Millikelvin Temperature Range Research Articles

Quantum paraelectric varactors for radiofrequency measurements at millikelvin temperatures

AbstractRadiofrequency reflectometry can provide fast and sensitive electrical read-out of charge and spin qubits in quantum dot devices coupled to resonant circuits. In situ frequency tuning and impedance matching of the resonator circuit using voltage-tunable capacitors (varactors) is needed to optimize read-out sensitivity, but the performance of conventional semiconductor- and ferroelectric-based varactors degrades substantially in the millikelvin temperature range relevant for solid-state quantum devices. Here we show that strontium titanate and potassium tantalate, materials which can exhibit quantum paraelectric behaviour with large field-tunable permittivity at low temperatures, can be used to make varactors with perfect impedance matching and resonator frequency tuning at 6 mK. We characterize the varactors at 6 mK in terms of their capacitance tunability, dissipative losses and magnetic field insensitivity. We use the quantum paraelectric varactors to optimize the radiofrequency read-out of carbon nanotube quantum dot devices, achieving a charge sensitivity of 4.8 μe Hz−1/2 and a capacitance sensitivity of 0.04 aF Hz−1/2.

Josephson-like Tunnel Resonance and Large Coulomb Drag in GaAs-Based Electron-Hole Bilayers.

Bilayers consisting of two-dimensional (2D) electron and hole gases separated by a 10nm thick AlGaAs barrier are formed by charge accumulation in epitaxially grown GaAs. Both vertical and lateral electric transport are measured in the millikelvin temperature range. The conductivity between the layers shows a sharp tunnel resonance at a density of 1.1×10^{10} cm^{-2}, which is consistent with a Josephson-like enhanced tunnel conductance. The tunnel resonance disappears with increasing densities and the two 2D charge gases start to show 2D-Fermi-gas behavior. Interlayer interactions persist causing a positive drag voltage that is very large at small densities. The transition from the Josephson-like tunnel resonance to the Fermi-gas behavior is interpreted as a phase transition from an exciton gas in the Bose-Einstein-condensate state to a degenerate electron-hole Fermi gas.

Superconductivity in Ce-based cage compounds

Cerium-based ternary compounds CeNi2Cd20 and CePd2Cd20 do not exhibit long-range order down to millikelvin temperature range. Given the large separation between Ce ions which significantly reduces the super-exchange interactions and vanishingly small Ruderman–Kittel–Kasuya–Yosida interaction, here we show that nodal superconductivity mediated by the valence fluctuations must be a ground state in these materials. We propose that the critical temperature for the superconducting transition can be significantly increased by applying hydrostatic pressure. We employ an extended periodic Anderson lattice model which includes the long-range Coulomb interactions between the itinerant electrons as well as the local Coulomb interaction between the predominantly localized and itinerant electrons to compute a critical temperature of the superconducting transition. Using the slave-boson approach we show that fluctuations mediated by the repulsive electron–electron interactions lead to the emergence of d-wave superconductivity.

High-Performance Cryogen-Free Platform for Microkelvin-Range Refrigeration

Accessibility to the microkelvin temperature regime is important for quantum technology research. Cryogen-free dilution refrigerators opened up the millikelvin temperature range, but the perception is that ultralow temperatures are difficult to reach and require specialist infrastructure. The authors report the design of a microkelvin platform based on a nuclear-demagnetization stage, engineered and optimized for operation on a cryogen-free dilution refrigerator. The study demonstrates temperatures as low as 395 $\ensuremath{\mu}$K, and a protocol that enables experiments operated below 1 mK for 95% of the time, providing an efficient cryogen-free microkelvin environment for a wide range of applications.

Intrinsic dissipation mechanisms in metallic glass resonators.

Micro- and nanoresonators have important applications including sensing, navigation, and biochemical detection. Their performance is quantified using the quality factor Q, which gives the ratio of the energy stored to the energy dissipated per cycle. Metallic glasses are a promising material class for micro- and nanoscale resonators since they are amorphous and can be fabricated precisely into complex shapes on these length scales. To understand the intrinsic dissipation mechanisms that ultimately limit large Q-values in metallic glasses, we perform molecular dynamics simulations to model metallic glass resonators subjected to bending vibrations at low temperatures. We calculate the power spectrum of the kinetic energy, redistribution of energy from the fundamental mode of vibration, and Q vs the kinetic energy per atom K of the excitation. In the harmonic and anharmonic response regimes where there are no atomic rearrangements, we find that Q → ∞ over the time periods we consider (since we do not consider coupling to the environment). We identify a characteristic Kr above which atomic rearrangements occur, and there is significant energy leakage from the fundamental mode to higher frequencies, causing finite Q. Thus, Kr is a critical parameter determining resonator performance. We show that Kr decreases as a power-law, Kr ∼ N-k, with increasing system size N, where k ≈ 1.3. We estimate the critical strain ⟨γr⟩∼ 10-8 for micrometer-sized resonators below which atomic rearrangements do not occur in the millikelvin temperature range, and thus, large Q-values can be obtained when they are operated below γr. We also find that Kr for amorphous resonators is comparable to that for resonators with crystalline order.

High Q value Quartz Tuning Fork in Vacuum as a Potential Thermometer in Millikelvin Temperature Range

The results of a newly developed pulse-demodulation (P-D) technique introduced to determine the resonant characteristics of a high Q value quartz tuning forks in vacuum and millikelvin temperature range are presented. Applying P-D technique to a standard 32 kHz quartz tuning fork with extremely low excitation energy of the order of a few femtojoules, we were able to measure the resonance frequency of the fork’s decay signal with resolution better than 10 $$\upmu $$ Hz. Using this highly sensitive measurement technique, we found a continuous and reproducible temperature dependence of the tuning fork’s resonance frequency in the millikelvin temperature range. The observed dependence suggests a potential application for the quartz tuning forks to be used as thermometers in the millikelvin temperature range. We also discuss the physical origin of the observed phenomenon.

Studies of scintillation properties of CaMoO4 at millikelvin temperatures

Application of CaMoO4 as a scintillation target in cryogenic rare event searches relies on the understanding of scintillation properties of the material at the temperatures at which these detectors operate. We devised and implemented a detection module with a low-temperature photomultiplier from Hamamatsu (model R8520-06) powered by a Cockcroft-Walton generator. The detector module containing the CaMoO4 crystal was placed in a 3He/4He dilution refrigerator and used to measure scintillation characteristics of CaMoO4 in the millikelvin temperature range. At the lowest temperature achieved, the energy resolution of CaMoO4 for 122 keV γ from a 57Co source is found to be 30%, and the fast and slow decay constants are 40.6 ± 0.8 μs and 3410 ± 50 μs, respectively. The temperature variation of the CaMoO4 decay kinetics is discussed in terms of a three-level model of the emission center.

Microwave Losses in YBCO Coplanar Waveguide Resonators at Low Power and Millikelvin Range

We have investigated the temperature dependence of microwave losses in the low-power limit and millikelvin temperature range of a thin-film YBa2Cu3O7-x (YBCO) λ/2 coplanar waveguide resonator patterned on a (La0.3Sr0.7) (Al0.65Ta0.35)O3 (LSAT) substrate covered by a CeO2 seed layer. The unloaded quality factor is mainly governed by the surface resistance of the YBCO film and the dielectric losses caused by resonant absorption due to a bath of two-level fluctuators in the dielectrics. The value of the unloaded quality factor at 20 mK and low power, i.e., Q0∼2000, allows for the realization of YBCO microwave quantum circuits implementing biepitaxial grain-boundary Josephson junctions.

Microwave losses in MgO, LaAlO3, and (La0.3Sr0.7)(Al0.65Ta0.35)O3 dielectrics at low power and in the millikelvin temperature range

We have investigated both the temperature and the power dependence of microwave losses for various dielectrics commonly used as substrates for the growth of high critical temperature superconductor thin films. We present measurement of niobium superconducting λ∕2 coplanar waveguide resonators, fabricated on MgO, LaAlO3, and (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT), at the millikelvin temperature range and at low input power. By comparing our results with the two-level system model, we have discriminated among different dominant loss mechanisms. LSAT has shown the best results as regards the dielectric losses in the investigated regimes.

Spin-Orbital Short-Range Order on a Honeycomb-Based Lattice

Frustrated magnetic materials, in which local conditions for energy minimization are incompatible because of the lattice structure, can remain disordered to the lowest temperatures. Such is the case for Ba(3)CuSb(2)O(9), which is magnetically anisotropic at the atomic scale but curiously isotropic on mesoscopic length and time scales. We find that the frustration of Wannier's Ising model on the triangular lattice is imprinted in a nanostructured honeycomb lattice of Cu(2+) ions that resists a coherent static Jahn-Teller distortion. The resulting two-dimensional random-bond spin-1/2 system on the honeycomb lattice has a broad spectrum of spin-dimer-like excitations and low-energy spin degrees of freedom that retain overall hexagonal symmetry.

Note: Ultra-high frequency ultra-low dc power consumption HEMT amplifier for quantum measurements in millikelvin temperature range

We have presented theory and experimentally demonstrated an efficient method for drastically reducing the power consumption of the rf/microwave amplifiers based on HEMT in unsaturated dc regime. Conceptual one-stage 10 dB-gain amplifier showed submicrowatt level of the power consumption (0.95 μW at frequency of 0.5 GHz) when cooled down to 300 mK. Proposed technique has a great potential to design the readout amplifiers for ultra-deep-cooled cryoelectronic quantum devices.

Level crossings and zero-field splitting in the{Cr8}-cubane spin cluster studied using inelastic neutron scattering and magnetization

Inelastic neutron scattering (INS) in variable magnetic field and high-field magnetizationmeasurements in the millikelvin temperature range were performed to gain insight into the low-energymagnetic excitation spectrum and the field-induced level crossings in the molecular spin cluster{Cr8}-cubane. These complementary techniques provide consistent estimates of the lowestlevel-crossing field. The overall features of the experimental data are explained using anisotropic Heisenberg model, based on three distinct exchange interactions linking the eightCrIII paramagneticcenters (spins s = 3/2), that is supplemented with a relatively large molecular magnetic anisotropy term for the lowestS = 1 multiplet. It is noted that the existence of the anisotropy is clearly evident from themagnetic field dependence of the excitations in the INS measurements, while themagnetization measurements are not sensitive to its effects.

Resistivity Anomaly in Structurally Disordered PbFCl-type Arsenide Selenides

Single crystals of the PbFCl-type arsenide selenides ThAsSe and ZrAs1.40 Se0.50 display intrinsic structural disorder, as inferred from X-ray and electron diffraction as well as transmission-electron-microscopy studies. This structural disorder is held responsible for the formation of tunneling states that are a cause of glass-type temperature dependencies of both the thermal conductivity and the specific heat in the millikelvin temperature range. The electrical resistivity increases as –AT 1/2 upon cooling to below 16 K. Furthermore, it is unaffected by the application of a magnetic field. These observations are considered as evidence for an experimental realization of a two-channel Kondo effect derived from structural two-level systems.

Magneto-structural correlation in Cu(NH 3) 2Ag 2(CN) 4. Crystal structure, magnetic and thermodynamic properties of an S = 1/2 low-dimensional Heisenberg antiferromagnet

Crystal structure, thermodynamic properties and electron spin resonance spectra of Cu(NH 3) 2Ag 2(CN) 4 have been studied. The structure of the studied compound consists of interpenetrating wave-shaped two-dimensional (2d) arrays, in which CuN 4N 2 octahedra axially elongated due to the Jahn–Teller effect are linked by diamagnetic [Ag(CN) 2] − anions forming a square lattice. Although the susceptibility and specific heat data confirm the existence of short-range order at about 2 K, quantitative analysis of the experimental data reveals that the magnetic behavior differs from that of the crystal structure expected for 2d Heisenberg magnet on the square lattice. Structural features responsible for the observed difference are discussed. In addition, it is suggested, that additional degrees of freedom contribute to the thermodynamic equilibrium properties in the millikelvin temperature range.

Study of the microwave-induced transport through a quantum dot inserted in a 35-GHz loop-gap resonator

We report on the design and performance of an experimental setup dedicated to the high-frequency manipulation of individual electron spins in quantum dots in the milliKelvin temperature range. Engel and Loss [Phys. Rev. Lett. 86 (2001) 004648] proposed how the spin-state can be read out via a charge current through the dot in the Coulomb blockade state. A major challenge is that fast manipulation compared to decoherence requires a large microwave (mw) magnetic field, which is technically difficult to produce on a nanometer scale without much power and without substantial capacitive coupling between the dot and its electromagnetic surrounding. We show that our setup overcomes these difficulties.

Magnetic ordering in the erbium honeycomb lattices of ErX 3 (X=Cl, Br, I)

Unusual two-dimensional (2D) and three-dimensional (3D) magnetic ordering phenomena were observed in the insulating, transparent erbium trihalides ErX 3 (X=Cl, Br, I) in the milli-Kelvin temperature range. The layer-type crystal structures of ErCl 3 (AlCl 3-type structure) and ErBr 3/ErI 3 (BiI 3-type structure) are closely related. All three compounds show a new type of two-sublattice 120° antiferromagnetic order in the erbium honeycomb layers with an infinite rotational degeneracy. The magnetic structures were determined by powder and single-crystal neutron diffraction. ErCl 3 shows short-range 2D magnetic order above 350 mK to several Kelvin and a transition to 3D magnetic order at 350 mK with a k -vector of ( 2 3 , 0, − 1 12 ). ErBr 3 and ErI 3 display short-range 2D magnetic order from 400 mK to several Kelvin, long-range 2D order between 280 and 400 mK, and short-range 3D order below 280 mK with a correlation length of 15 Å along the c-axis and k=( 1 3 , 1 3 , 0) . The differences between ErCl 3 and ErBr 3/ErI 3 are due to different geometrical layer stackings. For ErBr 3/ErI 3 a threefold ambiguity in the magnetic layer stacking causes the disorder along the c-axis. The ordered magnetic moments at saturation increase from 3.3 to 4.7 and 5.5 μ B/Er 3+ for ErCl 3, ErBr 3 and ErI 3, respectively.

Thermodynamics of 3He solid monolayers in the Heisenberg model

The two-dimensional Heisenberg model is applied to the interpretation of the experimental data on the thermodynamic and magnetic properties of 3He monoatomic films in the millikelvin temperature range, i.e., under conditions when these properties are completely governed by the dynamics of the nuclear spin subsystem. The theoretical results obtained make it possible to describe the internal energy E, the heat capacity Cs, and the magnetic susceptibility χ of the two-dimensional spin-1/2 Heisenberg ferromagnets and antiferromagnets on a triangular lattice within the unified approach over the entire range of temperatures. The data available in the literature on the heat capacity and magnetic susceptibility of 3He films are interpreted in the framework of the advanced theory. Most attention is concentrated on the layers characterized by the ferromagnetic exchange. Comparative analysis of theoretical and experimental data is carried out with the use of two fitting parameters: the exchange interaction constant J and the number of “active” spins n2 in the layer that is determined from the entropy of the system in the limit T → ∞. It is demonstrated that, for the ferromagnetic layers, the theoretical results obtained within the Heisenberg model are in very good agreement with the experimental data reported by different authors.

Noncollinear two- and three-dimensional magnetic ordering in the honeycomb lattices ofErX3(X=Cl,Br,I)

Unusual magnetic ordering phenomena have been observed in the three title compounds in the milli-Kelvin temperature range. The ratio of interlayer to intralayer coupling and thus the critical behavior and the nature of the magnetic order can be tuned by chemical variation. In all three lattices the two-dimensional (2D) magnetic order within the layers is a two sublattice 120\ifmmode^\circ\else\textdegree\fi{} antiferromagnetic order with an infinite rotational degeneracy. The ordered magnetic moments increase from 3.3 to 4.7 and $5.5{\ensuremath{\mu}}_{B}/{\mathrm{Er}}^{3+}$ for ${\mathrm{ErCl}}_{3},$ ${\mathrm{ErBr}}_{3},$ and ${\mathrm{ErI}}_{3},$ respectively. ${\mathrm{ErCl}}_{3}$ shows a transition to 3D magnetic order at 350 mK with a k vector of $(\frac{2}{3},\phantom{\rule{0ex}{0ex}}0,\ensuremath{-}\frac{1}{12}).$ ${\mathrm{ErBr}}_{3}$ and ${\mathrm{ErI}}_{3}$ are the first examples of rare-earth trihalides displaying short-range 2D magnetic order from 400 mK up to several Kelvin, long-range 2D order between 280 mK and 400 mK, long-range 2D and short-range 3D order with $\mathbf{k}=(\frac{1}{3},\frac{1}{3},0),$ and a correlation length of 15 \AA{} below 280 mK. The differences are attributed to the different layer stackings in ${\mathrm{ErCl}}_{3}$ and ${\mathrm{ErBr}}_{3}{/\mathrm{E}\mathrm{r}\mathrm{I}}_{3}$ as well as the variation of interlayer distances.

Design of RuO 2-based thermometers for the millikelvin temperature range

RuO 2-based thick film resistors (TFRs) were designed for use as low temperature thermometers in the millikelvin temperature range. The R- T characteristics and sensitivities of these thermometers below 1 K are comparable with germanium thermometers usually used in this temperature range. However, their low electrical resistance and their sensitivity are opposing properties which put certain technological limitations on their design. The temperature dependence of the resistance of RuO 2 sensors at the lowest temperatures investigated can be adequately described in terms of a variable-range hopping mechanism.

Advances in dilution refrigeration in high magnetic fields

3He/ 4He dilution refrigerators have become a standard laboratory tool used to cool experiments to the milli-kelvin temperature range. Many of these applications involve the use of high magnetic fields. One system includes a 20.7 T superconducting magnet (the highest field ever achieved using multi filamentary superconducting wire in compact magnet form) and it can cool samples to 20 mK. Other systems have been designed for use with rapidly sweeping superconducting magnets, and even for pulsed liquid nitrogen cooled magnets, with maximum fields up to 60 T.