Quantum critical systems derive their finite-temperature properties from the influence of a zero-temperature quantum phase transition1. The paradigm is essential for understanding unconventional high-Tc superconductors and the non-Fermi liquid properties of heavy fermion compounds. However, the microscopic origins of quantum phase transitions in complex materials are often debated. Here we demonstrate experimentally, with support from numerical renormalization group calculations, a universal crossover from quantum critical non-Fermi liquid behaviour to distinct Fermi liquid ground states in a highly controllable quantum dot device. Our device realizes the non-Fermi liquid two-channel Kondo state2,3, based on a spin-1/2 impurity exchange-coupled equally to two independent electronic reservoirs4. On detuning the exchange couplings we observe the Fermi liquid scale T*, at energies below which the spin is screened conventionally by the more strongly coupled channel. We extract a quadratic dependence of T* on gate voltage close to criticality, and validate an asymptotically exact description of the universal crossover between strongly correlated non-Fermi liquid and Fermi liquid states5,6.
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We are grateful to S. Amasha, Y. Oreg, A. Carmi, E. Sela, A. K. Mitchell and M. Heiblum for discussions; H. K. Choi, Y. Chung and J. MacArthur for electronics expertise; M. Heiblum for use of his laboratory during initial device characterization; H. Inoue, N. Ofek, O. Raslin and E. Weisz for fabrication guidance; F. B. Anders, E. Lebanon and the late A. Schiller for their calculations which guided prior experimental work; and M. Stopa for his SETE software for electrostatic quantum dot modelling. The device was fabricated in the Braun Submicron Center at the Weizmann Institute of Science, with final fabrication steps done at Stanford Nano Shared Facilities (SNSF) at Stanford University. This work was supported by the Gordon and Betty Moore Foundation grant no. GBMF3429, the Hungarian research grant OTKA K105149, the Polish National Science Centre project no. DEC-2013/10/E/ST3/00213, EU grant no. CIG-303 689, the National Science Foundation grant no. DMR-0906062, and the US-Israel BSF grant no. 2008149. A.J.K. and L.P. were supported by a Stanford Graduate Fellowship. SETE calculations were run on the Odyssey cluster supported by the FAS Division of Science, Research Computing Group at Harvard University. NRG calculations were performed at the Poznań Supercomputing and Networking Center.
The authors declare no competing financial interests.
Extended data figures and tables
Acceleration voltage in the SEM was 5 kV. The device is tilted 40° with respect to normal incidence.
Top panel, G(VSD,VBWT) at T = 20 mK, exactly as in Fig. 3. Middle and bottom panels, T* (middle) and δP (bottom) as functions of VBWT. Black points and red curves are exactly as in Fig. 3; the blue points correspond to the weighted mean of extracted T* and δP for an ensemble of fitting ranges, and error bars on the blue points correspond to the s.d. of the weighted mean.
Extended Data Figure 3 Measured (G(0, T) − G(VSD, T))/(kT)1/2 (symbols) and CFT fit (solid lines) of Fig. 2e broken out into separate panels for each T.
Temperature T in mK is shown centrally in each panel. The range in measured VSD is from −31.5 to 28.5 µV, resulting in a decreasing range on the (eVSD/kT)1/2 axis as T is increased. The single fit in Fig. 2e is plotted against the measured data for each T.
Top left, measured G(VLP, VBWT) from Fig. 2c. Panels at right, measured (G(0, T) − G(VSD, T))/(kT)1/2 at six points on 2CK lines in the (VLP, VBWT) plane; points are indicated by coloured stars. Black lines are fits to thermally broadened spectral functions from ref. 26 with small phase shifts from potential scattering.
a, Measured G(VSD, VLP) reveals two prominent linear features, the slopes of which are labelled as m and n. b, Measured G(VSD = 0, T) (crosses) and fits using equation (8) (lines). Every measurement is the average of 20 successive traces. c, Power-law fit of peak height to extracted electron temperature yields an exponent of −1.04(1). d, Residuals of the fit shown in c are all less than 0.001e2/h.
Within each Coulomb diamond we label the number of electrons on the dot (N) as determined by charge sensing techniques. The intersection of the lines indicate U ≈ 2.9 meV.
a, Measurement scheme. G(VSD, VBWT) is measured using the grain’s own pair of measurement leads (red pads), which are isolated from the measurement leads of the dot by depleting gate BR. Gate BL is depleted to avoid shorting conductance through the channel just left of the grain. The current path is the red dashed line. The grey stars indicate ohmic contacts which are floated during measurement. b, G(VSD, VBWT) through the grain in the Coulomb blockade regime (T = 20 mK). The intersections of the lines indicate EC ≈ 160 µeV.
a, G(VSD, VLP) through the dot in the Coulomb blockade regime, with both the dot and grain formed. Here VBWT is such that the grain is Coulomb-blockaded. From the slopes of the dashed lines overlaid on the peaks in the data, we determine lever arms αLP = 0.081 and αSD = 0.194. b, G(VBWT, VLP) through the dot at zero VSD. Peaks in G correspond to Coulomb blockade on the dot being lifted; the splitting implies finite Udg. For fixed VBWT the difference in peak positions (blue horizontal bar) gives the dot–grain charging energy Udg = (e)(αLP)(ΔVLP) = 0.081 × 0.26 meV = 21 µeV. Dot–grain tunnelling is negligible in this limit. c, Conductance through the grain appears where expected given the interpretation of b. The conductance is measured with gates BL and BR depleted, measuring through the two point contacts formed by gate pairs LBB/BWB and LBT/BWT.
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Keller, A., Peeters, L., Moca, C. et al. Universal Fermi liquid crossover and quantum criticality in a mesoscopic system. Nature 526, 237–240 (2015). https://doi.org/10.1038/nature15261
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