Searching for new phases of quantum matters with unique physical properties is a never-ending quest in condensed matter physics. Knowledge of these properties holds the key to the future advances in science and technology.Our group aims to unravel the underlying mechanism of these electronic/magnetic properties of novel quantum materials. By using spectroscopic imaging-scanning tunneling microscope (SI-STM) and related spectroscopy techniques, we investigate the electronic and magnetic structures in both real-space and momentum-space with atomic resolution. We also visualize the quantum phenomena on these interesting surfaces by various scanning probe microscopy techniques. Our current topics include topological materials, high Tc superconductors and transition metal dichalcogenides.

Scanning Tunneling Microscope(STM)

STM was invented by Gerd Binnig and Heinrich Rohrer, who sequently received Nobel Prize in physics in 1986 for their achievement. In a STM, a very sharp metal tip is brought very close to the sample surface without physical contact. As an electrical bias voltage is applied to the sample, the electrons tunnels through the vacuum barrier. The tunneling current depends on the tip-sample distance exponentially; i.e. 0.1nm change in tip-sample distance results in a 10 times difference in tunnel current. By precisely measuring this current at different location and controlling the tip-sample distance, we can image the sample surface with high spatial resolution.

Spectroscopic imaging by STM

We typically carry out the following measurements with STM:
(1) Topography
In this mode, a feedback loop is used to keep the tunnel current constant by adjusting the tip-sample distance with a piezo scanner. The adjustment (the voltage applied to the peizo scanner) is recored as function of location (x, y), which represents the relative variation in height of the sample surface, assuming no change in local density of states (LDOS).

(2) Scanning tunneling spectropy (STS)
The differential conductance, dI/dV at an energy of E=eV is proportional to LDOS(E) of the surface in an ideal case. By measuring dI/dV at the energy range of interest, we can obtain the dI/dV spectrum at a given location (x, y).

(3) Differential conductance map, dI/dV(x, y, E) and Fourier transform-STS
By taking the differential conductance, dI/dV(E) as function of location (x, y), a 3D map of LDOS(x, y, E) can be obtained and reveals the spatial variation of electronic structures with atomic resolution. In addition, the standing waves of quasiparticle interference, resulting from quasiparticles scattering off impurities or defects, can be measured by Fourier transform of LDOS map. This technique yeilds the key information on the electronic truncture in momentum space.

Our home-built spectroscopic-imaging STMs (SI-STMs)

It takes several days to acquire one spectroscopic map. Moreover, it is necessary to keep register to the same field of view with atomic precision when we change the experimental condition such as temperature or magnetic field. To achieve such conditions, the design of our STM system has to reach the ultimate mechanical stability.

1.6K-9Tesla cryogenic UHV SI-STM

In last several years, we have successfully constructed two powerful SI-STMs under the generous support from Ministry of Science and Technology and Academia Sinica. The picture below shows our first SI-STM system inside a NIC-56 soundroom, which can greatly reduce the acoustic noise. For our homemade vibration isolation system, three large heavy pillars (each is filled with ~500kg lead) connect to form a stable tripod structure. Three pneumatic isolators with neutral frequency of ~1Hz are mounted on each pillar to support a lead-filled table (~900kg), where our STM system is installed. The homemade cryostat has a cleaving mechanism so we can cleave single crystals at T<20K in UHV. The Pan-type STM head is mounted on a home-made low vibration 4He pot refrigerator, which can achieve the base temperature~1.6K at STM head. The system is fitted with a custom-made 9Tesla magnet dewar. The helium boil-off rate is ~4.8 L/day, which allows us to take spectroscopic maps continuously for ~10 days.

UHV 3He-16Tesla SI-STM and the Precision Measurement Lab

The second is our new 300mK-16Tesla UHV SI-STM as depicted below. The system is installed in the Precision Measurement Lab inside the Interdisciplinary Research Building at Academia Sinica. The Precision Measurement Lab is specially designed for ultra-sensitive experiments like SI-STM. The lab floor is a 75ton concrete block that is decoupled from the main building floor by a thick layer of sand. We further installed our homemade vibration isolation system to minimize the vibrational noise. The lab is acoustically shielded and temperature/humidity regulated so that the environmental impact on the measurements is minimized. We can operate this SI-STM system for more than 6 days at T=4.2K or ~5 days at T~270mK. We can now reach the energy resolution of ~110ueV at the base temperature. Combined with the 16Telsa magnetic field, our 3He-16Tesla SI-STM is not only the first in Taiwan but also unique in the world.

Low temperature scanning probe microscope

Iron-based high temperature superconductors (FeSC)

With superconducting transition temperature up to ~56K, FeSC are the second high Tc superconductor family known to date. Although there is a general consensus on the unconventional nature of the Cooper pairing state for FeSC, many issues remain unsolved, including the pairing symmetry and the electron-pairing mechanism. Our goal is that to understand the mechanism of high-Tc superconductivity by complementary studies on FeSC and cuprates.

If you'd like to learn more about our research, please contact Dr. Tien-Ming Chuang.