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Induced charge density wave and superconductivity in Cu-doped TaSe₃
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| Title: | Induced charge density wave and superconductivity in Cu-doped TaSe₃ |
| Other Titles: | CuドープしたTaSe₃における誘起された電荷密度波と超伝導 |
| Authors: | 野村, 温 Browse this author |
| Issue Date: | 25-Mar-2019 |
| Publisher: | Hokkaido University |
| Abstract: | The relationship between superconductivity (SC) and charge density wave (CDW) has been a major research topic in condensed matter physics and has been investigated in many materials. However, previous studies were actually limited to two cases: the first is where SC and a CDW intrinsically exist; the second is where SC is induced in a CDW material. To understand the whole picture of the relationship between SC and CDW, we investigated the relationship in a third case where a CDW is induced in a superconducting material. Transition metal trichalcogenides, MX3 (M: Nb, Ta; X: S, Se), has a structure consisting of chains made of transition metals and chalcogens, and the chains are weakly bonded by van der Waals forces. Owing to this structure, MX3 is a quasi-one-dimensional conductor in which an electric current travels well in the direction of the chain axis. In most materials belonging to MX3 (NbSe3, TaS3, and NbS3), the Fermi surface is close to a plane. Therefore, the nesting condition is good and a CDW develops below the transition temperature in these materials. On the other hand, TaSe3 which is one of MX3 exhibits no CDWtransition but the filamentary superconductivity transition at about 2 K because TaSe3 is more three-dimensional than the other MX3 compounds. If Cu atoms are doped in TaSe3, the Cu atoms are considered to be difficult to substitute for Ta atoms because the common valence that Cu takes (+1 or +2) is different from that of Ta (+5). On the other hand, the Cu atoms are expected to enter in the Van der Waals gap and to increase the distance between chains of TaSe3. Therefore, it is presumed that Cu doping decreases the dimensionality and improves the nesting condition in TaSe3. In this thesis, we tried to induce a CDW in TaSe3 with a superconducting material by Cu doping, and investigated the relationship between the induced CDW and SC. We have synthesized single crystals of Cu-doped TaSe3, and measured precisely the temperature dependence of the resistance from 4.2 to 280 K. We discover an anomalous sharp dip in the temperature derivative of the resistance (dR=dT) at about 91 K in Cu- doped TaSe3, which is never observed in pure TaSe3. The dip suggests that there is a sudden change in state with a relative increase in resistance. In addition, the dip is "γ”shaped. We reveal that the same "γ”-shaped dip in dR=dT is commonly observed at the CDW transition temperature in many CDW conductors, which is a universal consequence resulting from the opening and growth of a CDW gap on a Fermi surface. Furthermore, the result of the single-crystal X-ray diffraction (XRD) analysis implies that the lattice pa- rameters perpendicular the chain axis increase and that parallel to the chain axis decreases by Cu doping, leading to an improvement in the nesting condition. The "γ”-shaped dip and the result of the single-crystal XRD analysis show that a CDW emerges by Cu doping in TaSe3. We investigated the effect of Cu doping on SC in Cu-doped TaSe3 by measuring the temperature dependence of the resistance from 0.6 to 2 K. We observed an emergence of a region where the SC transition temperature (TC) decreased in samples with higher Cu concentrations, and found that the region tended to expand with increasing Cu concen- tration. These results and the fact that the SC of TaSe3 is filamentary show that SC is suppressed locally by Cu doping in Cu-doped TaSe3. From the above discussions, it was revealed that a CDW is induced while SC is sup- pressed in Cu-doped TaSe3. Hence, the induced CDW and SC would be in a competitive relationship. The locality of SC suppression suggests that the induced CDWs are local. The resis- tance anomaly due to the induced CDW transition was extremely small. Moreover, the size of the anomaly was enhanced with increasing Cu concentration but the temperature at which the anomaly appeared hardly changed. These results of the anomaly can be interpreted consistently from the short-range order of the induced CDWs in the vicinity of Cu atoms. From all the discussions, we conclude that the induced short-range order CDWs and SC are in a competitive relationship in Cu-doped TaSe3. The competitive relationship between short-range order CDWs and SC obtained in the present work is different from the non-competitive relationship previously reported in CuxTiSe2 where SC is induced in a CDW material. By comparing the detailed experimental results of these two materials, we will clarify the physics that defines the relationship between short-range order CDWs and SC. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第13634号 |
| Degree Level: | 博士 |
| Degree Discipline: | 工学 |
| Examination Committee Members: | (主査) 教授 丹田 聡, 教授 小田 研（本学大学院理学院）, 教授 戸田 泰則 |
| Degree Affiliation: | 工学院(応用物理学専攻) |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/74056 |
| Appears in Collections: | 課程博士 (Doctorate by way of Advanced Course) > 工学院(Graduate School of Engineering)
学位論文 (Theses) > 博士 （工学）
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