrier concentrations) and therefore raise the issue of the applicability of the ESP picture to describe the dynamical conductivity of graphene. These doubts are additionally fostered by a startling report on non-vanishing absorption below the 2|EF| threshold for interband processes [11].
Formally, the Drude approach follows from the commonly used random phase approximation (RPA) [14], which indeed grants equation (1), but, with the velocity parameter fixed at its bare value [15] vF,0 = 0.85 × 106 m s−1—the value of vF,0 can be accurately estimated, for example, from calculations based on density-functional theory at the local density approximation (LDA) level [16–18]. Markedly, vF,0 is smaller than the apparent (renormalized) Fermi velocity, vF = (1.0–1.1) × 106 m s−1, which is derived from most of the spectroscopy studies on graphene-based systems [19–27] (and from beyond-LDA theoretical approaches). In fact, the Drude weight determined by the bare velocity, , could partially explain the experimental claims [12, 13] of a suppressed
. However, the RPA approach leading to equation (1) with vF replaced by vF,0 misses some important physics [15]. A more adequate formalism, based on many-body diagrammatic perturbation theory [15], provides a more complex scenario in which self-energy (i.e. velocity enhancement) and vertex (i.e. excitonic) corrections compete with each other. Nevertheless, when the carrier concentration is sufficiently high, the approximate validity of the ESP model of non-interacting Dirac-fermion quasiparticles is expected to be recovered and equation (1), with the renormalized (apparent) velocity parameter, may be justified. Thus the issue of the strength of the Drude weight in graphene, one of its very basic electronic properties, remains a matter of controversy.
In this paper, we report on testing the ESP model against the results of cyclotron-resonance (CR) absorption experiments, performed on highly doped graphene specimens. Consistently following the ESP picture, we note that the Drude weight is fully transformed into the strength of the CR absorption in the classical limit and, with a good precision, into the strength of inter-Landau-level (LL) transitions in the quantum limit, i.e. into a quantity which is accurately determined from our absolute magneto-transmission measurements. The carrier concentration is deduced from the analysis of the oscillatory broadening of the CR response with LL filling factor, whereas the effective mass (and, in turn, the apparent Fermi velocity) is directly read from the dispersion of the CR with magnetic field. The Drude weight, as measured from the strength of the CR absorption, is found to be consistent with the estimate given by equation (1) in which the velocity parameter has the meaning of its apparent (spectroscopically measured or theoretically renormalized) value.
2. Experimental results
The investigated graphene sheet was prepared by thermal decomposition of the Si-terminated substrate of 6H–SiC. Subsequent hydrogenation of the 'zero layer' yields a quasi-free-standing epitaxial graphene monolayer [28]. Such specimens are typically highly p-doped with the Fermi energy EF ≈ −0.3 eV and carrier mobility
To measure the far-infrared transmittance, a macroscopic area of the sample (≈4 mm2) was exposed to the radiation of a globar, which was analyzed by a Fourier transform spectrometer and delivered to the sample via light-pipe optics. The light was detected by a composite bolometer placed directly below the sample, kept at T = 1.8 K. From the substrate-normalized transmission T(B), the real part of the optical conductivity, ℜe [
Experimental data are presented in figures 1 and 2 in the form of the conductivity spectra and as a false-color plot of ℜe [
Figure 1. The real part of the longitudinal conductivity
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Standard imageFigure 2. Upper panel: LL fan chart with schematically shown CR transitions in the quantum regime. Lower panel: a color plot of the real part of the experimentally determined longitudinal optical conductivity
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Standard imageFigure 3. (a) Maximum of the CR absorption as a function of the magnetic field. The dashed line denotes the theoretical position of CR for the effective mass of m = 0.058m0. The departure of the CR position from the linear-in-B behavior at low fields is consistent with the appearance of confined magneto-plasmons [34, 43]. (b) Drude weight (area) extracted using a Drude–Lorentzian formula (see [34, 43]) and the directly read-out amplitude of ℜe [
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Standard imageIn high magnetic fields, the CR absorption still follows an overall linear-in-B evolution, consistent with the moderate mobility of the sample. However, at the same time, it first shows clear indications of the approaching quantum regime, in which CR absorption corresponds to transitions between well-defined adjacent LLs. A number of CR studies of graphene in this regime have already been performed; see, e.g., [19–27]. A typical response in the quantum regime closely reflects the properties of the LL spectrum:
![Equation (2)](https://content.cld.iop.org/journals/1367-2630/14/9/095008/revision1/nj435878eqn2.gif)
Inter-LL excitations follow a clear -dependence, and the CR absorption gains a multi-mode character [35] due to the non-equidistant level spacing. In the low-temperature limit we always get at least two CR absorption modes unless the Fermi level is placed precisely between two LLs.
A closer inspection of the data in figures 1 and 2 (upper panel) reveals broadening of the CR line that appears at magnetic fields of Bm ≈ 27, 20, 16 and 13.5 T and possibly also at 11.5 T. This broadening is best manifested as a decrease in the peak intensity, see figures 1 or 2, whereas the total peak area remains fairly constant, see figure 3(b). This sequence of field values matches the simple rule mBm = const (with m = 3,4,5,6 and 7) and allows us to identify the filling factors
The sequence of the CR line broadening mBm = const is compatible with two possible scenarios. Either the Fermi level in graphene is pinned by electronic states in the SiC substrate, i.e. , or the carrier density n simply remains constant with varying B. In the latter case, the density is given by |n| = const = Nfm
Using the ESP expression for the cyclotron frequency, , we obtain vF and EF. We find that vF = (0.99 ± 0.02) × 106 m s−1, which is sensibly larger than the 'bare' value vF,0, and |EF| = (325 ± 5) meV, which is in excellent agreement with the position of onset of the interband absorption (not discussed in this paper), which provides |EF| = (320 ± 10) meV. According to the ESP model (1), the deduced Fermi level implies the expected Drude weight
.
On the other hand, the Drude weight can be directly extracted from our data by estimating the area under ℜe [, see figure 3(b).
To compare both estimates of the Drude weight, the following corrections should be taken into account: (i) the coverage of the substrate by graphene is not full (see, e.g., AFM measurements on similarly prepared samples [34]), (ii) the hydrogenization process is not always complete—a small part of the sample remains covered only by the non-graphene 'zero layer' [47], (iii) bilayer graphene may also appear at selected locations [48] and (iv) the area below ℜe [
To sum up, we find that our two independent estimates of the Drude weight agree with each other with a precision better than 10% and conclude that we do not observe any significant deviation from the validity of the ESP model (1). Our findings thus do not support recent transmission studies [12, 13] in which a significant suppression of the Drude weight (in comparison with the ESP expectation) has been reported. The reason for this discrepancy remains unclear at the moment, although we believe that (at least a part of) the suppression of the Drude weight found in [12] might stem from the normalization procedure used, in which the Drude-type absorption of graphene in the regime of electron–hole puddles (when the sample is neutral on average) is neglected.
3. Discussion
The validity of the ESP model (1) for the description of the strength of intraband absorption reminds us of Landau's theory of normal Fermi liquids, which establishes a 'mapping' between a system of interacting fermions and a gas of weakly interacting quasiparticles with renormalized parameters [14]. The renormalized quasiparticle velocity vF is precisely one of these parameters, which is completely controlled by the real part of the quasiparticle self-energy evaluated at the Fermi level EF. Quasiparticles, however, do interact among each other and their interactions, which are described by the dimensionless Landau parameters Fs,aℓ, modify physical observables such as the compressibility and the spin susceptibility or, more generally, the macroscopic response of the interacting electron liquid to external fields [14]. Our experimental test of the validity of the ESP model (1) tells us that interactions among quasiparticles in graphene, which diagrammatically are encoded in 'vertex' or 'excitonic' corrections, are weak, at least at large carrier densities.
Our experimental findings and the approximate validity of equation (1) can also be interpreted in terms of an enhancement of the Drude weight with respect to its non-interacting value
, which is found to be given by
. This result is in excellent agreement with the theoretical predictions of Abedinpour et al [15]. By taking into account screening at the Thomas–Fermi level, the authors of [15] predicted
at a carrier density |n| = 8 × 1012 cm−2 and for a dimensionless coupling constant
For the sake of completeness, we should note that the Drude weight is strictly fully transferred into the CR strength only in the quasi-classical limit. In the quantum regime, the total CR weight
may differ from
. When the Fermi level is placed just in between two LLs (|
, but for the half-filled last occupied LL (|
[50]:
![Equation (3)](https://content.cld.iop.org/journals/1367-2630/14/9/095008/revision1/nj435878eqn3.gif)
However, one can easily verify that this suppression, which is related to the transfer of the oscillator strength between intra- and inter-band absorptions, remains very small for m ⩾ 2 (below 2%). The approximation is thus very applicable as long as the zero LL is not involved in the CR absorption.
4. Conclusions
To conclude, an absolute magneto-transmission experiment on highly doped quasi-free-standing graphene allowed us to follow the oscillator strength of the CR, which exactly matches the zero-field Drude weight. We found that this weight remains very close to and that a single-particle-like picture is fully sufficient to account for our data, provided that the renormalized (quasiparticle) parameters EF,vF and/or mc are considered. This implies that the Drude weight in a doped graphene sheet is enhanced with respect to its bare value due to electron–electron interactions and that the enhancement is tied to the Fermi velocity enhancement [51, 52]. Our findings are in excellent agreement with a recent theoretical work [15], which predicted an enhancement of the Drude weight in doped graphene stemming from the interplay between broken Galilean invariance and electron–electron interactions.
Acknowledgments
We acknowledge valuable discussions with A H MacDonald, G Vignale and Li Yang. This work has been supported by projects GACR no. P204/10/1020 and GRA/10/E006 within the ESF EuroGraphene program (EPIGRAT) and by the Swiss National Science Foundation (SNSF), grant 200021-120347, through the National Centre of Competence in Research 'Materials with Novel Electronic Properties-MaNEP'. We also acknowledge funding received from EuroMagNETII under the European Union contract no. 228043. Work in Erlangen was supported by the DFG and the ESF within the EuroGraphene project GRAPHIC-RF and by the European Union within the project ConceptGraphene. MP was supported by the Italian Ministry of Education, University, and Research (MIUR) through the program 'FIRB—Futuro in Ricerca 2010' grant no. RBFR10M5BT ('PLASMOGRAPH: plasmons and terahertz devices in graphene').
Appendix.: Extraction of the optical conductivity from transmission spectra
The experimentally obtained substrate-normalized transmission T(B) spectra are used to extract the optical conductivity, Re{
![Equation (A.1)](https://content.cld.iop.org/journals/1367-2630/14/9/095008/revision1/nj435878eqnA.1.gif)
Here Ns = ns + i ks is the experimentally known complex refractive index and ds is the thickness of the substrate, Z0 = 377
Figure A.1. The schematic representation of the sample and the definitions of the complex coefficients of equation (A.1). A near-normal incidence was used; the rays on the figure are inclined for clarity.
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Standard imageIn the derivation of the ratio of the transmission coefficient of the sample to that of the bare substrate, all internal reflections in the substrate must be taken into account. The experimental spectral resolution was reduced to 4 cm−1 in order to suppress the Fabry–Perot interference in the SiC substrate. In this case, the internal reflected rays add incoherently. By plane-wave counting, the experimental transmission coefficients of the bare substrate and the sample are derived in terms of the complex coefficients:
![Equation (A.2)](https://content.cld.iop.org/journals/1367-2630/14/9/095008/revision1/nj435878eqnA.2.gif)
The substrate-normalized transmission follows from equations (A.1) and (A.2):
![Equation (A.3)](https://content.cld.iop.org/journals/1367-2630/14/9/095008/revision1/nj435878eqnA.3.gif)
In the experimental spectral range, ks ≪ ns, therefore ks is neglected in all complex coefficients (A.1), except in
![Equation (A.4)](https://content.cld.iop.org/journals/1367-2630/14/9/095008/revision1/nj435878eqnA.4.gif)
Evidently, from the experimental transmission spectra alone, the complex conductivity