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Register a new userTwo phase transitions induced by a magnetic field in graphite
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Different instabilities have been speculated for a three-dimensional electron gas confined to its lowest Landau level. The phase transition induced in graphite by a strong magnetic field, and believed to be a Charge Density Wave (CDW), is the only experimentally established case of such instabilities. Studying the magnetoresistance in graphite for the first time up to 80 T, we find that the magnetic field induces two successive phase transitions, consisting of two distinct ordered states each restricted to a finite field window. In both states, an energy gap opens up in the out-of-plane conductivity and coexists with an unexpected in-plane metallicity for a fully gap bulk system. Such peculiar metallicity may arise as a consequence of edge-state transport expected to develop in presence of a bulk gap.
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A detailed magnetoresistance study of bulk and microflake samples of highly oriented pyrolytic graphite with a thickness of 25 $mu$m to 23~nm reveals that the usually observed field-induced metal-insulator and electronic phase transitions vanish in thinner samples. The observed suppression is accompanied by orders of magnitude decrease of the magnetoresistance and of the amplitude of the Shubnikov-de-Haas oscillations. The overall behavior is related to the decrease in the quantity of two-dimensional interfaces between crystalline regions of the same and different stacking orders present in graphite samples. Our results indicate that these field-induced transitions are not intrinsic to the ideal graphite structure and, therefore, a relevant portion of the published interpretations should be reconsidered.
In the immediate vicinity of the critical temperature (T$_c$) of a phase transition, there are fluctuations of the order parameter, which reside beyond the mean-field approximation. Such critical fluctuations usually occur in a very narrow temperature window in contrast to Gaussian fluctuations. Here, we report on a study of specific heat in graphite subject to high magnetic field when all carriers are confined in the lowest Landau levels. The observation of a BCS-like specific heat jump in both temperature and field sweeps establishes that the phase transition discovered decades ago in graphite is of the second-order. The jump is preceded by a steady field-induced enhancement of the electronic specific heat. A modest (20 percent) reduction in the amplitude of the magnetic field (from 33 T to 27 T) leads to a threefold decrease of T$_c$ and a drastic widening of the specific heat anomaly, which acquires a tail spreading to two times T$_c$. We argue that the steady departure from the mean-field BCS behavior is the consequence of an exceptionally large Ginzburg number in this dilute metal, which grows steadily as the field lowers. Our fit of the critical fluctuations indicates that they belong to the $3DXY$ universality class, similar to the case of $^4$He superfluid transition.
Exploring new parameter regimes to realize and control novel phases of matter has been a main theme in modern condensed matter physics research. The recent discovery of 2D magnetism in nearly freestanding monolayer atomic crystals has already led to observations of a number of novel magnetic phenomena absent in bulk counterparts. Such intricate interplays between magnetism and crystalline structures provide ample opportunities for exploring quantum phase transitions in this new 2D parameter regime. Here, using magnetic field and temperature dependent circularly polarized Raman spectroscopy of phonons and magnons, we map out the phase diagram of CrI3 that has been known to be a layered AFM in its 2D films and a FM in its 3D bulk. We, however, reveal a novel mixed state of layered AFM and FM in 3D CrI3 bulk crystals where the layered AFM survives in the surface layers and the FM appears in deeper bulk layers. We then show that the surface layered AFM transits into the FM at a critical magnetic field of 2 T, similar to what was found in the few layer case. Interestingly, concurrent with this magnetic phase transition, we discover a first-order structural phase transition that alters the crystallographic point group from C3i to C2h and thus, from a symmetry perspective, this monoclinic structural phase belongs to the 3D nematic order universality class. Our result not only unveils the complex single magnon behavior in 3D CrI3, but also settles down the puzzle of how CrI3 transits from a bulk FM to a thin layered AFM semiconductor, despite recent efforts in understanding the origin of layered AFM in CrI3 thin layer, and reveals the intimate relationship between the layered AFM-to-FM and the crystalline rhombohedral-to-monoclinic phase transitions. These findings further open up opportunities for future 2D magnet-based magneto-mechanical devices.
We study the possibility to observe the two channel Kondo physics in multiple quantum dot heterostructures in the presence of magnetic field. We show that a fine tuning of the coupling parameters of the system and an external magnetic field may stabilize the two channel Kondo critical point. We make predictions for behavior of the scaling of the differential conductance in the vicinity of the quantum critical point, as a function of magnetic field, temperature and source-drain potential.
We present magnetization and magnetostriction studies of the insulating perovskite LaCoO3 in magnetic fields approaching 100 T. In marked contrast with expectations from single-ion models, the data reveal two distinct first-order spin transitions and well-defined magnetization plateaux. The magnetization at the higher plateau is only about half the saturation value expected for spin-1 Co3+ ions. These findings strongly suggest collective behavior induced by strong interactions between different electronic -- and therefore spin -- configurations of Co3+ ions. We propose a model of these interactions that predicts crystalline spin textures and a cascade of four magnetic phase transitions at high fields, of which the first two account for the experimental data.
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