You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
electron spin resonance ( esr ) has proven to be an important method in identifying the ground state of strongly correlated electron systems .
esr helped e.g. to identify the ordered spin - density wave ground state in the bechgaard salts @xcite and for carbonaceous materials , esr was key to discover the ac@xmath0 ( a = k , rb , cs ) fulleride polymer @xcite . +
a natural expectation is that esr can be applied for single - wall carbon nanotubes ( swcnts ) @xcite and graphene @xcite , which are the two novel members of the carbon nanostructure family .
the esr literature on graphene is yet restricted to a single report @xcite .
although there exists larger literature on the swcnts , the situation is yet unclear . in general , the esr signal on itinerant electrons yields a direct measurement of the spin - relaxation time ( often called as spin - decoherence time ) , @xmath1 , through the relation : @xmath2 , where @xmath3 is the homogeneous esr line - width and @xmath4 is the electron gyromagnetic ratio .
@xmath1 is the central parameter which characterizes the usability of the materials for spintronics .
this explains the motivation of the esr studies on graphene and swcnts .
+ one important question is whether the esr signal of the itinerant ( i.e. the conduction electrons ) can be observed at all .
it was argued on a theoretical basis @xcite that it can not be observed due to the tomonaga - luttinger liquid ground state of the metallic swcnts @xcite .
it seemed that the only way to explore the local magnetism in swcnts is to spin label it either by means of @xmath5c nuclei @xcite or by an electron spin label @xcite .
the literature situation on the swcnt esr studies is conflicting , and it is reviewed herein without any judgement on validity .
@xcite reported the observation of the esr signal of itinerant electrons .
@xcite reported that the esr signal occuring around @xmath6 is caused by defects in the swcnts .
@xcite reported that a similar signal is related to the itinerant electrons with a possible antiferromagnetic order at low temperature .
@xcite reported the observation of the itinerant electron esr in swcnt samples prepared by chemical vapor deposition .    often , the identification of the itinerant electron esr signal is based on two facts : the asymmetry of the esr lineshape ( also known as a dysonian ) and the temperature independence of the esr signal intensity .
the dysonian lineshape also occurs for localized spins ( e.g. for paramagnetic impurities ) which are embedded in a metal thus this property can not be used for the above identification .
this is discussed as eq .
3.6 in the seminal paper of feher and kip as the `` slowly diffusing magnetic dipole case '' @xcite .
the temperature independence of the esr intensity could be observed for localized paramagnetic spins when they are embedded in a metal with increasing conductivity , @xmath7 with decreasing temperature ; then the microwave penetration depth @xmath8 ( here @xmath9 is the permeability of the vacuum and @xmath10 is the frequency of the microwaves ) .
there has been remarkable progress in the quest for the intrinsic esr signal in swcnts using samples made of nanotubes separated according to their metallicity @xcite .
however , both kinds of samples , i.e. those made of purely metallic or semiconducting nanotubes shows similar esr signals @xcite , thus the situation remains unresolved .
a parallel situation happened for high @xmath11 superconductors : soon after their discovery @xcite several reports claimed to have observed the `` intrinsic '' esr signal in these compounds .
later it turned out for all studies that the signal of parasitic phases ( which happen to have strong paramagnetic signals ) , the so - called green and brown - phases were observed .
later , spin labeling ( e.g. gd substituting y in yba@xmath12cu@xmath13o@xmath14 ) turned out to be successful to study the electronic structure @xcite .
the esr signal of itinerant electrons in the swcnts is expected to have i ) a @xmath15-factor near 2 , ii ) a line - width , @xmath3 smaller than 1 mt , and iii ) a signal intensity corresponding to the low density of states ( dos ) with no temperature dependence .
all properties present a significant hindrance for the signal identification since most impurity in carbon have @xmath16 , a maximum 1 mt line - width , and the curie spin - susceptibility of even a small amount of impurity overwhelms the small pauli susceptibility of the itinerant electrons . since nothing is known about the @xmath15-factor and the line - width _ a priori _ , only the magnitude of the calibrated esr signal when compared to the theoretical estimates of the pauli spin - susceptibility provides a clear - cut esr signal identification in graphene or swcnts .    here , we outline the method to determine the calibrated esr signal intensity and the resulting dos in one- and two - dimensional carbon . the method is demonstrated for k doped graphite powder which is regarded as a model system of biased graphene @xcite .
a good agreement is obtained between the theoretical and expeirmental dos for the kc@xmath17 doped graphite system .
we note , that a similar program was applied successfully when the esr signals of rb@xmath13c@xmath0 @xcite and mgb@xmath18 @xcite were discovered .
we give benchmarks which can be used to decide whether the esr of the itinerant electrons is observed in graphene .
we used commercial graphite powder ( fischer scientific ) and potassium ( 99.95 % purity : sigma - aldrich ) for the intercalation experiments . the graphite powder ( 3 mg )
was mixed with 3 mg mno : mgo powder ( mn concentration 1.5 ppm ) and ground in a mortar .
mgo separates the graphite powder pieces , which enables the penetration of exciting microwave and its mn content acts as an esr intensity standard .
the mixture was vacuum annealed at 500 @xmath19 for 1 h in an esr quartz tube and inserted into an ar glove - box without air exposure .
alkali doping was performed by heating the esr quartz tube containing the graphite powder and potassium for 29 hours using the standard temperature gradient method in ref .
@xcite to obtain stage i , i.e. kc@xmath17 intercalated graphite .
esr measurements were performed with a jeol x - band spectrometer at room temperature .
cccccccccccc & & & & & & + & & & & & + & & & & & +    first , we discuss spin - susceptibility , @xmath20 , calculated from the esr signal in different dimensions .
esr spectroscopy measures the net amount of magnetic moments , which is an extensive thermodynamic variable , i.e. proportional to the sample amount .
the corresponding intensive variable , which characterizes the material is the spin - susceptibility , @xmath20 which reads as : @xmath21 where @xmath22 is the magnetic moment , @xmath23 is the magnetic field of the resonance , @xmath24 is the volume in @xmath25 dimension ( @xmath26 ) , and @xmath9 is the permeability of the vacuum . clearly , the unit of @xmath20 depends on the dimension @xmath25 .
@xmath20 is either due to the curie spin - susceptibility for non - interacting spins or the pauli spin - susceptibility for itinerant electrons in a metal .
the relevant expressions are given in table [ susceptibilities ] .
therein , @xmath27/@xmath28 denotes the unit area / volume , @xmath15 is the @xmath15-factor , @xmath29 is the bohr moment and @xmath30 is the boltzmann constant .
@xmath31 is the spin state of the non - interacting spins and @xmath32 is the dos at the fermi level in units of @xmath33 . here , @xmath34 refers to the unit chosen , e.g. for c@xmath0 fulleride salts , the unit could be 60 carbon atoms .
then the dos is larger but so is the unit volume which cancels in the result . for graphene
, the two atom basis is used as @xmath34 .
the esr intensity of a metal can be calibrated against a curie spin system with known amount of spins .
this leads to the comparison of the pauli and the curie spin - susceptibilities :    @xmath35    where @xmath36 denotes the esr signal . @xmath24 and @xmath37 are the volume of the sample and the unit cell in @xmath25 dimensions , respectively .
note that @xmath38 is the number of units in the metallic sample and @xmath39 is the number of curie spins .
( [ chi_curie_div_pauli ] ) is correct for both si and gaussian units and is independent of the choice of @xmath34 , as expected .    for @xmath40 and @xmath41 , eq .
( [ chi_curie_div_pauli ] ) simplifies to : @xmath42     esr spectrum of saturated k doped graphite powder sample at @xmath43300 k. the inset shows a zoom on the esr spectrum showing the presence of the six lines of the @xmath44 hyperfine structure .
the solid curve is a fit . ]    .
[ calib2 ] parameters of the @xmath20 calibration of kc@xmath17 . [ cols="<,^,^",options="header " , ]     we present the case of kc@xmath17 as an example of the esr intensity calibration . in fig .
[ graphdoped ] , we show the esr signal of the mixture of mno : mgo and saturated k doped graphite
. parameters of the calibration are given in table [ calib2 ] : @xmath45 is the spin concentration and the effective @xmath46 as only the @xmath47 transition is observed from the 5 zeeman transitions of the mn@xmath48 ( @xmath49 ) @xcite .
the sample content gives : @xmath50 and eq .
( [ chi_curie_div_pauli ] ) yields @xmath51 , in good agreement with @xmath52 obtained by specific heat measurements @xcite .    in the following ,
we analyze the case of graphene .
there , @xmath53 @xmath54 is the graphene elementary cell and the dos , at @xmath55 and @xmath56 ( @xmath57 is the damping parameter ) , reads as a function of the chemical potential @xmath58 @xcite : @xmath59 here , @xmath60 is the fermi velocity .
consequently , @xmath61 if @xmath58 is measured in ev .
thus eq .
( [ chi_c_div_p_simple ] ) ( in two dimensions ) at room temperature reads :    @xmath62 \frac{n(\text{gr})}{n(\text{curie } ) } \label{chi_graphene_div_curie } \end{split}\ ] ]    @xmath63 is the number of graphene unit cells in the sample .
finally , we assess the feasibility of esr spectroscopy on graphene .
esr spectrometer performance is given by the limit - of - detection ( lod@xmath64 ) i.  e.  the number of @xmath40 curie magnetic moments at room temperature which are required for a signal - to - noise ratio of @xmath65 for @xmath66 mt linewidth , and 1 s / spectrum - point time constant .
for modern spectrometers lod@xmath67 spins/0.1 mt . to calculate the lod for a broadened esr line , lod@xmath68 , we introduce a function to track the effect of broadening :    @xmath69    this function is 1 if @xmath70 and it is 10 if @xmath71 which is the usual maximum modulation amplitude . for line - widths above this value , the function grows quadratically , which describes that the amplitude of the derivative esr signal drops quadratically . using this function :
comparison with eq .
( [ chi_graphene_div_curie ] ) yields that numerically ( @xmath58 in ev units ) @xmath74 is the lod for graphene . we could conclude that @xmath75 which gives a lower bound for the area of the graphene sheet which enables the esr measurement . assuming a @xmath76 mt and a shift in chemical potential by gate bias of @xmath770.2 ev we estimate @xmath78 .
in summary , we detailed the method of obtaining the calibrated esr intensity and the dos in carbonaceous materials .
we argue that a similar analysis is required for the identification of the esr signal of itinerant electrons in swcnt and graphene .
work supported by the otka grant nr .
k 81492 , and nr .
k72613 , by the erc grant nr .
erc-259374-sylo , the marie curie erg project carbotron , and by the new szchenyi plan nr .
tmop-4.2.1/b-09/1/kmr-2010 - 0002 .
bd acknowledges the bolyai programme of the hungarian academy of sciences . the swiss nsf and its nccr `` manep '' are acknowledged for support .
26ifxundefined [ 1 ] ifx#1 ifnum [ 1 ] # 1firstoftwo secondoftwo ifx [ 1 ] # 1firstoftwo secondoftwo `` `` # 1''''@noop [ 0]secondoftwosanitize@url [ 0 ] 
+ 12$12  & 12#1212_12%12@startlink[1]@endlink[0]@bib@innerbibempty @noop * * ,   ( ) @noop * * ,   ( ) @noop * * ,   ( ) @noop * * ,   ( ) @noop * * ,   ( ) @noop * * ,   ( ) @noop * * , ( ) @noop * * ,   ( ) @noop * * ,   ( ) @noop * * ,   ( ) @noop * * ,   ( ) @noop * * ,   ( ) @noop * * ,   ( ) @noop * * ,   ( ) @noop * * ,   ( ) @noop * * ,   ( ) link:\doibase    10.1038/nnano.2006.52 [ * * ,   ( ) ] @noop * * ( ) @noop * * ,   ( ) @noop * * ,   ( ) link:\doibase { 10.1103/physrevb.79.205106 } [ * * ,   ( ) ] link:\doibase    10.1103/physrevlett.71.1091 [ * * ,   ( ) ] link:\doibase { 10.1103/physrevlett.87.047002 } [ * * ( ) , ] @noop * * ,   ( ) @noop _ _  ( ,  ,  ) link:\doibase    10.1103/revmodphys.81.109 [ * * ,   ( ) ]
electron spin resonance ( esr ) spectroscopy is an important tool to characterize the ground state of conduction electrons and to measure their spin - relaxation times . 
 observing esr of the itinerant electrons is thus of great importance in graphene and in single - wall carbon nanotubes ( swcnts ) . 
 often , the identification of cesr signal is based on two facts : the apparent asymmetry of the esr signal ( known as a dysonian lineshape ) and on the temperature independence of the esr signal intensity . 
 we argue that these are insufficient as benchmarks and instead the esr signal intensity ( when calibrated against an intensity reference ) yields an accurate characterization . 
 we detail the method to obtain the density of states from an esr signal , which can be compared with theoretical estimates . 
 we demonstrate the success of the method for k doped graphite powder . we give a benchmark for the observation of esr in graphene .