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[Ryo Matsumoto](https://orcid.org/0000-0001-6294-5403), Sayaka Yamamoto, Shintaro Adachi, Hiromi Tanaka, Toru Shinmei, Tetsuo Irifune, [Yoshihiko Takano](https://orcid.org/0000-0002-1541-6928)

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[Exploration of pressure-induced superconductivity in CuSe-based compounds under high pressure](https://mdr.nims.go.jp/datasets/7e735a4f-09cc-4f8a-b568-3fbfda7c7c2d)

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Exploration of pressure-induced superconductivity in CuSe-based compounds under high pressureEur. Phys. J. B          (2025) 98:154 https://doi.org/10.1140/epjb/s10051-025-00993-4THE EUROPEANPHYSICAL JOURNAL BRegular Article - Solid State and MaterialsExploration of pressure-induced superconductivityin CuSe-based compounds under high pressureRyo Matsumoto1,a , Sayaka Yamamoto1,2, Shintaro Adachi3, Hiromi Tanaka4, Toru Shinmei5, Tetsuo Irifune5,and Yoshihiko Takano1,21 Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Tsukuba,Ibaraki 305-0047, Japan2 Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba,Ibaraki 305-8577, Japan3 Department of Mechanical and Electrical Systems Engineering, Faculty of Engineering, Kyoto University of AdvancedScience (KUAS), Kyoto 615-8577, Japan4 National Institute of Technology, Yonago College, Yonago, Tottori 683-8502, Japan5 Geodynamics Research Center, Premier Institute for Advanced Studies, Ehime University, Matsuyama, Ehime 790-8577,JapanReceived 2 April 2025 / Accepted 25 June 2025© The Author(s) 2025Abstract. The replacement of conducting layers in known layered superconducting compounds is a signif-icant challenge in the search for novel superconductors. In particular, the exploration of superconductorscan be accelerated by combining the application of high pressure for modifying the crystal structure. Inthis study, we investigate the electrical transport properties of the layered CuSe-based compound BiO-CuSe, which has the same structure as the 1111-type iron-based superconductor La(O,F)FeAs, underhigh pressure exceeding 80 GPa. In addition, as related CuSe-based compounds, the electrical propertiesof YBi2O4Cu2Se2 and Cu2Se are measured under high pressure. Although no clear signatures of bulksuperconductivity are observed in these CuSe-based compounds, filamentary superconductivity emerges inBiOCuSe and Cu2Se. Given the rapid advancements in high-pressure techniques in recent years, furtherexploration of high-pressure effects on layered materials with novel conducting layers is expected to leadto the discovery of next-generation superconducting materials.1 IntroductionResearch on superconductors faces a critical stagein the quest for exploration of next-generationhigh-transition-temperature (T c) materials. Recentadvancements in high-pressure techniques have led tothe discovery of new high-T c superconductors, includ-ing hydrides [1, 2], borides [3], and nickelates [4, 5].Notably, superconductors are often synthesized sys-tematically following the discovery of a parent com-pound. For instance, after the report of superconductiv-ity above 200 K in H3S [1, 2], related high-T c hydridessuch as LaH10 [6, 7], YH9 [8], and CeH9 [9] were con-tinuously discovered. To accelerate the exploration ofsuperconducting materials, it is crucial to investigatethe physical properties of unexplored compounds underSupplementary Information The online versioncontains supplementary material available at https://doi.org/10.1140/epjb/s10051-025-00993-4.a e-mail: MATSUMOTO.Ryo@nims.go.jp (correspondingauthor)high pressure, particularly by modifying conductionlayers in layered superconductors to identify new parentmaterials.Recently, layered CuSe-based compounds haveattracted significant attention due to their superiorfunctionalities, such as thermoelectric properties. Inparticular, BiOCuSe has been highlighted as a promis-ing candidate for intermediate-temperature thermo-electric applications [10]. Although BiOCuSe is origi-nally a semiconductor with a bandgap of 0.8 eV, itselectronic properties can be tuned via Te substitu-tion at the Se site [11], oxygen deficiency [12], andhigh-pressure application [13]. Interestingly, BiOCuSeadopts a layered ZrCuSiAs-type structure, which is thesame as that of the 1111-type iron-based superconduc-tor La(O,F)FeAs [14]. The electronic band structuresat around Fermi energy (EF) of BiOCuSe have beeninvestigated by a density functional theory (DFT) [15].The DFT calculation indicates that Cu and Se orbitalsmainly contribute the valence band maximum, which0123456789().: V,-vol 123http://crossmark.crossref.org/dialog/?doi=10.1140/epjb/s10051-025-00993-4&domain=pdfhttp://orcid.org/0000-0001-6294-5403https://doi.org/10.1140/epjb/s10051-025-00993-4mailto:MATSUMOTO.Ryo@nims.go.jp  154 Page 2 of 11 Eur. Phys. J. B          (2025) 98:154 suggests the CuSe-plane in BiOCuSe is an electri-cal conducting layer. The investigation of the com-bined effects of the elemental substitution, the introduc-tion of deficiency, and the high-pressure application iscrucial for evaluating the potential emergence ofsuperconductivity in carrier-tuned BiOCuSe andrelated compounds. YBi2O4Cu2Se2, which consists ofalternating YBi2O4 blocking layers and Cu2Se2 con-ducting layers along one axis, has been reported as ametallic derivative of BiOCuSe [16, 17]. According tothe DFT calculations, Cu and Se orbitals consist ofthe band crossing EF in YBi2O4Cu2Se2 [17]. A high-pressure study of YBi2O4Cu2Se2 is particularly intrigu-ing from the perspective of the exploration of supercon-ducting material. The investigation of possible super-conductivity in these compounds opens new vein ofnew group of materials such as recently discovered lay-ered oxychalcogenide superconductor Na2CoSe2O [18].Among binary Cu–Se systems, Cu2Se is focused as athermoelectric material because of the ultrahigh ther-moelectric figure of merit (ZT), which exceeds 4.0 nearthe structural phase transition between α- and β-Cu2Sein the temperature range of 340 K < T < 400 K [19]. Inaddition, Cu2Se exhibits diverse functionalities, includ-ing a possible charge density wave (CDW) state at lowtemperatures [20, 21], various crystal structures [19],and a pressure-induced electronic topological transi-tion [22]. By analysis from DFT calculations, both Cuand Se orbital contribute to the valence band maxi-mum [23]. The investigation of the electrical transportproperties of Cu2Se under high pressure is essential tounderstand deeper insight into its physical behavior.One of the key differences between these CuSe-basedcompounds and their isostructural Fe-based supercon-ducting counterparts is the limited tunability of carrierconcentration. In this context, the application of highpressure emerges as a promising approach to modify theelectronic structure, serving as an alternative to carrierdoping via elemental substitution.In this study, we examine the emergence of pressure-induced superconductivity in the layered CuSe-basedcompounds BiOCuSe and its metallic derivativeYBi2O4Cu2Se2, and binary Cu2Se. The crystal struc-ture and valence state of the obtained samples werecharacterized using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). High-pressureelectrical transport measurements were performedusing a diamond anvil cell (DAC) with the micro-electrodes. Structural information under high pres-sure was obtained from Raman spectroscopy in theDAC. Although the three CuSe-based compounds showno bulk superconductivity under examined pressureregions, we observed several anomalies due to filamen-tary superconductivity. Because the observation of fil-amentary signal indicates possible existence of super-conducting phase in these materials, the insight in thisstudy accelerates the future exploration of the super-conducting family consisting of CuSe-based conductinglayer.2 Experimental proceduresA polycrystalline sample of YBi2O4Cu2Se2 was syn-thesized via a solid-state reaction in a vacuum-sealedquartz tube. The starting materials of Bi2O3 (98%),Y2O3 (99.99%), Bi2Se3 (99.9%), CuO (99.9%), and Cu(99.9%) were mixed in a molar ratio of Y:Bi:O:Cu:Se= 1:2:4:2:2, ground thoroughly, and pelletized under200 MPa. The pellet of 5 mm diameter was sealed inan evacuated quartz tube, heated at 830 °C for 12 h,and then cooled in the furnace. As a byproduct, a smalltip of single-crystalline BiOCuSe was obtained insidethe quartz tube during the synthesis of YBi2O4Cu2Se2.Single crystals of Cu2Se were grown using a similar pro-cedure. Cu (99.9%) and Se (99.9%) were mixed in anatomic ratio of Cu:Se = 2:1 and sealed in an evacuatedquartz tube. The sample was heated to 700 °C and heldfor 2 h, followed by an increase to 830 °C for 24 h. Itwas then naturally cooled to room temperature in thefurnace.The obtained products were characterized by chem-ical composition analysis using a scanning electronmicroscope (SEM) using a JSM-6010LA (JEOL). Struc-tural analysis was performed using XRD with a Mini-Flex 600 diffractometer (Rigaku) and Cu Kα radia-tion (wavelength = 1.5418 Å). The lattice constants ofthe samples were determined from the measured XRDpatterns using PDIndexer [24]. The VESTA softwarewas used for crystal structure visualization [25]. XPSanalysis was conducted using the AXIS-ULTRA DLD(Shimadzu/Kratos) with monochromatic Al Kα radi-ation (photon energy = 1486.6 eV) under a pressureof the order in 10–9 Torr. The active Shirley methodfrom COMPRO software [26] was used for a backgroundsubtraction for the XPS spectrum. The binding energyof the sample was calibrated by measuring the valenceband spectrum of an Au reference in the same chamber.Resistance–temperature (R–T ) measurements wereperformed on the three compounds under both ambi-ent and high pressures. At ambient pressure, a con-ventional four-probe method was used with Au wiresand silver paste. For high-pressure measurements, anoriginally designed DAC with boron-doped diamond(BDD) electrodes [27, 28] and an undoped diamond(UDD) insulating layer [29] was used. The typical setupof DAC consisted of a nano-polycrystalline (NPD) [30]box-type anvil and a culet-type single crystalline anvilwith a 300 µm culet. A sample was placed on theBDD electrodes of the anvil, and resistance was mea-sured using the four-probe method. The probe currentfor resistance evaluation was determined from the cur-rent–voltage characteristics before measurement. Thesample chamber was fabricated using a stainless-steel(316 L) gasket with a drilled hole of 200 µm in diam-eter. The UDD insulating layer electrically separated123Eur. Phys. J. B          (2025) 98:154 Page 3 of 11   154 Fig. 1 a Powder XRD patterns of the obtained BiOCuSe, YBi2O4Cu2Se2 and Cu2Se. b SEM images of single crystallineBiOCuSe and c Cu2Se. d Cu 2p and e valence band XPS spectra in BiOCuSethe gasket from the electrodes. Cubic boron nitride wasused as the pressure-transmitting medium (PTM), withruby powders as a pressure manometer. Such the solidPTM generally provides non-hydrostatic pressure forthe sample in the case of DAC. The pressure in thesample space of the DAC was estimated by detectingthe shift of peak position of ruby fluorescence [31] andthe Raman spectrum of the diamond anvil [32] usingan inVia Raman Microscope (RENISHAW), which iscalibrated by the peak positions of these spectra atambient pressure. The control of temperature, the mea-surements of sample resistance between 300 and 2 K,and the application of a magnetic field were performedusing a physical property measurement system (Quan-tum Design).3 Results and discussion3.1 Characterization at ambient pressureFigure 1a presents the XRD patterns of the obtainedBiOCuSe, YBi2O4Cu2Se2, and Cu2Se. Almost the allobserved peaks are indexed to the known phase in thesecompounds such as tetragonal structure (P4/nmm)with the lattice constants of a = 3.948 and c = 8.974Å in BiOCuSe, tetragonal structure (I 4/mmm) witha = 3.880 and c = 24.099 Å in YBi2O4Cu2Se2, andtrigonal structure (R-3 m) with a = 4.120 and c =20.533 Å in Cu2Se. In the case of BiOCuSe and Cu2Se,the single crystalline samples are obtained, as shown inthe SEM image of Fig. 1b, c. In the electrical trans-port measurements, the single crystalline samples areused for BiOCuSe and Cu2Se. Among the elements inthe synthesized samples, the valence states of Cu inYBi2O4Cu2Se2 and Cu2Se are reported to have fluc-tuated in Cu+ and Cu2+ via an XPS analysis [17,33]. In this study, the valence states of Cu in BiO-CuSe were also investigated, as shown in Fig. 1d whichshows a Cu 2p core-level XPS spectrum. There are twomain peaks at 952.5 and 932.7 eV corresponding to Cu2p3/2 and 2p1/2 with the valence state of Cu+, whichis consistent with the formal charge valence of BiO-CuSe. Figure 1e displays the valence band spectrumof BiOCuSe. The electronic band structure appears toapproach the Fermi level, suggesting a metallic char-acteristic. According to previous reports, the electri-cal transport properties of BiOCuSe change drasticallyfrom semiconducting to metallic depending on the syn-thesis conditions [34]. In particular, when the synthe-sis temperature exceeds 730 °C, metallic BiOCuSe isobtained, which is consistent with our synthesis condi-tions and the observed metallic behavior.3.2 Electrical transport properties at ambientpressureFigure 2 presents the R–T curves of (a) BiOCuSe,(b) YBi2O4Cu2Se2, and (c) Cu2Se at ambient pres-sure. BiOCuSe exhibits a positive slope in R–T curvefrom 300 to 50 K, followed by a slight upturn below50 K. This metallic behavior is consistent with theXPS analysis, which indicates that the valence bandcrosses the Fermi level. The upturn is considered to beinfluence from slight amount of Cu deficiency in metal-lic BiOCuSe [10]. Similarly, YBi2O4Cu2Se2, known asa metallic derivative of BiOCuSe, displays metallicconduction without any noticeable anomalies. Cu2Sealso exhibits metallic behavior; however, a hump-likeanomaly appears around 120 K, which is attributedto the formation of a CDW ground state [20, 21]. Inall the examined CuSe-based materials of BiOCuSe,YBi2O4Cu2Se2, and Cu2Se, no superconductivity isobserved at ambient pressure.123  154 Page 4 of 11 Eur. Phys. J. B          (2025) 98:154 Fig. 2 Temperature dependence of electrical resistance of a BiOCuSe, b YBi2O4Cu2Se2, and c Cu2Se at ambient pressureFig. 3 a Typical configuration of diamond anvil cell with boron-doped diamond (BDD) electrodes and undoped diamond(UDD) insulating layer. The attached photo is a microscope image of the typical sample, such as BiOCuSe. b Temperaturedependence of electrical resistance in BiOCuSe under various pressures up to 23 GPa and c 87 GPa4 High-pressure effects4.1 BiOCuSeFigure 3a illustrates a typical configuration of theDAC with the BDD electrodes and the UDD insulatinglayer. As shown in the attached microscope image, asmall piece of the sample is placed on the BDD elec-trodes. Upon compression, the sample and electrodesestablish electrical contact, allowing for R–T measure-ments under high pressure, as presented in Fig. 3b, c.At the lowest pressure of 0.6 GPa, BiOCuSe exhibitsmetallic behavior with a slight upturn below 50 K, con-sistent with its transport properties at ambient pres-sure. However, as pressure increases to 3.9 GPa, theabsolute resistance rises significantly, and the R–Tcurve changes to semiconducting behavior. With fur-ther compression up to 23 GPa, the semiconductingnature becomes more pronounced. Interestingly, above31 GPa, the resistance begins to decrease unexpectedly.No saturation trend in resistance reduction is observedeven at the highest pressure of 87 GPa, indicating thatBiOCuSe undergoes pressure-induced metallization inextremely high-pressure regions. However, no signature123Eur. Phys. J. B          (2025) 98:154 Page 5 of 11   154 Fig. 4 a Raman spectraunder various pressures.b Pressure dependence inthe peak positions ofRaman active modes of M3,M4, and M5of superconductivity is detected at the maximum pres-sure applied in this study. Further experiments withhigher pressures, potentially using smaller culet diam-eters in diamond anvils, are necessary to explore thepossibility of pressure-induced superconductivity.Figure 4a presents the Raman spectra of BiOCuSeunder various pressures. At ambient pressure, twoRaman-active modes, A1g1 and A1g2, labeled as M3and M4, respectively, are observed within the measure-ment range [13]. These modes correspond to out-of-plane vibrations of Bi and Se. At ambient pressure,the M3 and M4 modes are clearly identified at 145.6and 176.6 cm−1, respectively. According to previousreports on the high-pressure behavior of Raman-activemodes [13], the M3 and M4 peaks gradually shift tohigher wavenumbers due to phonon hardening, and anadditional M5 peak emerges above 18.3 GPa. More-over, synchrotron XRD measurements under high pres-sure, as reported in the literature, indicate no struc-tural phase transition at least up to 31.6 GPa. Inour Raman analysis, the pressure-dependent spectralchanges, including the emergence of the M5 mode, arefully consistent with previously reported behavior, asshown in Fig. 4b. Furthermore, in the previously unex-plored pressure range above 40 GPa, no new peaksappear in our data up to 71 GPa, indicating that nostructural phase transition occurs in BiOCuSe withinthis pressure range.Although the signature of metallization in BiOCuSeis observed at extremely high pressure, the diamondanvil was broken above 87 GPa, and the pressure natu-rally decreased to ambient condition. Instead of furtherincreasing the pressure, we investigated the emergenceof superconductivity by recompressing a BiOCuSe sam-ple that had previously been compressed to 87 GPa andthen recovered to ambient pressure. Figure 5a presentsthe R–T curves under various pressures from 2.3 to 8.4GPa for the recovered BiOCuSe. At the lowest pres-sure, the sample exhibits metallic behavior from 300to 150 K, followed by an upturn in resistance at lowertemperatures. As the pressure up to 8.4 GPa, the semi-conducting character, indicated by the negative slopeof the R–T curve, becomes more pronounced, similarto the behavior observed in the first compression shownin Fig. 3b. However, above 10 GPa, the negative slopein the R–T curve decreases significantly, indicating thesuppression of semiconducting behavior, as shown inFig. 5b. At 25 GPa, a steep drop in resistance appears atlow temperatures. Beyond 25 GPa, the slope of the R–Tcurve continues to decrease, and the sample exhibitsnearly metallic behavior at 51 GPa. Figure 6a shows thenormalized R–T curves in the low-temperature regionabove 25 GPa. The onset of the resistance drop shiftsto higher temperatures with increasing pressure, andthe transition becomes sharper. The plots of temper-ature dependence in dR/dT are presented in Fig. S1to indicate onset temperature of the resistance drop.As shown in Fig. 6b, the resistance drop is systemati-cally suppressed under applied magnetic fields, provid-ing evidence for pressure-induced superconductivity inthe recompressed sample. Figure 6c presents the pres-sure dependence of the T c in recovered BiOCuSe. In ourmeasurements, superconductivity emerges at 4.3 K at25 GPa and rapidly increases to a maximum T c of 5.8 Kat 51 GPa. One possible origin of the observed super-conductivity is the presence of elemental Se [35–37] asan impurity or byproduct due to partial decompositionduring the initial compression up to 87 GPa. However,if the observed superconductivity were due to elementalSe, the pressure dependence of T c would be expectedto follow that of pure Se. In contrast, our results show adistinct pressure-dependent T c trend [36], as indicatedin Fig. 6c. Therefore, at this stage, we conclude thatthe observed superconductivity in recovered BiOCuSe123  154 Page 6 of 11 Eur. Phys. J. B          (2025) 98:154 Fig. 5 a Temperature dependence of resistance under various pressures from 1.9 to 8.4 GPa in the recovered BiOCuSewhich experiences the compression up to 87 GPa. b Pressure range of 8.4 to 25 GPa, and c 25 to 51 GPaFig. 6 a R–T properties in recovered BiOCuSe at low temperature region. b Magnetic field dependence of the drop ofresistance at 51 GPa. c Pressure dependence in T c with a comparison of that in elemental Se [36]originates from BiOCuSe itself or an unknown deriva-tive formed through decomposition, rather than fromelemental Se.Figure 7 presents the pressure dependence of the elec-trical resistance at 300 K (R300K) in metallic BiOCuSeup to 87 GPa, compared to the data from previouslystudied semiconducting BiOCuSe [13]. For the initiallysemiconducting BiOCuSe, the resistivity decreases upto 10 GPa and then begins to increase as pressureis applied. In contrast, the initially metallic BiOCuSeshows an increasing trend of the resistance even below10 GPa. The high-pressure behavior in the semicon-ducting BiOCuSe appears to shift to lower pressuresin the metallic sample, likely due to differences in ini-tial carrier concentration. The metallic nature of BiO-CuSe is significantly suppressed by compression, witha metal-to-semiconductor transition occurring around4 GPa. With further compression above 31 GPa, thesemiconducting behavior is steeply suppressed, and theR300K becomes lower than the ambient value above60 GPa, indicating a semiconductor-to-metal transi-tion. After releasing pressure from 87 GPa to ambi-ent pressure, the initially metallic BiOCuSe is recom-pressed. The recovered sample exhibits a decreasingtrend in resistance as a function of pressure, even nearambient pressure. The origin of the hysteresis effect123Eur. Phys. J. B          (2025) 98:154 Page 7 of 11   154 Fig. 7 Electrical resistance at 300 K (R300K) as a functionof pressure in initially metallic BiOCuSe and re-pressed onefrom this work and the resistivity at 300 K (ρ300K) in ini-tially semiconducting one from literature [13]with respect to pressure application remains an openquestion. Although structural analyses using Ramanspectroscopy and X-ray diffraction (XRD) are impor-tant, such measurements are challenging to perform onmetallic single-crystalline samples. As a direction forfuture research, in-situ XRD measurements on poly-crystalline samples under high pressure are expected toprovide more accurate insights into the crystal struc-ture. Furthermore, once the crystal structure of thesuperconducting phase is clarified, discussions on thesuperconducting pairing mechanism based on densityfunctional theory (DFT) calculations becomes feasible.4.2 YBi2O4Cu2Se2 and Cu2SeFigure 8 shows the temperature dependence of resis-tance in YBi2O4Cu2Se2 under various pressures rang-ing from 2.1 to 44 GPa. As observed in the R–T curveat ambient pressure, YBi2O4Cu2Se2 exhibits metallicbehavior at the lowest pressure of 2.1 GPa. Althoughthere are slight changes in the absolute value of theresistance and the slope of the R–T curve with appliedpressure, no drastic change in electrical properties, suchas a metal-to-semiconductor transition or the emer-gence of superconductivity, is observed up to 44 GPa.While YBi2O4Cu2Se2 shows no signature of supercon-ductivity at this stage of research, exploring higherpressure regions remains valuable, particularly in lightof the recently reported high T c phase in highly com-pressed transition metal diborides above 100 GPa [3,38, 39].Figure 9a, b shows the temperature dependence ofresistance in Cu2Se under various pressures rangingFig. 8 Temperature dependence of resistance onYBi2O4Cu2Se2 under various pressures from 2.1 to 44GPafrom 5.5 to 80 GPa. At the lowest pressure of 5.5 GPa,Cu2Se exhibits metallic behavior with a large humparound 120 K, attributed to the formation of CDWstate, which is consistent with its transport proper-ties at ambient pressure. As pressure is applied, theabsolute value of resistance increases, and the slopeof the R–T curve gradually becomes negative, indicat-ing a pressure-induced semiconductor transition. In thisregion, the transition temperature of the CDW (TCDW)increases with pressure. Under further compression, theresistance begins to decrease, and the semiconduct-ing behavior is rapidly suppressed. The TCDW alsodecreases with increasing pressure. Around 50 GPa,the R–T curve shows partial metallic behavior, andthe CDW transition nearly disappears. This pressure-induced metal-to-semiconductor transition, followed bythe recovery of metallic features beyond the semicon-ducting phase, is similar to the behavior observed inBiOCuSe. Instead of full metallicity and suppressionof the CDW transition, a small drop in resistance isobserved at low temperatures, and this transition isgradually suppressed under applied magnetic fields, asshown in Fig. 9c–f, indicating the emergence of super-conductivity. Figure 10 presents the pressure phase dia-gram of Cu2Se, showing the pressure dependence ofR300K, TCDW, and T c. Under the non-hydrostatic pres-sure conditions, the multi-step transition of supercon-ductivity, due to a pressure distribution in the sam-ple, is often observed. The T c is determined using anonset temperature of the resistance drop. The TCDWincreases as the semiconducting feature is enhancedwith increasing pressure up to 20 GPa. The metallicbehavior progresses with a reduction in TCDW above 20123  154 Page 8 of 11 Eur. Phys. J. B          (2025) 98:154 Fig. 9 Temperature dependence of resistance on Cu2Se under various pressures from a 5.5 to 24 GPa, b 30 to 80 GPa.The enlarged plots around low-temperature regions under various magnetic fields are shown in c to fFig. 10 Pressure dependence of the resistance at 300 K, thetransition temperature in CDW, and the superconductingtransition temperature in Cu2SeGPa. Under further compression, Cu2Se exhibits metal-lic behavior without CDW features and shows super-conductivity. The pressure-induced tuning of TCDWand the subsequent emergence of superconductivityafter the suppression of CDW has been observed invarious materials, including transition metal chalco-genides [40], topological kagome metals [41, 42], andrecently discovered high-T c nickelates [4, 43]. The ori-gin of the observed superconductivity, however, needsfurther investigation, as the transition is highly fila-mentary. In a current stage of research, the filamentarytransition possibly originates from the non-hydrostaticpressure. While the observed T c differs from reportedvalues for elemental Se and other Cu–Se binary com-pounds, such as CuSe2 [44], partial decompositionunder extreme pressure may contribute to this effect.5 ConclusionThe exploration of novel superconducting families is acrucial challenge in the search for next-generation high-T c materials. One promising strategy is the examina-tion of conducting layer replacements, which can beaccelerated by combining high-pressure experiments, asdemonstrated by the recent discovery of high-T c nicke-lates. This study presents a series of electrical transportmeasurements under high pressure on CuSe-based com-pounds of BiOCuSe, YBi2O4Cu2Se2, and Cu2Se usinga custom-designed DAC with the BDD electrodes. Thehigh-pressure effects on the electrical properties of thesecompounds are summarized as follows:In BiOCuSe, metallic behavior rapidly disappearsunder pressure up to 3.9 GPa, and a semiconduct-ing feature appears. The pressure-induced semiconduct-ing nature is suppressed above 31 GPa, with metal-lic tendencies reappearing at higher pressures. Whilebulk superconductivity is not observed up to 87 GPa,the recovered and re-compressed BiOCuSe exhibitssuperconducting signals above 20 GPa. YBi2O4Cu2Se2,which remains metallic, shows a positive R–T curve upto 44 GPa without any anomalies in its electrical prop-erties. Cu2Se, initially exhibiting metallic behavior witha CDW transition at ambient pressure, changes to asemiconducting feature with an increase in TCDW up to24 GPa. At higher pressures, the CDW disappears, andmetallic properties are recovered, with a filamentarysuperconducting transition emerging above 40 GPa.Since the observed superconducting signals in BiO-CuSe and Cu2Se are filamentary, further microscopicanalysis, such as in-situ XRD and other local observa-tion techniques, is required to investigate the origin ofsuperconductivity. Our findings open new directions forfuture research in the field of CuSe-based superconduc-tors. Moreover, as our high-pressure experiments werelimited to below 100 GPa, further investigations beyond123Eur. Phys. J. B          (2025) 98:154 Page 9 of 11   154 100 GPa, using advanced high-pressure techniques, areexpected to provide insights into the potential emer-gence of high-T c superconductivity. In addition, thecarrier tuning in these compounds using a fabrication ofthe electric double layer transistor (EDLT) in the sam-ple space of DAC [45] is promising strategy to inducesuperconductivity at ambient condition or lower pres-sure region.Acknowledgements This work was partly supported byJSPS KAKENHI Grant Number 23H01835, 23K13549, and23KK0088. The fabrication process of diamond electrodeswas partially supported by the NIMS Nanofabrication Plat-form in the Nanotechnology Platform Project sponsored bythe Ministry of Education, Culture, Sports, Science andTechnology (MEXT), Japan. The calculations in this studywere performed on the Numerical Materials Simulator atNIMS. The nano-polycrystalline diamond was synthesizedand provided via the Visiting Researcher’s Program of theGRC with proposal No. 2023YB01. This work was sup-ported by the World Premier International Research CenterInitiative (WPI), MEXT, Japan.Author contributionsRyo Matsumoto: conceptualization, methodology, investiga-tion, and writing—original draft. Sayaka Yamamoto: inves-tigation, visualization, and reviewing manuscript. ShintaroAdachi: investigation and reviewing manuscript. HiromiTanaka: investigation and reviewing manuscript. Toru Shin-mei: investigation and reviewing manuscript. Tetsuo Irifune:investigation and reviewing manuscript. Yoshihiko Takano:supervision, funding acquisition, and reviewing manuscript.Data availability statement The manuscript has noassociated data. [Authors’ comment: The data that supportthe findings of this study are available from the correspond-ing author, upon reasonable request.]Code availability This study did not use any custom codeor software that is not publicly available.Open Access This article is licensed under a CreativeCommons Attribution 4.0 International License, which per-mits use, sharing, adaptation, distribution and reproductionin any medium or format, as long as you give appropriatecredit to the original author(s) and the source, provide a linkto the Creative Commons licence, and indicate if changeswere made. The images or other third party material inthis article are included in the article’s Creative Commonslicence, unless indicated otherwise in a credit line to thematerial. If material is not included in the article’s CreativeCommons licence and your intended use is not permittedby statutory regulation or exceeds the permitted use, youwill need to obtain permission directly from the copyrightholder. 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