document: the layered organic material 3 ( ai3 ) , which has been studied since the 1980s,@xcite has recently attracted renewed interest because it reveals low - energy massless dirac fermions under hyrdostatic pressure ( @xmath2 gpa).@xcite compared to graphene , certainly the most popular material with low - energy dirac fermions@xcite or electronic states at the surface of three - dimensional topological insulators,@xcite ai3 is strikingly different in several respects . 
   apart from the tilt of the dirac cones and the anisotropy in the fermi surface,@xcite its average fermi velocity is roughly one order of magnitude smaller than that in graphene . 
   this , together with an experimentally identified low - temperature charge - ordered phase at ambient pressure,@xcite indicates the relevance of electronic correlations . 
   indeed , because the effective coupling constant for coulomb - type electron - electron interactions is inversely proportional to the fermi velocity , it is expected to be ten times larger in ai3 than in graphene . 
   the material ai3 thus opens the exciting prospective to study strongly - correlated dirac fermions that are beyond the scope of graphene electrons.@xcite . 
   another specificity of ai3 is the presence of additional massive carriers in the vicinity of the fermi level , as recently pointed out in ab - initio band - structure calculations.@xcite however , the interplay between massless dirac fermions and massive carriers has , to the best of our knowledge , not yet been proven experimentally . 
   finally , one should mention a topological merging of dirac points that is expected for high but experimentally accessible pressure.@xcite    here , we present magneto - transport measurements of ai3 crystals under hydrostatic pressure larger than @xmath3 gpa where dirac carriers are present . 
   we show not only the existence of high - mobility dirac carriers as reported elsewhere,@xcite but we prove also experimentally the presence of low - mobility massive holes , in agreement with recent band - structure calculations.@xcite the interplay between both carrier types at low energy is the main result of our studies . 
   furthermore , we show that the measured mobilities for the two carrier types hint at scattering mechanisms due to strongly screened interaction potentials or other short - range scatterers . 
   the remainder of the paper is organized as follows . in sec . 
   [ sec:1 ] , we present the experimental set - up and the results of the magneto - transport measurements ( sec . 
   [ sec:1.1 ] ) under hydrostatic pressure . 
   the subsection [ sec:1.2 ] is devoted to a discussion of the temperature dependence of the carrier densities , in comparison with the model of ( a ) massless dirac fermions and ( b ) massive carriers . 
   furthermore thermopower measurements are presented to corroborate the two - carrier scenario . 
   the measured temperature dependence of the extracted carrier mobilities is exposed in sec . 
   [ sec:1.3 ] , and a theoretical discussion of the experimental results , in terms of short - range ( such as screened coulomb ) scatterers may be found in sec . 
   [ sec:2 ] . 
   we present our conclusions and future perspectives in sec . 
   [ sec:3 ] . 
   the single crystals of ai3 used in our study have been synthesized by electro - crystallization . 
   their typical size is @xmath4 mm@xmath5 ( @xmath6 plane ) x @xmath7 m ( @xmath8 direction ) . 
   six @xmath9 nm thick gold contacts were deposited by joule evaporation on both sides of the sample , allowing for simultaneous longitudinal and transverse resistivity measurements . 
   a picture of one of the three samples studied is shown in the inset of figure [ magneto ] . 
   the resistivities were measured using a low - frequency ac lock - in technique . 
   the magnetic field @xmath10 , oriented along the @xmath8 axis , was swept between @xmath11 and @xmath12 t at constant temperature between @xmath13 and @xmath3 k. to account for alignment mismatch of patterned contacts , the longitudinal ( transverse ) resistivity has been symmetrized ( anti - symmetrized ) with respect to the orientation of @xmath10 to obtain even [ @xmath14 and odd [ @xmath15 functions respectively . 
   hydrostatic pressure was applied at room temperature in a nicral clamp cell using daphne 7373 silicone oil as the pressure transmitting medium . 
   the pressure was determined , at room temperature , using a manganine resistance gauge located in the pressure cell close to the sample . 
   the values given below take into account the pressure decrease during cooling . 
   the analysis of our data is based on the study of the magneto - conductivity and is similar to the one presented in ref . for multi - carrier semiconductor systems . 
   the magneto - conductivity is obtained from the measured resistivity tensor by means of @xmath16 $ ] . for a single carrier system , its analytical expression reads@xcite @xmath17 where @xmath18 , @xmath19 is the electron charge , @xmath20 the mobility , and @xmath21 is the carrier density . 
   figure [ magneto ] displays a typical magneto - conductivity curve of ai3 under pressure , where two ` plateaus ' can be clearly seen . as conductivity in ai3 
   has a strong 2d character , conductivity is shown both as 3d conductivity ( @xmath22 ) and as 2d conductivity ( @xmath23 of each bedt - ttf plane ) according to @xmath24 . 
   as conductivity is additive , in a two - carrier system , the contributions of each carrier type a and b can be added , @xmath25    the two `` plateaus '' , observed in fig . 
   [ magneto ] , indicate the existence of two different carrier types ( @xmath26 or @xmath27 ) with significantly different mobilities . from this curve 
   , we can extract the mobilities , @xmath28 , of each carrier type , their zero - field conductivities , @xmath29 , and their carrier densities , @xmath30 , by @xmath31 . 
   regime at high fields . 
   the left axis shows the square ( 2d ) conductivity of each bedt - ttf plane while the right axis shows the `` bulk '' ( 3d ) longitudinal conductivity ( see text ) . 
   inset : photograph of one sample . ] 
   figure [ sigmaxxt ] shows magneto - conductivity curves of ai3 at a fixed pressure for several temperatures . 
   the previous analysis has been repeated for each of these magneto - conductivity curves to obtain the densities ( fig . 
   [ density ] ) and mobilities ( fig . [ mobility ] ) for each carrier type as a function of temperature and for three different pressures , @xmath32 , @xmath33 and @xmath34 gpa . 
   the strong temperature dependence of the carrier density is a signature that temperature is higher than @xmath35 for both a and b carriers even at the lowest measured temperature , @xmath36 k. this low fermi temperature hints at the absence of charge inhomogeneities that prevent the approach of the dirac point in graphene on si0@xmath37 substrates.@xcite      the carrier density can be calculated from @xmath38 , where @xmath39 is the fermi - dirac distribution and @xmath40 is the density of states for massive ( @xmath41 ) and dirac ( @xmath42 ) carriers:@xcite     gpa for different temperatures , from bottom to top : @xmath3 , @xmath43 , @xmath34 , @xmath44 , @xmath45 , @xmath46 , @xmath47 , @xmath48 , @xmath49 , @xmath50 and @xmath51 k. the left axis shows the square ( 2d ) conductivity of each bedt - ttf plane while the right axis shows the `` bulk '' ( 3d ) longitudinal conductivity . ] 
   gpa , triangles @xmath33 gpa and squares @xmath34 gpa ; blue thin symbols for a carriers and red thick symbols for b carriers ) . 
   the left axis shows the density for each bedt - ttf plane ( @xmath52 ) while the right axis shows the bulk density ( @xmath53 ) . 
   the lines represent power - law fits of to the a and b carrier densities that yield exponents 0.9 and 2.2 , respectively . 
   inset : band structure calculations at @xmath54 gpa where both dirac cones ( a ) and parabolic bands ( b ) cross the fermi level ( adapted from ref . ) . ]     gpa , triangles : @xmath33 gpa and squares : @xmath34 gpa ; blue thin symbols for a carriers and red thick symbols for b carriers ) . 
   the continuous lines represent power laws fits for the mobilities dependences with temperature , which gives exponents @xmath55 ( a carriers ) and @xmath56 ( b carriers ) . 
   the low temperature dispersion of a carriers mobility is due to a decrease of the saturating mobility ( dotted line ) by increasing pressure . ] 
   @xmath57    @xmath58    where @xmath59 and @xmath60 are valley and spin degeneracies and @xmath61 is the effective mass of massive carriers described by a schrdinger equation . in ai3 under pressure , 
   two dirac cones but only one massive band exist at the fermi level.@xcite for large temperatures , @xmath62 , the carrier density depends linearly on temperature for massive carriers and quadratically for dirac carriers :    @xmath63    @xmath64    figure [ density ] represents the measured temperature dependence of the mobilities and reveals a power - law behavior , @xmath65 . 
   indeed one obtains an exponent of @xmath66 for the low - mobility carriers ( b ) , in good agreement with what [ eq . 
   ( [ nmassif ] ) ] expected for massive carriers , whereas one finds @xmath67 for the high - mobility carriers ( a ) , as roughly expected for massless dirac particles [ eq . ( [ ndirac ] ) ] . 
   besides , the nature of the carriers can be extracted from hall measurements . 
   furthermore , we have performed thermopower measurements under pressure on a second sample ( figure [ thermopower ] ) . 
   these data show a sign change for the seebeck coefficient ( s ) around 5k . 
   thermopower is the voltage per unit of temperature produced by a thermal gradient . 
   the carrier type determines the sign while the density and mobility of the carriers establish the amplitude . 
   thus , a sign change of the thermopower indicates that the relevant carriers at low temperature have a different charge than those at high temperature , requiring a two - carrier scenario .        in agreement with ref . 
   , a carriers which dominate the low - field conduction are electrons . on the contrary , at large fields 
   the conduction is dominated by holes ( b carriers ) . 
   notice that our results are consistent with ab - initio calculations of the band structure of ai3 under a pressure of @xmath68 gpa ( inset of figure [ density])@xcite and do not depend on pressure ( within the range @xmath69 gpa ) . 
   this supports the idea that massless and massive particles coexist in a broad pressure range 
   . however , since @xmath70 in the whole temperature range under study , both dirac electrons and dirac holes are excited . 
   thus there are indeed not two but three carrier types : dirac holes , dirac electrons and massive electrons . for @xmath62 , 
   the electron and hole densities are actually identical ( semimetal with symmetric band structure ) : @xmath71 . 
   the absence of a third ` plateau ' in the magneto - conductivity data allows us to consider that dirac electrons and holes have roughly the same mobilities : @xmath72 . 
   therefore , the results obtained in figure [ density ] and [ mobility ] still hold when we consider two types of dirac carriers ( electrons and holes ) in addition to the massive holes . 
   this analysis allows us to avoid using hall effect measurements for the determination of carrier densities . indeed , hall effect interpretation becomes challenging as dirac electron and hole contributions partially compensate , leading to the determination of only an ` effective ' dirac carrier density , and they are both mixed with massive carriers contribution . 
   this problem is solved here by analyzing the magneto - conductivity where all carriers contributions are additive . 
   the effective mass of the massive carriers has been extracted from eq . 
   ( [ nmassif ] ) . 
   the obtained value is quite small @xmath73 ( @xmath74 is the free electron mass ) . 
   meanwhile , from eq . 
   ( [ ndirac ] ) , @xmath75 m / s can be extracted , in agreement with previous theoretica@xcite and experimental estimates.@xcite in fig . [ density ] , no significant variation of this argument is observed upon sweeping pressure ( which should appear as a vertical shift of the @xmath76 line ) . 
   this indicates that @xmath77 does not change with pressure . in principle 
   , pressure should enhance hopping while reducing the unit cell volume . 
   thus , an enhancement of the @xmath77 with pressure could be expected according to the approximate expression @xmath78 , where @xmath79 is the hopping integral . 
   this expression can be simplified by means of harrison s law ( @xmath80 ) into @xmath81 . as pressure slightly modifies the lattice constant ( @xmath82gpa @xcite ) 
   , @xmath77 is expected to vary by the same order of magnitude which is smaller than our current experimental uncertainty . 
   this accounts for the apparent absence of pressure effects on the carrier density in the range @xmath83 gpa .    in fig . 
   [ mobility ] , the mobility of the dirac carriers ( a ) reaches @xmath84 cm@xmath5/vs at low temperatures ( @xmath86 k ) , a value comparable to already published values.@xcite it is quite high compared to typical graphene on 2 values ( @xmath87 to @xmath88 cm@xmath5/vs ) but similar to suspended graphene and graphene on bn mobilities at very low carrier density.@xcite on the other hand , the mobility for massive carriers is @xmath89 cm@xmath5/vs at @xmath86 k , which is two orders of magnitude smaller than for dirac carriers . 
   the temperature dependence of the mobility follows power laws for both massive ( exponent @xmath56 ) and dirac carriers ( exponent @xmath55 ) . 
   moreover , the dirac carrier mobility seems to saturate at @xmath90 . 
   a similar saturation has been reported in others dirac systems.@xcite table [ tabcomp ] summarizes the main parameters of massive and dirac carriers in ai3 , in comparison with graphene on 2 . 
   .dirac and massive carriers parameters in ai3 at high pressure , in comparison with graphene electrons . [ cols="<,<,<,<",options="header " , ] 
   in order to better understand the difference in the mobility , we investigate the ratio @xmath91 , in terms of the scattering times @xmath92 and @xmath93 for the massless dirac and massive carriers , respectively . 
   furthermore , @xmath94 is the density - dependent cyclotron mass of the dirac carriers , in terms of the fermi energy @xmath95 . 
   the scattering times may be obtained from fermi s golden rule ( for @xmath96 ) @xmath97 in terms of the impurity density @xmath98 , the matrix element @xmath99 , and the density of states ( [ ddirac ] ) for dirac and ( [ dmassif ] ) for massive carriers . 
   we consider implicitly that both carrier types are affected by the same impurities , and the matrix element is independent of @xmath100 for short - range impurity scattering . 
   apart from atomic defects , screened coulomb - type impurities approximately fulfill this condition , as it may be seen within the thomas - fermi ( tf ) approximation . 
   indeed , the screening length of the coulomb interaction is dominated by the thomas - fermi wave vector @xmath101 m@xmath102 of the massive carriers , for an effective bohr radius @xmath103 , whereas the thomas - fermi wave vector for massless dirac carriers @xmath104 m@xmath102 , for a density @xmath105 cm@xmath102 and a fine - structure constant @xmath107 . 
   the thomas - fermi wave vector is thus roughly one order of magnitude larger than the fermi wave vector of the massive carriers , which is itself much larger than that of the dirac carriers . 
   the screened coulomb potential for @xmath100-type carriers may therefore be approximated by its @xmath108 value , @xmath109 , which is thus the same for both carrier types , as mentioned above . 
   here , @xmath110 is the permittivity of the dielectric environment and @xmath111 is the dielectric function calculated within the thomas - fermi approximation .    in view of the above considerations , we thus obtain , for the mobility ratio in the limit @xmath112 @xmath113 which does neither depend on the form of the matrix element nor on the impurity density . 
   one expects a ratio in the @xmath114 range , whereas the measured ratio is @xmath115 at @xmath116 k. notice that for @xmath62 , that is in the experimentally relevant regime here , one may replace the energy dependence in the density of states of the massless dirac carriers by a linear dependence in temperature , @xmath117 , such that one expects a linear temperature dependence of the mobility ratio ( [ eq : mu ] ) , in agreement with our experimental findings ( @xmath118 for @xmath119 , see fig . [ mobility ] ) . 
   to conclude , we present an interpretation of magneto - transport in ai3 that indicates that both massive and dirac carriers are present even at high pressures . thermopower measurements performed on one of the three studied samples are also in agreement with this two carrier scenario . 
   so far in the literature , the conduction in this system has been attributed solely to dirac carriers.@xcite moreover , this coexistence holds with little perturbation in the whole range of pressure under study . as dirac carriers have high mobility , they dominate the conduction at low magnetic field and high temperatures . on the contrary , for high magnetic fields and low temperatures , 
   the massive holes drive the conduction properties . 
   this crossover can be clearly seen from our magneto - conductivity curves and is responsible for their peculiar ` plateau ' shape . 
   it should also be noted that a proper separation of massive carriers has to be done prior to using any expression that concerns solely dirac carriers . in order to confirm the picture of coexisting dirac and massive carriers , complementary studies , such as spectroscopic measurements , 
   are highly desirable but beyond the scope of the present paper . 
   k. s. novoselov , a. k. geim , s. v. morozov , d. jiang , m. i. katsnelson , i. v. gregorieva , s. v. dubonos , and a. a. firsov , nature * 438 * , 197 ( 2005 ) ; y. zhang , y .- w . tan , h. l. stormer , and p. kim , nature * 438 * 201 , ( 2005 ) . 
   castro neto , f. guinea , n.m.r . 
   peres , k.s . 
   novoselov , and a.k . 
   geim , rev . 
   phys * 81 * , 109 ( 2009 ) ; m. o. goerbig rev . 
   phys * 83 * , 1193 ( 2011 ) ; v. n. kotov , b. uchoa , v. m. pereira , f. guinea , and a. h. castro neto , rev . 
   * 84 * , 1067 .
summary: transport measurements were performed on the organic layered compound 3 under hydrostatic pressure . 
   the carrier types , densities and mobilities are determined from the magneto - conductance of 3 . 
   while evidence of high - mobility massless dirac carriers has already been given , we report here , their coexistence with low - mobility massive holes . 
   this coexistence seems robust as it has been found up to the highest studied pressure . 
   our results are in agreement with recent dft calculations of the band structure of this system under hydrostatic pressure . 
   a comparison with graphene dirac carriers has also been done 
   .    3 @xmath0 2 @xmath1