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Syuma Yasuzuka, [Shinya Uji](https://orcid.org/0000-0001-9351-6388), [Taichi Terashima](https://orcid.org/0000-0001-9239-0621), [Takako Konoike](https://orcid.org/0000-0002-6037-5782), David Graf, Eun Sang Choi, James S. Brooks, Hiroshi M. Yamamoto, Reizo Kato

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[Interplay between Angular-Dependent Magnetoresistance Oscillation and Charge-Ordered States in the Organic Conductor <i>β</i>′′-(ET)(TCNQ)](https://mdr.nims.go.jp/datasets/e0546169-e7ed-4902-b908-c5b84c30c434)

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Journal of the Physical Society of JapanInterplay between Angular-Dependent Magnetoresistance Oscillationand Charge Ordered States in the Organic Conductor β”-(ET)(TCNQ)Syuma Yasuzuka1,2 ∗, Shinya Uji3, Taichi Terashima3, Takako Konoike3, David Graf4, EunSang Choi4, James S. Brooks4 †, Hiroshi M. Yamamoto5,6, and Reizo Kato51Research Center for Applied Quantum Physics, Hiroshima Institute of Technology,Hiroshima 731-5193, Japan2National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0003, Japan3Research Center for Materials Nanoarchitectonics (MANA), National Institute forMaterials Science (NIMS), Tsukuba, Ibaraki 305-0003, Japan4National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida32310, USA5Condensed Molecular Materials Laboratory, RIKEN, Wako, Saitama 351-0198, Japan6Research Center of Integrative Molecular Systems, Institute for Molecular Science,Okazaki, Aichi 444-8585 JapanThis paper reports on the effect of pressure on the angular-dependent magnetoresistance andShubnikov-de Haas (SdH) oscillation for β”-(ET)(TCNQ), where ET and TCNQ stand forbis(ethylenedithio)tetrathiafulvalene and tetracyanoquinodimethane, respectively. At 0 kbar,the temperature dependence of the interlayer resistance shows three hump anomalies ataround T1 = 174 K, T2 = 72 K, and T3 = 22 K. At low temperatures below T3, both theYamaji oscillation and the periodic dip structure due to a commensurability effect are clearlyobserved in the angular dependence of the interlayer resistance at high magnetic fields. At 2.0kbar, the T1-anomaly is suppressed and the T2- and T3-anomalies shifts to lower temperatures.Below T3, similar Yamaji oscillation and the dip structure are evident. At 5.1 kbar, the T3-anomaly is removed, and only the dip structure is clearly observed at low temperatures. Fromthis finding, the dip structure is attributable to a commensurability effect between the possible4kF charge-density-wave in the TCNQ layers and the interlayer lattice potential. Although noYamaji oscillation is observed at 5.1 kbar, a higher SdH frequency is detected compared tothat at 0 kbar, suggesting the emergence of a magnetic breakdown orbit in the significantlyundulated Fermi surface originated from the ET layers. The temperature-pressure phase dia-gram of β”-(ET)(TCNQ) is determined from the resistance measurements.1/19J. Phys. Soc. Jpn.1. IntroductionLayered organic conductors, which are often regarded as strongly correlated electronsystems, have provided various interesting electronic states. Examples include charge- andspin-density-waves (CDW and SDW), charge ordering (CO) state driven by electron cor-relations, and unconventional superconductivity.1–8 Reflecting softness arising from van derWaals nature of the molecular packing, these electronic states can be controlled by apply-ing moderate pressure.7–18 To deeply understand such physical properties, their Fermi sur-faces (FSs) have been intensively studied by various techniques, e.g., quantum oscillations[de Haas-van Alphen (dHvA) or Shubnikov-de Haas (SdH) effect], periodic orbit resonance,angle-resolved photoelectron spectroscopy, and angular-dependent magnetoresistance oscil-lation (AMRO).6, 19–29In this paper, we report on the effect of pressure on the angular-dependent magnetore-sistance and Shubnikov-de Haas (SdH) oscillation of a quarter-filled layered organic con-ductor β”-(ET)(TCNQ), where ET and TCNQ stand for bis(ethylenedithio)tetrathiafulvaleneand tetracyanoquinodimethane, respectively.30, 31 The crystal structure has triclinic symmetryconsisting of ET and TCNQ layers and there exist one ET and one TCNQ in the unit cell asschematically depicted in Fig. 1(a).30, 31 According to the band structure calculation, a highlyone-dimensional (1D) band of TCNQ stacked along the c-axis and a warped 1D band of ETextended along the a-axis are formed. Thus, the FS with rectangular cross-sections, originallyformed of two pairs of orthogonal 1D FSs, are predicted as shown in Fig. 1(b).30, 31The temperature dependence of the resistivity for β”-(ET)(TCNQ) is found to have metal-lic behavior with three hump anomalies around T1 ∼ 175 K, T2 ∼ 80 K, and T3 ∼ 20 K.30, 31Optical spectroscopy and X-ray scattering studies revealed a checkerboard pattern of the COstate in the ET layers at room temperature.32, 33 Interestingly, this CO state disappears belowT1,32, 33 where the magnetic susceptibility also shows a kink.31 According to the optical con-ductivity measurement,33, 34 the coherence of the quasiparticle in the ET layer grows mono-tonically below T1, and the electronic state in the ET layer finally behaves as a metal withFermi liquid nature below T3, where several 2D SdH frequencies were observed. Below T3,the magnetic susceptibility shows a sharp drop and is independent of the magnetic field direc-tion.30, 31 Thus, the resistivity anomaly at T3 may be related to a density wave (DW) transitiondriven by an imperfect nesting of the FSs originated from the ET bands. In fact, an X-rayscattering study35 has found new satellite reflections with a modulation vector.∗yasuzuka@cc.it-hiroshima.ac.jp†Deceased in September 20142/19J. Phys. Soc. Jpn.(a)abcET layer TCNQ layerholeelectron(b)kckaET originTCNQ origin -orbitFig. 1. (Color online) (a) A schematic crystal structure of β”-(ET)(TCNQ). (b) Calculated Fermi surface (FS)of β”-(ET)(TCNQ), where the FSs originated from the ET and TCNQ layers are indicated by the dashed andsolid curves, respectively.30, 31 FS pocket labeled α-orbit in the same scale as the first Brillouin zone is shownon the bottom right side.On the other hand, the vibronic bands are observed in the optical conductivity polarizedparallel to the TCNQ stacking direction (i.e., c-axis) at 280 K and no additional vibronicbands are found down to 6 K.33, 34 This observation suggests a 4kF lattice modulation in theTCNQ layers below room temperature. Although the formation of a 4kF-CDW and the anni-hilation of the FSs originated from the TCNQ band are expected, the electronic state in theTCNQ layers is not well characterized in the infrared and Raman studies.32–34Below T3, measurements of the AMRO were performed extensively by the authors.27The AMRO consists of a long- and a short-period oscillations. The long period oscillation is3/19J. Phys. Soc. Jpn.ascribed to the Yamaji oscillation corresponding to the small FS pocket labeled α-orbit [seeFig. 1(b)].27, 36 The Yamaji oscillation arises from the orbital motion of the electrons on thecorrugated cylindrical FS.24 The peaks in the Yamaji oscillation appear when the averagedelectronic group velocity along the interlayer direction vanishes as the field is tilted. For asimply corrugated cylindrical FS, the peaks appear periodically as a function of tanθ, whereθ is a angle between the magnetic field and the least conducting axis. The shape of the α-orbit is found to be strongly elongated approximately along the c-axis, in agreement withprevious magneto-optical measurements.37 Judging from the shape of the FS, the α-orbitmay be generated by the imperfect nesting of the FSs originated from the ET layers.The short period oscillation, the periodic behavior of the resistance dips rather than thepeaks, has been discussed in terms of the Yoshioka model.38 When a periodic potential witha wave vector Q is turned on by a DW formation, the additional scattering of the carriersby the double periodicity (lattice and DW) increases the interlayer resistance. However, thisscattering effect is suppressed when the magnetic field direction satisfies a commensurabilitycondition depending on Q. This commensurability effect leads to periodic dips as a functionof the field angle. Based on the Yoshioka model,38 the superlattice vector Q is found to benearly parallel to the c-axis. One of the DW from the ET band or the possible 4kF-CDWfrom the TCNQ band will be the origin of the dip structures.27 However, it remains an openquestion as to which band plays a decisive role.To further investigate the electronic state, we have performed the measurements of theangular-dependent magnetoresistance under pressure for β”-(ET)(TCNQ) since the pressurecould selectively suppress the superlattice potentials associated with the DW and possible4kF-CDW.39, 40 In this paper, we will show that the angular-dependent magnetoresistance isdrastically changed by the pressure. In addition, a high frequency SdH oscillation above 30T is observed at 7.0 kbar, indicating a significant difference between the FSs at 0 kbar and7.0 kbar. The temperature-pressure phase diagram of β”-(ET)(TCNQ) is determined from theresistance measurements. These results provide important interplay between the electronicstate and pressre in β”-(ET)(TCNQ).2. ExperimentalTo obtain single crystals of β”-(ET)(TCNQ), a CH2Cl2 (or CH2Br2) solution of ET,TCNQ, and TIE(= tetraiodoethylene) was placed at room temperature, and the solvent wasallowed to evaporate slowly to dryness within 24 hours.30, 31 The interlayer resistance wasmeasured by a conventional four-probe AC technique with electric current along the b∗-axis4/19J. Phys. Soc. Jpn.100101102103R 300250200150100500T (K)0 kbar2.0 kbar5.1 kbarT1T2T3-0.6-0.4-0.20.00.20.40.6dR / dT (arb. unit)3002001000T (K)T3 T2T1P = 0 kbarFig. 2. (Color online) Resistance of β”-(ET)(TCNQ) as a function of temperature under three pressures.40 Theinset shows the temperature dependence of dR/dT at P = 0 kbar. Three resistance anomalies at T1, T2, and T3are indicated by the arrows.(the least conducting axis). Pressure was generated by use of a WC piston and a beryllium-copper clamp type cylinder. Daphne 7373 oil was used as a pressure transmitting liquid.41The pressures were calibrated from the NH4F I-II transition at room temperature. Since thepressure decreases by about 1.5 kbar between room temperature and 200 K,41 the pressreshown here is reduced by 1.5 kbar from the value at room temperature. The clamped pressurecell with samples inside was rotated in the b∗-c plane using a single-axis rotator. The angle θis the angle between the magnetic field and the least conducting b∗-axis within the b∗-c plane.The experiments were made with a 4He cryostat with a 15-T superconducting magnet at theNational Institute for Materials Science, Tsukuba, Ibaraki, Japan. Higher magnetic field ex-periments were performed in a 33-T resistive magnet at the National High Magnetic FieldLaboratory, Tallahassee, Florida.5/19J. Phys. Soc. Jpn.2.52.01.51.00.5R 1209060300-30-60-90(degrees)P = 5.1 kbarT = 1.7 K13.5 T12.0 T10.0 T8.0 T6.0 T4.0 T2.0 T1.0 T(c)c cb*50403020100R 1209060300-30-60-90(degrees)P = 2.0 kbarT = 1.8 K 13.5 T12.0 T10.0 T8.0 T6.0 T4.5 T3.0 T1.5 T(b)c cb*2520151050R 1209060300-30-60-90 (degrees)P = 0 kbarT = 1.8 K 13.8 T12.0 T10.0 T8.0 T6.0 T4.0 T2.0 T1.0 T(a)c cb*Fig. 3. (Color online) Angular-dependent magnetoresistance of β”-(ET)(TCNQ) for various magnetic fielldsunder pressures (a) P = 0 kbar, (b) P = 2.0 kbar, and (c) P = 5.1 kbar. In Figs. 3(a) and 3(b), the crosses anddashed lines show peaks of Yamaji oscillation and dips due to the commensurability effect, respectively. Thepositions of peaks and dips are independent of applied magnetic fields.3. Results and discussion3.1 Pressure effect on resistance anomaliesThe temperature dependence of the interlayer resistance under pressure reported in ref.40 is reproduced in Fig. 2. At 0 kbar, three hump anomalies are clearly observed. The insetshows the temperature dependence of dR/dT at 0 kbar. Three resistance anomalies at T1 =174 K, T2 = 72 K, and T3 = 22 K are indicated by the arrows. At 2.0 kbar, the anomaliesat T2 and T3 are slightly sifted to lower temperatures, while the anomaly at T1 is completelyabsent. The significant increase of the resistance with decreasing temperature below roomtemperature at 2.0 kbar shows that T1 is higher than room temperature. Since it is reportedthat the CO state in the ET layers disappears below T1,33 we could conclude that there is noCO state below room temperature at 2.0 kbar. At 5.1 kbar, the T3-anomaly is not observed andonly the T2-anomaly is still observed. The absence of the T3-anomaly shows that there is nonesting of the pair of the warped 1D FSs originating from the ET bands. Thus, we found thatthe electronic state of β”-(ET)(TCNQ) is drastically changed by the application of pressure.Although the origin of the T2-anomaly is still unclear, the semiconductor-like behaviorbelow T1 at 0 kbar or below room temperature under pressures may be associated with apossible 4kF-CDW gap. According to the optical conductivity measurement,33 the CO statein the ET layers collapses and bad metal (or charge disproportionation) with the incoher-ent electron transport appears below T1. Interestingly, the electron transport in the ET layersshows a crossover behavior into a coherent Fermi liquid state below T2, leading to the en-6/19J. Phys. Soc. Jpn.hanced conductive behavior below T2 (Note that T2 is defined as an inflection point in theeach R(T ) curve in Fig. 2.). Thus, the T2-anomaly doesn’t represent a thermodynamic phasetransition but the crossover behavior. The absence of the anomalies in the magnetic suscep-tibility around T231 is consistent with the above discussion. The temperature-pressure phasediagram for β”-(ET)(TCNQ) is discussed later.3.2 Angular-dependent magnetoresistanceFigure 3(a) shows the θ-dependence of the interlayer resistance in magnetic fields rotatedon the b∗-c plane. As pointed out previously,27 the oscillatory behavior consists of the resis-tance peaks due to the Yamaji oscillation and the dips due to the commensurability effect atambient pressure. As shown in Fig. 3(a), the Yamaji oscillation marked by crosses is domi-nantly observed at 4.0 T. This oscillation arises from the small pocket created by the imperfectnesting of the FSs originated from the ET layers.27, 37 Above 10 T, the dip structure is super-imposed, as indicated by the dashed lines. With increasing magnetic field, the dip structuresbecome sharper, whose positions are independent of the magnetic field strength. The resultsclearly show that the dips are not caused by quantum oscillation but by the commensurabil-ity effect.38 Similar dips due to the commensurability effect are observed in a DW phase ofα-(ET)2MHg(SCN)4 (M = K, Rb, Tl).42–44Figure 3(b) shows the θ-dependence of the interlayer resistance at 2.0 kbar, where T1 isprobably higher than room temperature. The oscillatory behavior at 2.0 kbar is very similarto that at 0 kbar, suggesting no significant change in the electronic structure at 2.0 kbar. Ataround θ = 0o, a new hump is induced above 6.0 T and is enhanced with increasing magneticfield. A similar hump has been reported in an organic superconductor with incommensuratesuperlattice potential45 and a high-Tc cuprate superconductor.46 The humps have been dis-cussed in terms of p-type or dxy-type staggered warping in the 2D FSs.47 Such staggeredwarping in 2D FS will be likely induced by applying pressure in β”-(ET)(TCNQ).Figure 3(c) shows the results at 5.1 kbar, where only the T2-anomaly is observed. Manysharp dips, whose positions are independent of the magnetic field strength, are observed. Thedips will be attributable to the commensurability effect, as observed in Figs. 3(a) and 3(b). NoYamaji oscillations are observed. In addition, the peak at θ = ±90o is evident at high fields,showing the coherent interlayer transport.48, 49 The pressure effect on the resistance peak at θ= ±90o is discussed later.To see the oscillations more clearly, we plot the second derivative curves of the magne-toresistance as a function of tanθ at three pressures in Fig. 4. At 0 kbar and 2.0 kbar, the dips7/19J. Phys. Soc. Jpn.with a short period are evident in a low angle region whereas the peaks with a long periodare observed more clearly in a high angle region. Both dips and peaks are periodic in tanθ.The observation of the dips and peaks clearly shows that the DW from the ET band or thepossible 4kF-CDW from the TCNQ band coexist with a 2D pocket at 2.0 kbar below T3; theelectronic structure at 2.0 kbar is essentially the same as that at ambient pressure.At 5.1 kbar, the peaks disappear and only the dips are visible. Since the peaks (Yamajioscillation) are ascribed to the α-orbit generated by the imperfect nesting of the FSs on theET layers, the absence of the peaks shows no FS nesting at 5.1 kbar: the DW state on the ETlayers is totally suppressed. On the other hand, the T2-anomaly is still observed, as seen inFig. 2. Therefore, we conclude that the dip structure is not attributable to the DW state on theET layers but to the possible 4kF-CDW phase in the TCNQ band.Note that the sharp dip structure in the angular-dependent magnetoresistance was reportedin Ref. 40, where the mechanism of the dip structure was discussed in terms of a 1D FS with- d2R/d (arb. units)-8 -6 -4 -2 0 2 4 6 8tan  + + +++ + +++++++++ + + ++++0 kbar2.0 kbar5.1 kbarFig. 4. (Color online) Second derivative curves of magnetoresistance as a function of tanθ at 13.8 T for 0 kbarand at 13.5 T for 2.0 kbar and 5.1 kbar. Both peaks (crosses) and dips (lines) are visible for 0 kbar and 2.0 kbar,although only dips are observed at 5.1 kbar. All peaks and dips are periodic in tanθ.8/19J. Phys. Soc. Jpn.-8-6-4-202468tandip ,    tanpeak-20 -10 0 10 20dip or peak number, N: 0 kbar: 2.0 kbar: 5.1 kbar: 0 kbar: 2.0 kbarYamaji oscillationsCommensurability effect       (Yoshioka model)Fig. 5. (Color online) Plot of tanθdip and tanθpeak versus N (dip or peak number) obtained from Figs. 3 and 4,where θdip and θpeak are the dip and peak angles, respectively.higher order corrugation, whose model is theoretically discussed by Blundell and Singleton.50An explanation based on the 1D FS scenario was not entirely convincing because it requiredan unphysical long-range transfer integral between the ET chains parallel to the a-axis.40Figure 5 shows the plot of tanθdip and tanθpeak versus N (dip or peak number), where θdipand θpeak, obtained from Figs. 3 and 4, represent the angles of dips and peaks, respectively.The slopes in these plots correspond to the periods of the dip and peak with respect to tanθ.As can be seen in Fig. 5, there is no significant change in the dip period under pressure,indicating that the superlattice in the possible 4kF-CDW phase is almost unchanged underpressure. The periodicity ∆ of the peaks associated with the Yamaji oscillation as a functionof tanθ is given by ∆ = π/(dkF),24 where d (= 20.36 Å) is the interlayer spacing and kF isFermi wave number. From the slope of the tanθ associated with the peak positions versus N,the periodicity ∆ is obtained and then kF is estimated to be 0.182 ± 0.010 Å−1 at 0 kbar and0.199 ± 0.004 Å−1 at 2.0 kbar, respectively. Although the shape of the α-orbit at 2.0 kbar isunknown, we expect that a similar nesting of the FSs from the ET bands occurs down to 2.0kbar.9/19J. Phys. Soc. Jpn.1.641.621.601.581.561.541.52R 10095908580(degrees)5.1 kbar(b)2 c42.542.041.541.040.540.039.5R 10095908580(degrees)2.0 kbar(a)2 cFig. 6. (Color online) Enlarged view at 13.5 T around θ = 90o of Figs. 3(b) and 3(c) for P = 2.0 kbar (a) andP = 5.1 kbar (b), respectively.Let us discuss the peak effect appearing at θ = ±90o for P = 2.0 kbar. Figure 6(a) showsthe enlarged view at 13.5 T around θ = 90o in Fig. 3(b), where a small peak can be seen. Sucha peak, as observed in various low dimensional conductors, is known to result from the closedorbits on the side of the warped 1D or cylindrical 2D FS.48, 49 The peak provides evidence forcoherent interlayer transport in the layered systems.48, 49 The width of the peak is proportionalto the interlayer transfer integral.48, 49 From Fig. 6(a), the peak width 2∆θc is estimated to be0.8o, which is nearly equal to that at ambient pressure.27 Since the magnetic field is rotatedon the b∗-c plane, the peak will be ascribed to the small closed orbit on the long-axis side ofthe cross section of the corrugated cylindrical FS related to the α-orbit.27Next, we discuss the peak effect for P = 5.1 kbar. Figure 6(b) shows the enlarged view at13.5 T around θ = 90o in Fig. 3(c). Compared with the resistance peak at 2.0 kbar, the width ofthe resistance peak is remarkably enhanced at 5.1 kbar. Since the resistance peak at 2.0 kbaris associated with the small pocket created by the nesting of the FSs originated from the ETlayers, it is likely that the enhanced peak structure at 5.1 kbar is caused by the corrugation ofthe warped 1D FSs formed by the ET band. From Fig. 6(b), the peak width 2∆θc is estimatedto be 2.8o, which is three times larger than that at ambient pressure and 2.0 kbar. This resultsuggests a significant enhancement of the interlayer transfer integral and corrugation of theFS formed by the ET band at 5.1 kbar.3.3 Shubnikov-de Haas effectFigure 7 shows the magnetic field dependence of the interlayer resistance over a temper-ature range from 0.5 to 4.2 K at 7.0 kbar. The interlayer resistance increases with increasing10/19J. Phys. Soc. Jpn.43210R 403020100B (T)"-(ET)(TCNQ)    P = 7.0 kbar0.5 K1.4 K1.7 K2.0 K2.5 K4.2 K3.703.653.603.553.503.453.40R 3433323130B (T)0.5 K0.6 K0.7 K0.8 K0.9 K1.1 K1.3 KFig. 7. (Color online) Interlayer resistance of β”-(ET)(TCNQ) as a function of magnetic field at various tem-peratures ranging from 0.5 to 4.2 K. The inset is the interlayer resistance for high-field range between 30 and33 T below 1.3 K, where high frequency SdH oscillations are found.magnetic field strength. The SdH oscillation resulting from the α-orbit with the frequency of137 T is observed at 0 kbar.27 In contrast, as shown in the inset of Fig. 7, high frequency SdHoscillations above 30 T are evident at 7.0 kbar.Figure 8(a) shows the Fourier transform spectrum of the SdH oscillations in the magneticfield range of 30 to 33 T. The high frequency of 4183 T indicates that the cross-sectional areaof the FS is approximately 30% of the first Brillouin zone. The Fourier transform amplitudedivided by temperature versus temperature, called the mass plot, is shown in Fig. 8(b). Asolid curve is the calculated result by the Lifshitz-Kosevich formula.19 The numerical fit ofthe data provides the effective mass of me = 4.5m0, where m0 is the free electron mass. Thiseffective mass obtained here is about four times larger than that in the α-pocket at 0 kbar.The observation of a larger FS with a heavier effective mass indicates a significant difference11/19J. Phys. Soc. Jpn.678910-523456789Amplitude / T (arb. units)1.21.00.80.60.4T  (K)"-(ET)(TCNQ)     P = 7.0 kbarme = 4.5m0 Amplitude (arb. units)80006000400020000F (T)"-(ET)(TCNQ)     P = 7.0 kbar     30 T < B < 33 T0.5 K0.6 K0.7 K0.8 K0.9 K1.1 K1.3 K(a)(b)Fig. 8. (Color online) (a) Fourier transform spectra of the SdH oscillations at 7.0 kbar. (b) The Fourier trans-form spectrum amplitude divided by temperature versus temperature (mass plot). A solid curve is the calculatedresult by the Lifshitz-Kosevich formula.between the FSs at 0 kbar and 7.0 kbar. The observations of the dip structure [Fig. 3(c)]and the T2-anomaly in R(T ) show the existence of the possible 4kF-CDW and the absenceof the FS originated from the TCNQ layers at 7.0 kbar. Therefore, the ET derived FSs areresponsible for the high frequency SdH oscillations. It is likely that the transfer integralalong the c-axis increases with increasing pressure, leading to a significant undulation of the12/19J. Phys. Soc. Jpn.FSs originated from the ET layers. No Yamaji oscillation at 5.1 kbar in Figs 3(c) and 4 showsthat the undulated FSs are not yet closed. Thus, a magnetic breakdown effect may play acrucial role for the observation of the SdH oscillations, which were not recognized in theprevious SdH study.39 The observation of the large effective mass of me = 4.5m0 suggests themass enhancement by the strong electron correlation in the ET band, which is a source of theCO state in the ET layers.3.4 T-P phase diagramFigure 9 shows the T -P phase diagram of β”-(ET)(TCNQ), derived from the resistancemeasurements (Fig. 2). There are four temperature regions I (T > T1), II (T1 > T > T2), III(T2 > T > T3), and IV (T < T3) as previously defined and discussed at 0 kbar by Uruichiet al.33 In region I, the CO state in the ET layers coexists with the possible 4kF-CDW in theTCNQ layers. In region II, the CO state in the ET layers collapses and the bad metal (orthe charge disproportionation) with the incoherent electron transport appears. According tothe optical conductivity measurements,33 the incoherent transport in the ET layers show acrossover behavior into a coherent Fermi liquid state across T2 with decreasing temperature.In region III, the coherence of the quasiparticles in the ET layers grows monotonically withdecreasing temperature. In region IV, the DW phase is formed in the ET layers, which coexistswith the possible 4kF-CDW in the TCNQ layers. In this region, small FS pockets are formedby the imperfect nesting of the FSs originated from the ET layers. A critical pressure for theDW phase, where T3 goes to zero, is estimated to be ∼ 5 kbar. Thus, we consider that theground state above 5 kbar is the possible 4kF-CDW, where the 1D FSs originated from theET layers are strongly undulated.Finally, let us discuss the similarities and differences between the pressure-inducedchanges of β”-(ET)(TCNQ) and other organic conductors. As shown in Fig. 9, T1 increaseswith increasing pressure. This behavior is completely different from the pressure effect on thetypical charge ordered insulators,51–53 where a metal-insulator transition temperature accom-panied with a CO state decreases with increasing pressure because of the enhanced electronicband overlap. The pressure dependence of the Raman spectra at room temperature showedthat the pressure plays the same role as lowering the temperature, i.e., thermal contractionof the lattice.33 Therefore, lowering the temperature or increasing the pressure around roomtemperature corresponds to decrease of V/t, where V is the inter-site Coulomb interactionand t is the inter-site hopping energy, leading to destabilization of the CO state and a resul-tant increase in T1 with increasing pressure. As the temperature decreases across T2, there13/19J. Phys. Soc. Jpn.1246810246810024681000Temperature (K)6543210Pressure (kbar)T1T2T3IVIIIIIIFig. 9. (Color online) T -P phase diagram of β”-(ET)(TCNQ) derived from Fig. 2. At 2.0 kbar, the T1-anomalymay occur at a temperature above room temperature. A critical pressure for the DW phase, where T3 goes tozero, is estimated to be ∼ 5 kbar. The dashed line dividing the regions II and III means the pressure dependenceof T2, where the bad metal with the incoherent electron transport show a crossover behavior into a coherentFermi liquid state with decreasing temperature. See section 3.4 for the electronic states in the regions I, II, III,and IV.is a crossover behavior from incoherent to coherent transport in the ET layers.33 This phe-nomenon is very similar to the crossover behavior from a high-temperature bad metal to alow-temperature Fermi liquid metal in the half-filled band systems such as κ-(ET)2X, whereX stands for monovalent anions.54, 55 On the lower temperature side, T3 decreases with in-creasing pressure and the DW state is completely suppressed around 5 kbar. Similar pressureeffects are reported for the SDW phase in the Bechgaard salts56, 57 and the CDW phase in theα-(ET)2KHg(SCN)4.42, 584. SummaryWe have studied the effect of pressure on the resistance anomalies, the angular-dependentmagnetoresistance, and SdH effect for β”-(ET)(TCNQ). When the T3-anomaly is observed,the Yamaji oscillations and the dips explained by the Yoshioka model are clearly observed.However, when the T3-anomaly is completely suppressed at 5.1 kbar, the Yamaji oscillations14/19J. Phys. Soc. Jpn.simultaneously disappear but the dips are more evident. Since the semiconductor-like be-havior below T1 at 0 kbar or below room temperature under pressure survives at 5.1 kbar,it is likely that the sharp dips are attributable to the possible 4kF-CDW in the TCNQ lay-ers. The observation of the high frequency SdH oscillation with the heavier effective massshows a significant difference between the FSs at 0 kbar and 7.0 kbar. The high frequencySdH oscillation likely results from the magnetic breakdown effect on the strongly undulatedFS originated from the ET layers under high pressure. The experimental results reported heredemonstrate that the measurement of angular-dependent magnetoresistance provides a pow-erful tool to investigate charge ordered states such as a DW and a 4kF-CDW states as well asthe conventional 2D FS studies.AcknowledgmentsThe authors thank Prof. M. Kimata, Prof. K. Yakushi, Dr. K. Enomoto, and Dr. T.Kawamoto for useful discussion and suggestion. The authors would like to thank Ms. M.Nishimura for her hearty co-operation in our experiments. This work was supported by WorldPremier International Research Center Initiative (WPI), MEXT, Japan. A portion of this workwas performed at the National High Magnetic Field Laboratory, which is supported by Na-tional Science Foundation Cooperative Agreement Nos. DMR-1644779, DMR-2128556 andthe State of Florida.15/19J. Phys. Soc. Jpn.References1) H. 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