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M. Nakayama, [S. Nakagawa](https://orcid.org/0000-0003-4539-6953), T. Yamaguchi, H. Minami, [K. Kadowaki](https://orcid.org/0000-0001-9391-1150), [H. Nakao](https://orcid.org/0000-0003-4020-537X), [T. Mochiku](https://orcid.org/0000-0003-2208-4279), [M. Tsujimoto](https://orcid.org/0000-0003-4296-5137), [S. Ishida](https://orcid.org/0000-0001-9445-8079), [H. Eisaki](https://orcid.org/0000-0002-8299-6416), [T. Kashiwagi](https://orcid.org/0000-0002-4839-9247)

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[Study on the effect of Bi/Sr ratio on the device characteristics of high-<i>T</i>c superconducting terahertz wave emitters made of Bi2Sr2CaCu2O8+<i>δ</i> single crystals](https://mdr.nims.go.jp/datasets/bb4c4a94-a9e6-489c-9c9f-6b420900a02a)

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Study on the effect of Bi/Sr ratio on the device characteristics of high-Tc superconducting terahertz wave emitters made of Bi2Sr2CaCu2O8+δ single crystalsViewOnlineExportCitationRESEARCH ARTICLE |  FEBRUARY 16 2024Study on the effect of Bi/Sr ratio on the devicecharacteristics of high-T c superconducting terahertz waveemitters made of Bi2Sr2CaCu2O8+δ single crystalsM. Nakayama; S. Nakagawa  ; T. Yamaguchi; H. Minami; K. Kadowaki  ; H. Nakao  ; T. Mochiku  ;M. Tsujimoto  ; S. Ishida  ; H. Eisaki  ; T. Kashiwagi  J. Appl. Phys. 135, 073902 (2024)https://doi.org/10.1063/5.0188077Articles You May Be Interested InImproved excitation mode selectivity of high- T c superconducting terahertz emittersJ. Appl. Phys. (July 2018)Study of device characteristics of intrinsic Josephson junction terahertz emitters related to annealingconditions of the crystalsJ. Appl. Phys. (April 2023)Numerical simulation of quench initiation and propagation in multi-filamentary Bi2Sr2CaCu2Ox round wiresat high magnetic fieldsJ. Appl. Phys. (April 2019) 08 October 2024 06:40:49https://pubs.aip.org/aip/jap/article/135/7/073902/3265792/Study-on-the-effect-of-Bi-Sr-ratio-on-the-devicehttps://pubs.aip.org/aip/jap/article/135/7/073902/3265792/Study-on-the-effect-of-Bi-Sr-ratio-on-the-device?pdfCoverIconEvent=citejavascript:;javascript:;https://orcid.org/0000-0003-4539-6953javascript:;javascript:;javascript:;https://orcid.org/0000-0001-9391-1150javascript:;https://orcid.org/0000-0003-4020-537Xjavascript:;https://orcid.org/0000-0003-2208-4279javascript:;https://orcid.org/0000-0003-4296-5137javascript:;https://orcid.org/0000-0001-9445-8079javascript:;https://orcid.org/0000-0002-8299-6416javascript:;https://orcid.org/0000-0002-4839-9247https://crossmark.crossref.org/dialog/?doi=10.1063/5.0188077&domain=pdf&date_stamp=2024-02-16https://doi.org/10.1063/5.0188077https://pubs.aip.org/aip/jap/article/124/3/033901/156111/Improved-excitation-mode-selectivity-of-high-Tchttps://pubs.aip.org/aip/jap/article/133/16/163904/2885247/Study-of-device-characteristics-of-intrinsichttps://pubs.aip.org/aip/jap/article/125/16/163901/156726/Numerical-simulation-of-quench-initiation-andhttps://servedbyadbutler.com/redirect.spark?MID=176720&plid=2589059&setID=592934&channelID=0&CID=925664&banID=522406050&PID=0&textadID=0&tc=1&rnd=2687373186&scheduleID=2507799&adSize=1640x440&data_keys=%7B%22%22%3A%22%22%7D&matches=%5B%22inurl%3A%5C%2Fjap%22%5D&mt=1728369649476089&spr=1&referrer=http%3A%2F%2Fpubs.aip.org%2Faip%2Fjap%2Farticle-pdf%2Fdoi%2F10.1063%2F5.0188077%2F19832238%2F073902_1_5.0188077.pdf&hc=a895d0dbace0d8413497a5f8bc76b2ef169708f0&location=Study on the effect of Bi/Sr ratio on the devicecharacteristics of high-Tc superconductingterahertz wave emitters made of Bi2Sr2CaCu2O8+δsingle crystalsCite as: J. Appl. Phys. 135, 073902 (2024); doi: 10.1063/5.0188077View Online Export Citation CrossMarkSubmitted: 18 November 2023 · Accepted: 24 January 2024 ·Published Online: 16 February 2024M. Nakayama,1,2 S. Nakagawa,1,2 T. Yamaguchi,1 H. Minami,1,3 K. Kadowaki,3 H. Nakao,4 T. Mochiku,5M. Tsujimoto,6 S. Ishida,2 H. Eisaki,2 and T. Kashiwagi1,3,a)AFFILIATIONS1Graduate School of Pure & Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan2Research Institute for Advanced Electronics and Photonics, National Institute of Advanced Industrial Science and Technology(AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan3Division of Materials Science, Faculty of Pure & Applied Sciences, University of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki 305-8573, Japan4Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba,Ibaraki 305-0801, Japan5National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan6Global Research and Development Center for Business by Quantum-AI Technology (G-QuAT), National Institute of AdvancedIndustrial Science and Technology (AIST), Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japana)Author to whom correspondence should be addressed: kashiwagi@ims.tsukuba.ac.jpABSTRACTTo obtain high-performance THz-wave-emitting devices made of single crystals of Bi2Sr2CaCu2O8þδ (Bi2212), a high-temperature super-conductor, an understanding of the device characteristics based on crystal characteristics can be a key issue because, in principle, the electri-cal properties of the intrinsic Josephson junctions (IJJs) constructed in Bi2212 crystals highly depend on crystal conditions, such as carrierconcentration, crystal homogeneities, and crystal defects. To evaluate the tendencies of the device characteristics associated with crystal char-acteristics, we prepared Bi2212 crystals with different Bi/Sr ratios (x ¼ 0:05, 0.15, and 0.25) and δ values (annealed under N2 or O2 gas flowconditions). The unit cell parameter c decreased as the Bi/Sr ratio or δ increased. For the same annealing conditions under N2 gas flow, thesuperconducting transition temperature as well as the size of the hysteresis loop of the current–voltage characteristics and emission charac-teristics were significantly suppressed for the sample with x ¼ 0:25 compared with the corresponding values for the samples with x ¼ 0:05and 0.15. The experimental results clearly indicate that parameters, such as the Bi/Sr ratio and annealing conditions, are crucial factors indetermining the electrical characteristics of a device. This information can be a useful guide for the preparation of crystals for IJJ THz-wavedevices that can be fine-tuned according to the desired device characteristics.© 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International (CC BY-NC-ND) license (https://creativecommons.org/licenses/by-nc-nd/4.0/). https://doi.org/10.1063/5.0188077I. INTRODUCTIONElectromagnetic waves in the terahertz frequency range(THz wave) have great potential in fundamental and appliedresearch fields in material sciences because the vibrationmodes of molecules, polymers, and proteins exist in thefrequencies of the THz region. High-performance THz waveemitters, detectors, and related devices have been used inmany applications, such as nondestructive inspections, securitychecking, identification of chemical substances, and cancerdetection.1–4Journal ofApplied PhysicsARTICLE pubs.aip.org/aip/japJ. Appl. Phys. 135, 073902 (2024); doi: 10.1063/5.0188077 135, 073902-1© Author(s) 2024 08 October 2024 06:40:49https://doi.org/10.1063/5.0188077https://doi.org/10.1063/5.0188077https://pubs.aip.org/action/showCitFormats?type=show&doi=10.1063/5.0188077http://crossmark.crossref.org/dialog/?doi=10.1063/5.0188077&domain=pdf&date_stamp=2024-02-16https://orcid.org/0000-0003-4539-6953https://orcid.org/0000-0001-9391-1150https://orcid.org/0000-0003-4020-537Xhttps://orcid.org/0000-0003-2208-4279https://orcid.org/0000-0003-4296-5137https://orcid.org/0000-0001-9445-8079https://orcid.org/0000-0002-8299-6416https://orcid.org/0000-0002-4839-9247mailto:kashiwagi@ims.tsukuba.ac.jphttps://creativecommons.org/licenses/by-nc-nd/4.0/https://doi.org/10.1063/5.0188077https://pubs.aip.org/aip/japAmong THz wave emitters, semiconducting solid-statedevices, such as resonant tunneling diodes (RTDs)5 and quantumcascade lasers (QCLs),6,7 have also been developed simultaneously.RTDs have the advantage of operating at room temperature withsub-milliwatt-level output power up to a frequency of approxi-mately 1 THz.8,9 Recently, a 6� 6 array of RTDs that operates at0.45 THz with a maximum radiation power of 11.8 mW wasreported.10 Although low-temperature operation below 30 K isrequired to generate frequencies of 1 THz, QCLs can generate fre-quencies ranging from 1.2 to 5.4 THz with milliwatt-level power.6,7However, THz-QCLs using different frequency generation tech-niques operating at room temperature have recently beendeveloped.11In the last decade, THz wave emitters based on the ACJosephson effect12 have been developed using a single crystal of thehigh-Tc superconductor Bi2Sr2CaCu2O8þδ (Bi2212). Single crystalsof Bi2212 consist of insulating Bi2O2 layers and superconductingCuO2 layers stacked alternately along the crystallographic c axisand are known as intrinsic Josephson junctions (IJJs).13–15 Waveswith frequencies in the terahertz range can be obtained by process-ing single crystals of Bi2212 into mesa structures and applying DCbias voltages across the IJJs.16 According to previous studies,emission frequencies in the 0.15–2.4 THz range can be generatedin free space by changing the size and shape of Bi2212 mesastructures.16–21 Josephson emissions with a frequency range of1–11 THz have been reported for small Bi2212 mesa structures.22The maximum power from a single-mesa device is approximately30–100 μW.17,20,23–26 An output power of approximately 0.6 mWcan also be achieved from an array of three mesa structures.26 Moredetailed characteristics of Bi2212-THz-wave emitters have beenreviewed in several studies.27–31We are interested in developing the IJJ device characteristicsfrom the perspective of their crystal properties. Bi2212 single crys-tals are nonstoichiometric compounds, expressed asBi2þxSr2�xCaCu2O8þδ . The nonstoichiometry of Bi/Sr, representedby x, significantly affects the superconducting characteristics of thecompound; a smaller x results in a higher superconducting transi-tion temperature.32–34 In addition, the superconducting transitiontemperatures and the conductivities of the compound significantlydepend on the amount of oxygen, thus depending on δ.35 Based onthese studies,32–35 the IJJ device characteristics are likely to be con-trolled by varying the values of x and δ because the electrical prop-erties of the IJJ device significantly depend on crystal conditions,such as carrier concentration and crystal defects. Clarifying theeffects of x and δ on the device characteristics will help improvethe performance of IJJ devices.Although some studies have investigated the characteristics ofIJJ emitters using crystals prepared under different annealing con-ditions36 and Pb substitution,37 to the best of our knowledge, theeffects of the nonstoichiometry of Bi/Sr on the device characteris-tics have not been studied previously. In this study, Bi2212 singlecrystals with different x and δ values were prepared, and the varia-tions in the crystal properties and device characteristics with thesevalues were studied as a first step toward understanding the tenden-cies of the device characteristics associated with the crystal charac-teristics depending on the values of x and δ. The crystal and devicecharacteristics of Bi2212 single crystals grown at x ¼ 0:05,x ¼ 0:15, and x ¼ 0:25 as nominal compositions were evaluated.These x values were chosen because it is empirically known thatlarger single crystals can be grown in slightly Bi-rich phases. Inaddition, from the perspective of crystal characteristics, previousstudies32–34 have reported a reduction in the maximum supercon-ducting transition temperature up to x ¼ 0:3. Therefore, clear dif-ferences in the crystal properties and device characteristics wereexpected by comparing the samples with x ¼ 0:05 and x ¼ 0:25.Furthermore, the grown crystals were annealed under O2 or N2 gasflow conditions to change the amount of δ in each crystal and gaina broader understanding of the effect of annealing on the IJJdevices.In this study, we first explain the compositional characteristicsof crystals grown with different x values. Second, we compare thesuperconducting transition temperatures and unit cell parametersof the samples. Third, the characteristics of IJJ devices fabricatedusing the grown crystals are discussed. The experimental resultsreveal that the superconducting transition temperature and unit cellparameters highly depend on parameters, such as the Bi/Sr ratioand annealing conditions. The crystal characteristics, which dependon the parameter set, are reflected in the electrical characteristics ofthe Bi2212 THz-wave emitters. The data obtained from thesestudies can serve as a useful guide for the preparation of the crystalsrequired for constructing IJJ THz-wave devices.II. SAMPLE PREPARATIONS AND EXPERIMENTALMETHODSBi2þxSr2�xCaCu2O8þδ single crystals prepared with nominalcomposition ratios of x ¼ 0:05 (Bi/Sr = 2.05/1.95), x ¼ 0:15 (2.15/1.85), and x ¼ 0:25 (2.25/1.75) were grown using a floating zonefurnace as described in previous studies.38,39 To evaluate thesample characteristics and fabricate crystal chips for Bi2212 THzwave emitters, the grown crystals were cut into square shapes in thesquare-millimeter scale. Then, to adjust the oxygen content δ ofthe crystals, they were annealed for 4–6 days, either under O2 gasflow at 400�C and a flow rate of 0.10 l/min or under N2 gas flow at600�C and a flow rate of 1.0 l/min. At the end of the annealingprocess, the crystals were quenched to fix their oxygen content.In Sec. III, samples prepared using O2- or N2-annealedcrystals with Bi/Sr ratios of 2.05/1.95 are referred to as Bi2.05-O2and Bi2.05-N2, respectively. Similarly, the samples fabricated usingthe O2- or N2-annealed crystals with Bi/Sr ratios of 2.15/1.85 or2.25/1.75 are referred to as Bi2.15–O2, Bi2.15–N2, Bi2.25–O2, andBi2.25–N2, respectively.To evaluate the chemical compositions of the grown crystals, ascanning electron microscope (SEM, TM3000, Hitachi) equippedwith an energy dispersive spectroscopy (EDS) system was used. ForEDS measurements, Bi2212 crystal chips fabricated using wet etchingwere used. The superconducting transition temperatures, Tcs, of thecrystals were determined from the temperature dependences of theirmagnetic susceptibilities using a SQUID magnetometer (MPMS,Quantum Design). For the magnetic susceptibility measurements,single crystals with a size of 2mm2 were used, and a magnetic fieldof 2 Oe was applied parallel to the c axis of the sample.Wet etching was used to fabricate the Bi2212 crystal chip,which was established by our group40,41 based on previousJournal ofApplied PhysicsARTICLE pubs.aip.org/aip/japJ. Appl. Phys. 135, 073902 (2024); doi: 10.1063/5.0188077 135, 073902-2© Author(s) 2024 08 October 2024 06:40:49https://pubs.aip.org/aip/japstudies.42–45 Briefly, the crystals were initially cleaved on both sidesto prepare thin single-crystal plates with thicknesses of 3–5 μm.Subsequently, silver and gold were deposited on both sides of thecrystal. The total thickness of the metal films was in the range of20–50 nm. The crystal was then glued onto a sapphire plate, andphotolithography techniques were used to create photoresist maskpatterns on the crystal surface. The crystals were processed intorectangular chips using a wet-etching method. Subsequently, thesechips were assembled as Bi2212 THz wave emitters using a sand-wich structure developed by our group.20In the sandwich structure, a crystal chip is simply sandwichedby two sapphire plates on which metallic electrodes are patterned.These parts are held by metallic rigs and fixed with screws. TheJoule heat generated at the crystal chip can be removed using thesestructures. The interface between the crystal chips and the sapphiresubstrate is important for thermal management, as well as for elec-trical contact between the chip and electrodes. By properly config-uring them, for example, a dc bias voltage with 2.2 mV/junctioncan be applied to a crystal chip with dimensions of58–66� 350� 4:7 μm3 at 15 K.20 Recently, thicker mesa structuresfor high power emission have been developed considering a goodheat-removal interface structure, and 0.7 mV/junction at 4.2 K wasobtained from a cylindrical mesa with 400� 13 μm2.45 The contin-uous development of thermal management is an important subjectto improve the emission intensity and frequency.To characterize the fabricated crystal chips, x-ray diffractionwas performed with an ω–2θ scan to measure the distribution ofthe unit cell parameters along the c axis of the samples. A four-circle diffractometer was used at BL-4C, the Photon Factory of theHigh Energy Accelerator Research Organization (KEK). The beamshape at the sample position was elliptical, with 0.6 mm verticallyand 0.8 mm horizontally. The incident x-ray energy was set to be8.8 keV using a Si(111) double-crystal monochromator. Under thepresent measurement conditions, a precise distribution of the unitcell parameters on the order of 10�2 Å can be obtained.To characterize the Bi2212 THz emitter, it was mounted onthe cold finger of a helium-flow type of cryostat (OxfordInstruments, CF1104) equipped with optical windows. An FeRhthermometer was placed on the cold finger. The measured temper-ature Tb directly indicates the temperature of the cold finger andnot that of the crystal chip itself. Electrical characteristics of thedevices were measured using a conventional two-terminal method.An InSb hot-electron bolometer (HEB) (QMC Instruments, QFI/2BI) was used to detect electromagnetic waves emitted from thedevices.46It is sometimes difficult to use the same piece of crystal forthe above measurements, primarily because of the technical limita-tions of sample preparation. For example, to fabricate crystal chipsusing a crystal that was evaluated by a magnetic susceptibility mea-surement, there are sometimes sample losses in the fabrication pro-cesses, such as failure of crystal cleaving and over-etching of thesamples. Therefore, different crystal pieces were used to evaluatethe samples. The single crystals used for the THz wave emittersand magnetic susceptibility measurements were different; however,they were obtained from the same site on the grown crystal rodand were annealed simultaneously. The crystal chips used for x-raydiffraction and device characteristic measurements were differentbut were fabricated simultaneously from the same single-crystalfragment.III. RESULTS AND DISCUSSIONHerein, the results related to the crystal characteristics depend-ing on x and δ are presented. First, we explain the compositionalcharacteristics of the crystals grown with different x values. Next,we compare the superconducting transition temperatures and unitcell parameters of the samples. The crystal characteristics evaluatedin this study exhibit some differences depending on the sample.The main purpose of this evaluation is to determine the trends inthe crystal conditions as x and δ varied.Table I lists the chemical compositions of the fabricatedcrystal chips as determined by the EDS analysis. The typical chipdimensions were �80� 200–400� 2–6 μm3. The values evaluatedin Table I were normalized such that the sum of Bi, Sr, and Ca wasfive. For normalization, Cu was not included because the observedEDS data for Cu had a larger standard deviation than those for theother elements. Unfortunately, the reason for the large standarddeviation is unknown. To estimate the precise chemical composi-tion, a quantitative analysis, such as that performed on an electronprobe microanalyzer (EPMA), is required.The results showed that the Bi/Sr ratios of the grown crystalswere higher than those of the nominal compositions and increasedwith increasing Bi/Sr ratios of the nominal compositions. Thegrown crystals tended to prefer Bi-rich phase conditions, asreported in previous studies.32,33 The samples with x ¼ 0:05showed a decrease in the amount of Ca. This feature may bereflected in the Tc characteristics, which will be discussed later.Figure 1 shows the temperature dependence of the normalizedmagnetic susceptibility of bulk crystals. The onsets of the supercon-ducting transition temperatures Tc of the crystals estimated fromthe measurements are summarized in Table II.The O2-annealed crystals with different Bi/Sr ratios showsimilar Tc values, whereas the Tc values of the N2-annealed crystalssignificantly depend on the Bi/Sr ratios, even under the sameannealing conditions. According to a previous study,34 the Tcvalues of samples with higher x values were strongly suppressed forsmall values of δ. The observed trend was consistent with thatreported in the previous study. The difference in Tc between thesamples with x ¼ 0:05 and x ¼ 0:15 may be reflected in the differ-ence in the amount of Ca. If the Ca2þ sites are substituted withTABLE I. EDS analysis data of fabricated Bi2212 crystal chips. Numbers in paren-theses are standard deviations of the last significant digit.Nominal EvaluatedSample Bi/Sr ratio Bi Sr Ca Bi/Sr ratioBi2.05–N2 1.051 2.24(3) 1.85(3) 0.91(4) 1.21–O2 2.24(3) 1.87(3) 0.89(2) 1.20Bi2.15–N2 1.162 2.23(2) 1.76(2) 1.02(2) 1.27–O2 2.21(4) 1.80(4) 0.98(4) 1.23Bi2.25–N2 1.286 2.27(2) 1.70(1) 1.02(2) 1.33–O2 2.27(2) 1.69(2) 1.05(2) 1.34Journal ofApplied PhysicsARTICLE pubs.aip.org/aip/japJ. Appl. Phys. 135, 073902 (2024); doi: 10.1063/5.0188077 135, 073902-3© Author(s) 2024 08 October 2024 06:40:49https://pubs.aip.org/aip/japBi3þ, the valence difference between these ions reduces the holedensity in the CuO2 plane.33,48 This is related to the decrease in theTc value of the sample with x ¼ 0:05. We will discuss the tendencyof the valence difference in the sample later.The information of Tc of as-grown crystals and typical anneal-ing conditions of the optimal doping for the samples would behelpful for understanding the characteristics of prepared crystals aswell as for preparing the crystals for the emitters. Table II lists theTc values of the as-grown crystals for x ¼ 0:05, x ¼ 0:15, andx ¼ 0:25. Based on previous studies32,35 and our experimentalexperience, the expected annealing conditions of optimal dopingfor x ¼ 0:05, x ¼ 0:15, and x ¼ 0:25 are �0:1%-O2 at �700�C,�0:1%-O2 at �500�C, and 100%-O2 with a flow rate of 0.10 L/minat �600�C, respectively. To prepare the gas condition of�0:1%-O2, a mixture of two gases, N2 with a flow rate of0.60 l/min and O2 with a flow rate of 0.20 ml/min, were used, andthese flow rates were then adjusted to �0:1%-O2 by monitoring anoxygen sensor installed downstream of the gas flow path. Theseconditions depend on the size of the crystals and atmospheric gasesowing to the degassing of oxygen from the samples.Figures 2(a)–2(c) show the ω–2θ scans measured around 0026for the crystal chips with different x values. Data were normalizedto the maximum peak intensities of the scans. The N2-annealedsamples exhibited almost a single peak in the ω–2θ scan. Withincreasing Bi/Sr ratio, the peak position shifted to a higher anglefrom 2θ ¼ 72:64� to 72:82�, indicating a reduction in the unit cellFIG. 1. Temperature dependence of normalized magnetic susceptibilities of thegrown crystals with different Bi/Sr ratios: (a) annealed under N2 gas flow at600�C and (b) annealed under O2 gas flow at 400�C. The inset of Fig. 1(b) dis-plays the magnification plot of the O2 annealed samples around Tc. Theobserved susceptibility data were normalized to be �1 using the valuesobtained at the lowest measurement temperatures. The results for the zero-fieldcooling data are plotted.TABLE II. Tc-onsets of the prepared crystals.Bi/Sr ratio N2 annealed (K) O2 annealed (K) As-grown (K)2.05/1.95 63.0 82.7 89.12.15/1.85 70.0 81.0 84.02.25/1.75 37.0 78.2 72.0FIG. 2. Normalized data plot of the ω–2θ scan measured around 0026 for the(a) x ¼ 0:05, (b) x ¼ 0:15, and (c) x ¼ 0:25 samples. The data for the N2 andO2 annealed samples are displayed in blue and red colored symbols, respec-tively. The data plots for O2 annealed samples are shifted vertically to avoidoverlapping those for the N2 annealed ones.Journal ofApplied PhysicsARTICLE pubs.aip.org/aip/japJ. Appl. Phys. 135, 073902 (2024); doi: 10.1063/5.0188077 135, 073902-4© Author(s) 2024 08 October 2024 06:40:49https://pubs.aip.org/aip/japparameter c. In general, the curve profile shows the distribution ofunit cell parameters and strain in the samples. The curve profiles ofeach N2-annealed sample show that these distributions are almostcomparable, although the curve profile of Bi2.25–N2 exhibited ashoulder on the lower-angle side.The O2-annealed samples differ from the N2-annealedsamples and show two distinct peaks. The weak signal observed onthe lower side of 2θ originates from the contribution of the surfaceof the crystals. The oxygen content on the crystal surface of theO2-annealed sample was easily reduced during metal evaporation,forming electrodes on the crystal surface. This is related to thestability of the oxygen in the compound, as discussed in our previ-ous study.47The stronger peak observed at a higher 2θ shifted to a higherangle from 2θ ¼ 72:94� to 73:22� with increasing Bi/Sr ratio. Theweak signal also shifted to a higher angle from 2θ ¼ 72:66� to73:00�. The linewidth of each O2-annealed sample was broaderthan that of the N2-annealed sample. For the Bi2.25–O2 sample, aclear broadening of the linewidth was observed, indicating a lowerhomogeneity of the unit cell parameter c.A shift in the peak position of the ω–2θ scan to a higher angleindicates a decrease in the unit cell parameter c. Figure 2 showsthat the unit cell parameter c decreased not only for theO2-annealed samples, but also when the Bi/Sr ratio was increased.These tendencies are consistent with those observed in previousstudies34,48,49 and are discussed later. The unit cell parameters cestimated from the ω–2θ scans are summarized in Table III.Figure 3 shows a comparison of the device characteristics fab-ricated using N2-annealed crystals. The dimensions of the fabri-cated crystal chips for the THz wave emitters are78� 174� 4:0 μm3 for Bi2.05-N2, 55 μm� 181 μm� 2:9 μm forBi2.15–N2 and 52� 385� 2:5 μm3 for Bi2.25–N2, respectively. Forthe device characteristics, we focused on the results of theN2-annealed samples to obtain a meaningful comparison of theircrystal characteristics. In our previous study, the device characteris-tics of the O2-annealed sample were complicated because of thehigher carrier concentration and inhomogeneity of the oxygencontent of the surface part of the crystal chips.47 For the sampleswith x ¼ 0:05 and 0.15, we also confirmed that the O2-annealedsamples prepared in this study show similar tendencies to thoseobtained in a previous study.47In addition, we only discuss whether these devices exhibitelectromagnetic wave emission and compare the device characteris-tics by using the above crystal chips, which we have fabricated sofar. Particularly, to obtain further understanding of the device char-acteristics based on the cavity resonance condition depending onthe chip size, a comparison of chips with similar dimensions isrequired. Therefore, to precisely understand the differences in theradiation characteristics of the samples, we plan to perform furtherexperiments using crystal chips with similar dimensions and anincreased number of sample measurements.The insets of Fig. 3 show the temperature dependence of the caxis resistance (RT) of the Bi2212 crystal chips assembled in asandwich structure. In all cases, the c axis resistance increased asthe temperature decreased from room temperature to lower tem-peratures but decreased significantly around Tc. Tc of Bi2.25–N2was lower than that of Bi2.05–N2 and Bi2.15–N2. This trend of Tcis consistent with the temperature dependence of the magnetic sus-ceptibility (MT) data, as shown in Fig. 1(a). The differences in thevalues of Tc between the RT and MT data likely originated fromthe differences in the crystal batches used for each measurement.However, a precise discussion of the sample conditions based onthe RT data is difficult because the data include extrinsic resistance,other than the resistance of the electrode of a metallic thin filmprepared on a sapphire plate;20 for example, the effects of thesurface conditions of the crystal chips prepared after wet etchingand the contact between the chips and electrodes may be includedin the resistance.Figures 3(a-1), 3(b-1), and 3(c-1) show the current–voltagecharacteristics (IVCs) of Bi2.05-N2, Bi2.15–N2, and Bi2.25–N2measured at various bath temperatures TB. In the IVCs, thevoltage generated as the applied current increased was low butsuddenly increased to a large voltage value when the currentexceeded the critical current Ic. When the current decreased, thevoltage state persisted up to a certain current value, and a hystere-sis loop was observed in the IVCs. The size of the hysteresis loopvaried with TB and decreased with increasing TB. The criticalcurrent Ic varied with the Bi/Sr ratio and decreased with anincreasing ratio, as shown in Fig. 3. To evaluate the difference inthe Ic values, the critical current density values Jc at low TB valueswere estimated from the Ic values and the areas of the crystalchips. The estimated values of Jc for Bi2.05–N2, Bi2.15–N2, andBi2.25–N2 were �140, �160, and �15 A/cm2, respectively. The Jcvalues of the Bi2.05-N2 and Bi2.15–N2 samples were comparableto those reported in previous studies;36 however, the Jc value ofthe Bi2.25–N2 sample was strongly suppressed. This is a charac-teristic feature of the substitution effect on the devicecharacteristics.In Figs. 3(a-2), 3(b-2), and 3(c-2), the radiation intensity detectedby the HEB, denoted Vbol, is plotted as a function of the bias voltageapplied to the crystal chip. Data were acquired simultaneously duringIVC measurements. As shown in Figs. 3(a-2) and 3(b-2), the devicesmade of Bi2.05-N2 and Bi2.15–N2 show the emission of electromag-netic waves at the return branches of the IVCs. According to theprevious studies,20,24 the detected emission power P can be estimatedby P ¼ 2ffiffiffi2pVbol=α, where α ¼ 3:3 mV/nW is the system opticalresponsivity calibrated with blackbody radiation. Therefore, theobserved Vbol of �10 mV for Bi2.05–N2 and Bi2.15–N2 correspondto P � 9 nW.Conversely, no electromagnetic-wave emission was observedfor Bi2.25–N2. The maximum applied bias voltage on the hysteresisloop of Bi2.25–N2 was smaller than that of the other two devices.In this case, it was difficult to generate high-frequency currentbased on the AC Josephson effect. The generated current did notmatch the cavity resonance frequencies determined by the shapesTABLE III. Estimated unit cell parameter c for the prepared crystal chips.Bi/Sr ratio N2 annealed (Å) O2 annealed (Å)2.05/1.95 30.91 30.822.15/1.85 30.88 30.752.25/1.75 30.86 30.69Journal ofApplied PhysicsARTICLE pubs.aip.org/aip/japJ. Appl. Phys. 135, 073902 (2024); doi: 10.1063/5.0188077 135, 073902-5© Author(s) 2024 08 October 2024 06:40:49https://pubs.aip.org/aip/japand dimensions of the crystal chips. This was most likely thereason for the lack of emissions in the Bi2.25–N2 sample.In the final part of this paper, for basic understanding of theexperimental results, we discuss the variation in the device charac-teristics owing to the Bi/Sr substitution and annealing effects,although these two effects are connected to each other.48,49 In addi-tion, we consider a simple case in which the influence of the substi-tution or introduction of Bi, Sr, and O atoms in Bi2212 crystals isobserved mainly along the crystallographic c axis.To understand the effect of Bi/Sr substitution, information onthe valency and ionic radii of Bi and Sr in Bi2212 crystals ishelpful. The valency of the elements in the compound is Bi3þ andSr2þ. The ionic radii of Bi3þ (eight coordinates) and Sr2þ (ninecoordinates) are 1.17 and 1.31 Å.50 In addition, according to previ-ous studies,32,33,48 this compound has a tendency for the Sr2þ siteslocated next to the apical oxygen to be easily substituted with Bi3þbecause of the greater stability of the crystal structure.Based on the above information, an increase in the ratio ofBi3þ substitutions at the Sr2þ sites in the compound is expected toreduce the unit cell parameter c because of the substitution of thesmall ionic radius of Bi3þ. In addition, owing to the valence differ-ence between Bi3þ and Sr2þ, the hole content in the CuO2 planedecreases. This behavior can also be understood from the substitu-tion effect of the Ca2þ site by Bi3þ and Y3þ.32,33,48 Moreover, anexcessive increase in the Bi content caused inhomogeneity in thecrystal structure and crystal defects, such as stacking faults in thecompound. These effects would appear in transport characteristics.The dependence of the experimental results on the Bi/Sr ratiocan be understood from these perspectives. As shown in Fig. 2, thesamples with higher x values exhibited a smaller unit cell parame-ter c. The strong suppression of the Tc value of Bi2.25–N2 ismainly from the reduction in the hole content of the CuO2 planedue to the substitution of Sr2þ sites by Bi3þ. The device characteris-tics are shown in Fig. 3. The lower hole content characteristics ofthe Bi2.25–N2 sample were reflected in the lowest Jc and Tc values.Based on the results for the unit cell parameter c, the Jc value ofBi2.25–N2 is expected to be larger than those of the other samplesbecause Bi2.25–N2 is expected to have the shortest IJJ tunnelingbarrier. However, the experimental results indicate that the domi-nant effect on the Jc value of the sample was the hole content ofthe CuO2 plane.Next, we discuss the effect of oxygen content in the crystal.The oxygen content can be tuned by varying the annealing condi-tions, such as atmospheric gas and heating temperatures.35According to previous studies,32,33,48 excess oxygen atoms arelocated in or near the Bi2O2 insulating layers. In addition, anincrease in the oxygen content is reflected as a reduction in the dis-tance between the BiO double layers and an increase in the holecontent of the CuO2 layers owing to the electric charge balance.As shown in Figs. 1 and 2, as the oxygen content increases,the transition temperature Tc increases, and the unit cell parameterc decreases. This feature is reflected in the device characteristicsas an increase in the maximum critical current Ic owing to theincrease in the carrier concentration of the sample as well asthe reduction in the distance between the BiO double layers. Thedevice characteristics of the oxygen-annealed samples are not pre-sented here. The characteristics mentioned above can be found notonly in our previous paper47 but also in a previous study in whichslightly underdoped crystals were used.36The data shown in Fig. 2 are beneficial for the preparation ofcrystal chips. The data indicate that the oxygen-annealed sampleshad an inhomogeneous oxygen content. This factor has an effectFIG. 3. Typical temperature dependence of the IVCs for Bi2.05-N2 (a-1), Bi2.15–N2 (b-1), and Bi2.25–N2 (c-1). The radiation intensities detected by the HEB, Vbol, areplotted as a function of the applied bias voltages for Bi2.05-N2 (a-2), Bi2.15–N2 (b-2), and Bi2.25–N2 (c-2). The temperature shown with the curve indicates Tb of thesample. The insets in the IVC-plots show the temperature dependence of the c axis resistance.Journal ofApplied PhysicsARTICLE pubs.aip.org/aip/japJ. Appl. Phys. 135, 073902 (2024); doi: 10.1063/5.0188077 135, 073902-6© Author(s) 2024 08 October 2024 06:40:49https://pubs.aip.org/aip/japon various device characteristics. Therefore, during the fabricationprocesses of crystal chips, care must be taken to maintain thehomogeneity of the oxygen content in the crystals.Finally, based on previous discussion and the Josephson rela-tion, we discuss how the characteristics of the material are reflectedin the device characteristic. According to the Josephson relation fora conventional superconducting Josephson junction, the currentdensity Jac in the resistive state is Jac ¼ Jc sin[(2eV/�h) t + const.],where e is the electric charge, �h is Planck’s constant divided by 2π,t is the time, and V is the bias voltage applied to the junction.Typically, Jc is proportional to junction tunnel conductance.51Hence, in the case of IJJs, Jc is characterized by material character-istics, such as the unit cell parameter c, carrier density, and Bi/Srnonstoichiometry.The experimental results discussed here show that the value ofJc can be tuned by controlling the Bi/Sr ratios and the oxygen con-tents of the compound. The primary effect of the change in thesevalues on the IJJs is the adjustment of the carrier content of thesample. To increase the emission power of terahertz waves fromthe devices, a higher value of Jc would be better according to theJosephson relation. This feature can be clearly seen in the resultsfor the Bi2.25–N2 sample, which shows no emission owing to thestrong suppression of the IVCs related to lower Jc and Tc. For thesuppression of emission, the inhomogeneity of the crystal will alsobe reflected, although further studies are needed to clarify its effecton IJJ devices. As discussed in a previous study,32 the decrease inthe value of the maximum Tc with increasing x value originatesfrom the inhomogeneity of the crystal structure due to Bi/Srnonstoichiometry.The high Jc sample prepared by increasing the oxygen contentsis unsuitable for the current device structure because of the require-ment of the application of a large amount of bias current to obtaina resistive state of IJJs, the requirement of good heat-removal struc-tures owing to the higher current application, and the inhomogene-ity of the oxygen content of the crystal surface. Consequently, it isexpected that a crystal prepared with a lower x value and slightlyunder-doped conditions would be better for the device to operate ata higher temperature with higher emission power.In summary, we demonstrated that the crystal properties ofBi2212 affect the characteristics of THz-wave-emitting devicesmade from Bi2212. The results of this study indicate that thecarrier concentration in a crystal chip can be adjusted by control-ling the Bi/Sr ratio and δ. In addition, the results suggest that thepreparation of crystals, including annealing under appropriate con-ditions depending on the Bi/Sr ratio, is crucial for producing thedesired IJJ devices. Although further evaluation of the device char-acteristics with a similar number of carriers for each Bi/Sr ratio isrequired to fully understand the effect of Bi/Sr substitution on IJJdevices, the experimental results presented herein provide a helpfulguide for preparing crystals for THz-wave-emitting devices.IV. CONCLUSIONBi2212 crystals with different Bi/Sr ratios and δ values wereprepared, and the crystal and device characteristics were comparedas a first step toward understanding the device characteristicsrelated to the crystal characteristics. No significant difference wasobserved in the electric device characteristics between samples withBi/Sr = 2.05/1.95 and 2.15/1.85. However, for the sample with Bi/Sr= 2.25/1.75, a significant change in device properties was observed.This feature can be understood primarily from the change in thenumber of carriers in the crystals, which can be adjusted by con-trolling the Bi/Sr ratio and δ. These results indicate that the prepa-ration of crystals, including annealing under appropriate conditionsdepending on the Bi/Sr ratio, is crucial for producing desired IJJdevices. The experimental results presented herein provide ahelpful guide for preparing crystals for THz-wave-emitting devices.ACKNOWLEDGMENTSThis study was supported by the Japan Society for thePromotion of Science Grant-in-Aid for Scientific Research (C) No.JP17K05018 and Research (B) Nos. JP20H02590, JP21H01377, andJP23H01819. This study was also supported by TIA-Kakehashigrants (Nos. 2018-43 and 2019-47). The x-ray diffraction measure-ments were performed with the approval of the Photon FactoryProgram Advisory Committee (Proposal No. 2019G634).AUTHOR DECLARATIONSConflict of InterestThe authors have no conflicts to disclose.Author ContributionsM. Nakayama: Conceptualization (supporting); Investigation(equal); Resources (equal); Visualization (lead); Writing – originaldraft (lead); Writing – review & editing (equal). S. Nakagawa:Conceptualization (equal); Investigation (equal); Resources (equal);Writing – review & editing (supporting). T. Yamaguchi:Investigation (equal). H. Minami: Conceptualization (supporting);Project administration (equal); Resources (equal); Supervision(supporting); Writing – review & editing (equal). K. Kadowaki:Resources (equal); Supervision (supporting). H. Nakao:Conceptualization (supporting); Investigation (supporting); Projectadministration (supporting); Resources (equal); Supervision (sup-porting); Writing – review & editing (supporting). T. Mochiku:Conceptualization (supporting); Project administration (support-ing); Supervision (supporting); Writing – review & editing (sup-porting). M. Tsujimoto: Project administration (supporting);Resources (supporting); Supervision (supporting); Writing – review& editing (supporting). S. Ishida: Conceptualization (supporting);Project administration (supporting); Resources (supporting);Supervision (equal); Writing – review & editing (supporting).H. Eisaki: Conceptualization (supporting); Project administration(supporting); Resources (equal); Supervision (equal); Writing –review & editing (supporting). T. Kashiwagi: Conceptualization(equal); Funding acquisition (lead); Investigation (equal); Projectadministration (equal); Resources (equal); Supervision (equal);Writing – original draft (lead); Writing – review & editing (lead).DATA AVAILABILITYThe data that support the findings of this study are availablewithin the article.Journal ofApplied PhysicsARTICLE pubs.aip.org/aip/japJ. Appl. Phys. 135, 073902 (2024); doi: 10.1063/5.0188077 135, 073902-7© Author(s) 2024 08 October 2024 06:40:49https://pubs.aip.org/aip/japREFERENCES1B. Ferguson and X. C. Zhang, Nat. Mater. 1, 26 (2002).2M. Tonouchi, Nat. Photonics 1, 97 (2007).3S. S. Dhillon, M. S. Vitiello, E. H. Linfield, A. G. Davies, M. C. Hoffmann,J. Booske, C. Paoloni, M. Gensch, P. Weightman, G. P. Williams et al., J. Phys.D: Appl. Phys. 50, 043001 (2017).4K. Okada, K. Serita, Q. 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