太赫兹辐射源:

We report on a Stirling-cooled compact Bi2Sr2CaCu2O8+δ intrinsic Josephson-junction stack with very high critical current density and improved cooling, operating at bath temperatures Tb up to 86 K. The square stand-alone stack is embedded between two sapphire substrates. For bath temperatures between 27.8 and 86 K emission is observed at frequencies from 0.356 to 2.09 THz. The emission power exceeds 1 μW at bath temperatures between 60 and 80 K for emission frequencies between 0.5 and 0.88 THz. A record high value of 0.577 THz is obtained for the emission frequency at Tb =80 K, which is important for potential applications using liquid nitrogen as a coolant. We also compare our experimental results with numerical simulations based on three-dimensional coupled sine-Gordon equations combined with heat diffusion equations.

Physical Review Applied 10, 024041 (2018)
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We developed a Bi2Sr2CaCu2O8 (BSCCO) intrinsic Josephson junction stacks with improved cooling, allowing for a remarkable increase in emission frequency compared to the previous designs. We started with a BSCCO stack embedded between two gold layers. When mounted in the standard way to a single substrate, the stack emits in the range of 0.43–0.82 THz. We then glued a second, thermally anchored substrate onto the sample surface. The maximum voltage of this better cooled and dimension-unchanged sample was increased and, accordingly, both the emission frequencies and the tunable frequency range were significantly increased up to 1.05 THz and to 0.71 THz, respectively. This double sided cooling may also be useful for other “hot” devices, e.g., quantum cascade lasers.

Appl. Phys. Lett. 105, 122602 (2014)
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We developed a liquid-nitrogen-cooled compact source for continuous terahertz

(THz) emission. The emitter is a Bi2Sr2CaCu2O8-δ intrinsic Josephson-junction stack embedded between two gold layers and sandwiched between two MgO substrates. The radiation is emitted to free space through a hollow metallic tube acting as a waveguide. The maximum emission power is 1.17 μW. The tunable emission frequency bandwidth is up to 100 GHz with a maximum emission power at 0.311 THz. Since the operation voltage is about 1 V and the current is less than 30 mA, we are able to drive this terahertz source at 77 K with only one commercial 1.5-V battery, just like a torch. This convenient and economical setup may find applications in fields like tracer-gas detection or nondestructive evaluation.

Phys. Rev. Applied 3, 024006 (2015)  
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We measured the linewidth f of terahertz radiation emitted from intrinsic Josephson junction stacks, using a Nb/AlN/NbN integrated receiver for detection. Previous resolution-limited measurements indicated that f may be below 1 GHz—much smaller than expected from a purely cavity-induced synchronization. While at low bias we found f to be not smaller than ∼500 MHz, at high bias, where a hot spot coexists with regions which are still superconducting, f turned out to be as narrow as 23 MHz. We attribute this to the hot spot acting as a synchronizing element. f decreases with increasing bath temperature, a behavior reminiscent of motional narrowing in NMR or electron spin resonance (ESR), but hard to explain in standard electrodynamic models of Josephson junctions.

Phys. Rev. B 86, 060505(R) (2012) (Editors' Suggestion)
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太赫兹辐射源原理探索:

We report on the electrothermal behavior and the terahertz emission properties of a stand-alone Bi2Sr2CaCu2O8 intrinsic Josephson junction stack contacted in a three-terminal configuration. One terminal is used as a collective ground while the other two, contacting the stack from its right and left side, allow to vary the current injection profile. At high bias, a hot spot forms in the stack. Its appearance and position can be controlled by varying the ratios of the injected currents. Depending on this ratio, the emitted power can vary by an order of magnitude. Further, for a given total injection current, the device allows to vary the emission frequency on a 10% level by altering the injection profile. The overall tunability of the emission frequency, varying also the total bias current, is on the order of 20%.

Appl. Phys. Lett. 107, 122206 (2015)
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We investigated on tuning the THz emission of a Bi2Sr2CaCu2O8 (BSCCO) intrinsic Josephson-junction stack by a focused laser beam which is scanned across the stack. The emission power Pe increases by up to 75% upon laser irradiation for a bath temperature near 22 K. The laser-induced changes in the voltage Vdc across the stack and in the emission power are measured simultaneously. The maximum of the laser-induced changes in emission power ΔPe is achieved by irradiating the stack on the location where the local temperature is about the critical temperature Tc. However, ΔPe  is found to be proportional to the laserinduced global voltage change ΔVdc, irrespective of the laser position. This unexpected global response is likely to be related to a change in the average stack temperature and is consistent with the change in Pe when increasing the bath temperature by about 0.2 K. This tuning method can be employed in the application of BSCCO THz sources.

Phys. Rev. Applied 3, 044012 (2015)
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We studied on THz emission measurements and low temperature scanning laser imaging of Bi2Sr2CaCu2O8 intrinsic Josephson junction stacks. Coherent emission is observed at large dc input power, where a hot spot and a standing wave, formed in the ‘‘cold’’ part of the stack, coexist. By changing bias current and bath temperature, the emission frequency can be varied by more than 40%; the variation matches the Josephson-frequency variation with voltage. The linewidth of radiation is much smaller than expected from a purely cavity-induced synchronization. Thus, an additional mechanism seems to play a role. Some scenarios, related to the presence of the hot spot, are discussed.

Phys. Rev. Lett. 105, 057002 (2010)
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At high enough input power in stacks of Bi2Sr2CaCu2O8 intrinsic Josephson junctions a hot spot a region heated to above the superconducting transition temperature coexists with regions being still in the superconducting state. In the “cold” regions cavity resonances can occur, synchronizing the ac Josephson currents and giving rise to strong and stable coherent terahertz THz emission. We investigated the interplay of hot spots and standing electromagnetic waves by low-temperature scanning laser microscopy and THz emission measurements, using stacks of various geometries. Standing electromagnetic wave patterns and THz emission are observed for a disk-shaped sample. The growth of a hot spot with increasing input power is monitored by small detector junctions surrounding a large rectangular mesa. For two rectangular mesas equipped with two current injectors and one arrow-shaped structure we show that the standing wave can be turned on and off in various regions of the stack structure, depending on the hot-spot position. The results support the picture of the hot spot acting as a reflective termination of the cavity, formed by the cold part of the mesa.

Phys. Rev. B 82, 214506 (2010)
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Large stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8 can emit synchronous THz radiation, the synchronization presumably triggered by a cavity resonance. To investigate this effect we use low temperature scanning laser microscopy to image electric field distributions. We verify the appearance of cavity modes at low bias and in the high input-power regime we find that standing-wave patterns are created through interactions with a hot spot, possibly pointing to a new mode of generating synchronized radiation in intrinsic Josephson junction stacks.

Phys. Rev. Lett. 102, 017006 (2009)
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We have experimentally investigated the fluxon dynamics in a narrow Bi2Sr2CaCu2O8 stack with junction length L of 1.8 μm. As an evidence of high-frequency excitation by a collective cavity mode, under an inplane external magnetic field, the current-voltage characteristics show prominent Fiske steps with the corresponding resonance frequencies of 75–305 GHz. Further study of flux-flow resistance oscillation with various c-axis currents clarifies the correlation with Fiske steps by distinguishing two different regions, i.e., the static flux-flow region at low bias current level and dynamic Fiske step region at high bias current level.

Phys. Rev. B 72, 140504 (2005)
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太赫兹检测:

We have combined a stand-alone Bi2Sr2CaCu2O8 intrinsic Josephson junction stack, emitting terahertz radiation, with a YBa2Cu3O7 grain boundary Josephson junction acting as detector. The detector is mounted on a lens, positioned 1.2 cm away from the emitter on a similar lens. With the emitter radiating at 0.5 THz, we observed up to 7 Shapiro steps on the current-voltage characteristic of the detector. The ac current induced in this junction was 0.9 mA, and the dissipated power was 1.8 μW. The setup, although far from being optimized, may be considered as a first step towards an integrated high-Tc receiver.

Appl. Phys. Lett. 102 , 092601 (2013)
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High frequency responses of intrinsic Josephson junctions up to 2.5 THz, including the observation of Shapiro steps under various conditions, are reported and discussed in this Letter. The sample was an array of intrinsic Josephson junctions singled out from inside a high Tc superconducting Bi2Sr2CaCu2O8-δ single crystal, with a bow-tie antenna integrated to it. The number of junctions in the array was controllable, the junctions were homogeneous, the distribution of applied irradiation among the junctions was even, and the junctions could synchronously respond to high frequency irradiation.

Phys. Rev. Lett. 87, 107002 (2001)
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太赫兹器件及制作工艺:

We demonstrate the operation of a deterministic fluxon ratchet made of a stack of 30 intrinsic Bi2Sr2CaCu2O8 Josephson junctions. The ratchet has the shape of a gear with 20 asymmetric teeth periods. It produces a rectified voltage of about 100 V at a 12 GHz drive frequency. The effect of coupling between intrinsic junctions, i.e., the mode of fluxon motion for ratchet operation, has been studied within the framework of the two-dimensional coupled sine-Gordon equations. Further, we used low-temperature scanning laser microscopy to demonstrate that voltage rectification indeed is due to directed fluxon motion, in agreement with numerical simulations.

Phys. Rev. B 80, 224507 (2009)
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Modifying the double-sided fabrication method we invented a few years ago, a new type of Bi2Sr2CaCu2O8 intrinsic Josephson junction (IJJs) array is explored in which, in addition to the indispensable top and bottom electrodes, there is an electrode only 100 nm thick fabricated in the middle of the array. This electrode provides easy access to the inside of the IJJs, enabling us to understand the device physics better than was possible before. As the first application of such a new device, we have clearly shown how “hot” a junction stack can be under various bias conditions.

Appl. Phys. Lett. 86, 023504 (2005)
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铁基超导单晶纳米线对超导对称性研究:

The determination of the pairing symmetry is one of the most crucial issues for the iron-based superconductors, for which various scenarios are discussed controversially. Non-magnetic impurity substitution is one of the most promising approaches to address the issue, because the pair-breaking mechanism from the non-magnetic impurities should be different for various models. Previous substitution experiments demonstrated that the non-magnetic zinc can suppress the superconductivity of various iron-based superconductors. Here we demonstrate the local destruction of superconductivity by non-magnetic zinc impurities in Ba0.5K0.5Fe2As2 by exploring phase-slip phenomena in a mesoscopic structure with 119×102 nm2 cross-section. The impurities suppress superconductivity in a three-dimensional ‘Swiss cheese’-like pattern with in-plane and out-of-plane characteristic lengths slightly below ~1.34 nm. This causes the superconducting order parameter to vary along abundant narrow channels with effective cross-section of a few square nanometres. The local destruction of superconductivity can be related to Cooper pair breaking by non-magnetic impurities.

Nat. Commun. 6, 7614 (2015)
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铁基超导单晶纳米线研究临界磁场临界载流密度:

We investigated the upper critical fields of Ca10(Pt4As8)(Fe2–xPtxAs2)5 superconducting whiskers. The whiskers consist of several wire-like grains with diameter of around 200 nm, joined by grainboundaries whose misorientation angles are less than 58. The upper critical fields along c-axis andin ab-plane were observed as 49 T at 12K and 50 T at 22K, respectively, which can be extrapolatedto 81 and 133 T at 0 K. The whisker demonstrated weak anisotropic factor and almost constantvalue of 2 below 15 K. The impressive transport properties of the whisker may find applicationsin fields like superconducting micro- and meso-structure systems.

Appl. Phys. Lett. 106, 262601 (2015)
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铁基超导单晶微桥研究本征载流特性:

We investigated the normal-state resistivity ρxx(T)  and the Hall effect in Zn- and Co-doped Ba0.5K0.5Fe2As2 single-crystalline microbridges. A crossover temperature T* was observed in the temperature dependency of the longitudinal resistivity ρxx(T), which separates ρxx(T) into temperature-linear and temperature-nonlinear regions. Above T*, the carriers in Ba0.5K0.5Fe2As2 and Co-doped Ba0.5K0.5Fe1.94Co0.06As2 demonstrate electronlikebehavior and an anomalous nonlinear magnetic field dependence of the Hall voltage with a sign reversal. By contrast, the Zn-doped Ba0.5K0.5Fe1.94Co0.06As2 behaves like a hole type and the Hall coefficient is independent of the magnetic field. The field-induced sign reversal of the Hall coefficient of undoped and Co-doped samples depends on the field modification on the mobility and hole/electron concentration ratio. The T2-dependent Hall angle of a nonmagnetic Zn-doped crystal is observed as a nearly parallel shift from that of the impurity-free crystal in the low-temperature region, indicating that the Zn induces a weak change of the spinons excitations, while increasing the number of scattering centers. The Co works as a nonmagnetic impurity as well, while it provides both spinons excitations and impurity scattering.

Phys. Rev. B 90, 024512 (2014)
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铁基超导单晶纳米线的制备和研究:

Single-crystalline  Ca10(Pt4As8)(Fe1.8Pt0.2As2)5  superconducting (SC) nanowhiskers with widths down to hundreds of nanometers were successfully grown in a Ta capsule in an evacuated quartz tube by a flux method. Magnetic and electrical properties measurements demonstrate that the whiskers have excellent crystallinity with critical temperature of up to 33 K, upper critical field of 52.8 T, and critical current density of Jc of 6.0×105 A/cm2 (at 26 K). Since cuprate high-Tc SC whiskers are fragile ceramics, the present intermetallic SC whiskers with high Tc have better opportunities for device applications. Moreover, although the growth mechanism is not understood well, the technique can be potentially useful for growth of other whiskers containing toxic elements.

J. Am. Chem. Soc. 134, 4068-4071 (2012)
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