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Register a new userNonequilibrium quantum phase transition in itinerant electron systems
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D. E. Feldman
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We study the effect of the voltage bias on the ferromagnetic phase transition in a one-dimensional itinerant electron system. The applied voltage drives the system into a nonequilibrium steady state with a non-zero electric current. The bias changes the universality class of the second order ferromagnetic transition. While the equilibrium transition belongs to the universality class of the uniaxial ferroelectric, we find the mean-field behavior near the nonequilibrium critical point.
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Quantum-critical behavior of the itinerant electron antiferromagnet (V0.9Ti0.1)2O3 has been studied by single-crystal neutron scattering. By directly observing antiferromagnetic spin fluctuations in the paramagnetic phase, we have shown that the characteristic energy depends on temperature as c_1 + c_2 T^{3/2}, where c_1 and c_2 are constants. This T^{3/2} dependence demonstrates that the present strongly correlated d-electron antiferromagnet clearly shows the criticality of the spin-density-wave quantum phase transition in three space dimensions.
To harness technological opportunities arising from optically controlled quantum many-body states a deeper theoretical understanding of driven-dissipative interacting systems and their nonequilibrium phase transitions is essential. Here we provide numerical evidence for a dynamical phase transition in the nonequilibrium steady state of interacting magnons in the prototypical two-dimensional Heisenberg antiferromagnet with drive and dissipation. This nonthermal transition is characterized by a qualitative change in the magnon distribution, from subthermal at low drive to a generalized Bose-Einstein form including a nonvanishing condensate fraction at high drive. A finite-size analysis reveals static and dynamical critical scaling, with a discontinuous slope of the magnon number versus driving field strength and critical slowing down at the transition point. Implications for experiments on quantum materials and polariton condensates are discussed.
We propose a static auxiliary field approximation to study the hybridization physics of Kondo systems without the sign problem and use the mutual information to measure the intersite hybridization correlations. Our method takes full account of the spatial fluctuations of the hybridization fields at all orders and overcomes the artificial (first-order) phase transition predicted in the mean-field approximation. When applied to the two-impurity Kondo model, it reveals a logarithmically divergent amplitude mutual information near the so-called Varma-Jones fixed point and a large phase mutual information manifesting the development of intersite phase coherence in the Kondo regime, with observable influences on physical properties. These highlight the importance of hybridization fluctuations and confirm the mutual information as a useful tool to explore the hybridization physics in Kondo systems.
Complete expressions of the thermal-expansion coefficient $alpha$ and the Gr{u}neisen parameter $Gamma$ are derived on the basis of the self-consistent renormalization (SCR) theory. By considering zero-point as well as thermal spin fluctuation under the stationary condition, the specific heat for each class of the magnetic quantum critical point (QCP) specified by the dynamical exponent $z=3$ (FM) and $z=2$ (AFM) and the spatial dimension ($d=3$ and $2$) is shown to be expressed as $C_{V}=C_a-C_b$, where $C_a$ is dominant at low temperatures, reproducing the past SCR criticality endorsed by the renormalization group theory. Starting from the explicit form of the entropy and using the Maxwell relation, $alpha=alpha_a+alpha_b$ (with $alpha_a$ and $alpha_b$ being related to $C_a$ and $C_b$, respectively) is derived, which is proven to be equivalent to $alpha$ derived from the free energy. The temperature-dependent coefficient found to exist in $alpha_b$, which is dominant at low temperatures, contributes to the crossover from the quantum-critical regime to the Curie-Weiss regime and even affects the quantum criticality at 2d AFM QCP. Based on these correctly calculated $C_{V}$ and $alpha$, Gr{u}neisen parameter $Gamma=Gamma_a+Gamma_b$ is derived, where $Gamma_a$ and $Gamma_b$ contain $alpha_a$ and $alpha_b$, respectively. The inverse susceptibility coupled to the volume $V$ in $Gamma_b$ gives rise to divergence of $Gamma$ at the QCP for each class even though characteristic energy scale of spin fluctuation $T_0$ is finite at the QCP, which gives a finite contribution in $Gamma_a=-frac{V}{T_0}left(frac{partial T_0}{partial V}right)_{T=0}$. General properties of $alpha$ and $Gamma$ including their signs as well as the relation to $T_0$ and the Kondo temperature in temperature-pressure phase diagrams of Ce- and Yb-based heavy electron systems are discussed.
In spite of its ubiquity in strongly correlated systems, the competition of paired and nematic ground states remains poorly understood. Recently such a competition was reported in the two-dimensional electron gas at filling factor $ u=5/2$. At this filling factor a pressure-induced quantum phase transition was observed from the paired fractional quantum Hall state to the quantum Hall nematic. Here we show that the pressure induced paired-to-nematic transition also develops at $ u=7/2$, demonstrating therefore this transition in both spin branches of the second orbital Landau level. However, we find that pressure is not the only parameter controlling this transition. Indeed, ground states consistent with those observed under pressure also develop in a sample measured at ambient pressure, but in which the electron-electron interaction was tuned close to its value at the quantum critical point. Our experiments suggest that electron-electron interactions play a critical role in driving the paired-to-nematic transition.
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